Portland Cement Association Research and Development Bulletin RDlOST by Robert L. Day
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Portland Cement Association
Research and Development Bulletin RDlOST
by Robert L. Day
KEYWORDS: accelerated testing, alkali-aggregate reaction, alkalis, calcium aluminates, calcium hydroxide,carboaluminates, carbonation, cement, cement chemistry, chlorides, chloroaluminates, cracking, delayed ettringiteformation, Duggan Test, durability, ettringite, expansion, gypsum, heat treatment, hydration, ions in solution,microcracking, monosulphoaluminate, pH, pore solution, porosity, precast concrete, precuring period, preferredorientation, secondary ettringite formation, strength, sulfates, sulfate/aluminate ratio, swelling pressure, tempera-ture, testing, thaumasite, railroad ties, transition zone.
ABSTRACT: The report comprises a review and analysis of the available literature pertaining to the causes, effectsand prevention ofsecondary (delayed) ettringite in concrete. Over 300publications have been examined. Case studiesof damage in concrete possibly caused by secondary ettringite formation are examined first. Fundamental researchon secondary ettringite formation, its chemistry, and deposition mechanisms is then reviewed. Key investigationson the topic are analyzed in detail. Next, the potential importance of (a)method of heat-curing and (b) the chemistryof cement is outlined. In the final chapter, a rapid test for evaluation of potential secondary ettringite susceptibility(the “Duggan” test) is evaluated. The analysis indicates that there appears to be a potential for a secondary ettringiteformation problem in North America; it is highly probable that secondary ettringite formation can lead to significantdeterioration of heat-treated concrete. However, it is unlikely that secondary ettringite formation is, or will be, thesole mechanism responsible for premature deterioration. The critical factors that determine extent of damage due tosecondary ettringite formation are(a) duration of delay period before heating the concrete;(b) severity of the heatingand/or cooling regime; and (c)the S03/A120~ ratio of the cement. There isno evidence that non heat-treated concreteis susceptible to this phenomenon. Further research and improvements to the Duggan test may result in thedevelopment ofa useful standard test method to assess the long-term dimensional stability and durability ofconcrete.
REFERENCE: Day, R.L.,TheE~ecto~SecondwyEttringite Fonmfion on theDurabi~ityof Concrete: A Literature Analysis,Research and Development Bulletin RD108T,Portland Cement Association, Skokie, Illinois, U.S.A. 1992.
MOTS CLI%: alcali, aluminate de calcium, bi%onpr6fabriqu6, carboaluminate, carbonatation, ciment, chimie duciment, chlorures, chloroaluminates, dormant de chemin de fer, durability, essai, essai acc416r4 essai Duggan,ettringite, expansion, fissuration, formation retardde d’ettringite, formation secondaire d’ettringite, gypse, hydratation,hydroxyde de calcium, ions en solution, microfissuration, monosulfoaluminate, orientation pr4f4rentielle, PH,pc%iodeavant cure, porositd, pression de gonflement, rapport sulfate/aluminate, rdaction alcali-granulat, rkisistance,solution de pore, sulfate, temp&ature, thaumasite, traitement h la chaleur, zone de transition.
Rl%UME: Le rapport consiste en une revue et une analyse de la littt%ature disponible en ce qui a trait aux causes, auxeffets et 21la pn%ention de la formation secondaire (retard6e) d’ettringite clans le bdton. Plus de 300publications ontdtd examin~es. Des &udes de cas sur des b6tons endommagds, possiblement Acause de la formation secondaired’ettringite, ont dtdexamindesenpremier. La recherche fondamentalesurla formation retardde d’ettringite, sa chimieet ses mdcanismes de deposition est ensuite passde en revue. Les investigations d’importance sur le sujet sentanalysdes en d&ail. Puis, l’importance potentielle de (a) la m&hode de cure iila chaleur et (b) la chimie du ciment estddcrite. Dans le dernier chapitre, un essai accdldrdpour 6valuer le potentiel de formation retardde d’ettringite (1’essai“Duggan”) est dvalud. L’analyse indique qu’il est possible qu’il y ait des probl$mes de formation secondaired’ettringite en Am6rique du Nerd. 11est tri% probable que la formation secondaire d’ettringite produise uned&6rioration importance du b6ton traitd Ala chaleur. Cependant, il est tr&speu probable que la formation secondaired’ettringite, soit ou devienne, le seul mdcanisme responsible de d&6rioration pr6matur6e. Les facteurs critiques quid&erminent l’ampleur des dommages dus ?ila formation secondaire d’ettringite sent (a) le ddlai avant chauffage dub&on, (b) la st%%itddu rdgime de chauffage ou de refroissement et (c)le rapport SO~/AlzO~du ciment. 11n’y a aucunindite ~ l’effet que le b6ton non chauff6 soit susceptible ?Ice ph~nomhe. Dautres recherches et des ameliorations ~I’essaiDugganpourraient mener iil’elaboration d’un essainormalisd utile pour determiner lastabilitd dimensionnelleAlong terme et la durability du bdton.
R~FfiRENCE: Day, R.L., TheEject ofSecondary Ettringite Formation on the Durability of Concrete: A literature Analysis,Research and Development Bulletin RD108T, Portland Cement Association [L’effet de la formation secondaired’ettringite sur la durability! du btiton une analyse bibliographique. Bulletin de Recherche et D4veloppementRD108T,Association du Ciment Portland], Skokie, Illinois, U.S.A. 1992.
Cover Illustrations: Ettringite (white needle-like crystals) in concrete exposed to phenolphthalien stain; atmospheric steam-curing cycle; and precast structure under construction.
PCA R&D Serial No. 1929
PCA Research and Development Bulletin RD108T
The Effect of Secondary EttringiteFormation on the Durability of Concrete:
A Literature Analysis
by Robert L. Day
Professor of Civil EngineeringCivil Engineering De artment
PThe University o Cal a2500 Universit Drive
J%x
Calgary, Alberta, Cana a T2N 1N4
ISBN 0-89312-1 69-X
42Portland Cement Association 1992
Page i
Summary of Major Conclusions
A review and analysis was performed of available literature pertaining to the causes, effects andprevention of secondary, or delayedl, ettringite in concrete. Over 300 publications were exam-ined The principal findings from this project are:
Given current Cement-prduction and concrete-construction practices there appears to bea potential for a secondary ettringite formation problem in North America. It is highlyprobable that secondary ettringite formation can lead to significant deterioration of heat-treated cxmcrete.The extent of the potential problem cannot be predicted with accuracy,but it is likely to involve a small fraction of the pre-cast concrete cast in North America.
There is no evidence that secondary ettringite formation has been a principal cause of de-terioration in non heat-treated concrete. There is some suggestion that formation of et-tringite after initial deterioration by other means can accelerate the deterioration process.Massive formations of ettringite observed in huge cracks and pores appear, for the mostpart, to be innocuous.
The critical factors that determine extent of damage due to secondary ettringite formation(all other factors being equal) are (a) duration of delay period prior to heating; (b) sever-ity of the heating &Jor cooling r6gime; (c) the S03/A1203 ratio of the cement.
Use of cements with an WA ratio greater than about 0.7 or an ~/A ratio greater thanabout 2.0 may result in concrete which+under particular curing and exposure conditions,is susceptible to secondary ettringite formation and subsequent deterioration.
Good quality laboratory research confirms that secondary ettringite formation can pro-duce expansion and cracking of concrete made with certain North American commercialcements. In North America it is not unusual to find Type 30 cements with S03 contents inthe range 3.5 to 4.0% — which could be as high as 4.5Y0.It is not unusual to find alumina
contents of 5% or more and C3A contents of 9% or more. Calculation of the ~/A or ~/Aratios for such cements places them in the maximum expansion ranges determined by He-inz and Ludwig and by Gillott.
Rapid heating and/or cooling and/or an inadequate delay period can result in microcracks— predominantly at the aggregate-paste or steel-paste interface. These cracks can act asnucleation sites for the later formation of ettringite.
‘Ihe aggregate-paste and steel-paste interface is a weak zone in the concrete that is high inboth calcium hydroxide and ettringite contents. The ready availability of sulphates nearthe cracked interface provides pessimum conditions for secondary ettringite formation.
The transition zone appears to be of higher quality when the interface involves limestoneaggregate. This suggests that, all other factors being equal, concrete made with limestoneaggregate may be less susceptible to secondary ettringite damage.
Saturated, or almost saturated, concrete or concrete subjected to frequent wetting/dryingcycles is essential to the formation of secondary ettringite and concrete darnage.
Early excessive heat treatments at tem~ratures above approximately 70”C results in sub-stantial amounts of sulphates being bound in an unusual form. These sulphates can be
1 see Section 1.3for an explanationof the terminology“secondaryetlringite”
Page ii
slowly released back into solution at later ages. This slow release process provides a sup-ply of sulphate ions for secondary ettringite formation.
The effect of secondary ettringite formation is substantially reduced by inclusion of anadequate air-entrainment void system in the concrete.
Where potential expansion due to secondary ettringite formation can occur, time to startof expansion increases and rate of expansion decreases as the size of the member or speci-men increases.
The 14 day sulphate expansion test does not provide an adequate prediction of cement sta-bility at later ages,
Specifications for Heat-Treated Concrete by the German Committee for Reinforced Con-crete (Table 6.2) should be serious]y considered for adoption in North America,
A new test method to determine optimum gypsum content of cement should be developedwhich considers long-term stability of the cement as well as optimum strength and settingtime,
Further research and improvements to the Duggan test may result in the development of ausefhl standard test method to assess the long-term dimensional stability and durability ofconcrete,
Page ill
Table of Contents
Summary of Major Conclusions . . . . . . . . . . . . . . . . . . . . . . . . ..i
TableofContents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..iii
ListofFigures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..vi
ListofTables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..viii
Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..ix
Chapter 1— Introduction 1
1,1 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...1
l.20utlineofReport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...1
l.3Terrninology— “SecondaryEttringite’’. . . . . . . . . . . . . . . . . ...2
Chapter 2 — Damage due to Secondary EttringiteFormation in Ordinary and Precast Concrete 4
2.1 CombinationsofAttackMechanisms . . . . . . . . . . . . . . . . . . . . . 4
2.2PelrographicObservations.. . . . . . . . . . . . . . . . . . . . . . . ...7
2.3RoblemswithPrecastConcreteandTies . . . . . . . . . . . . . . . . . . . 7
2.4RecastConcrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...9
2.4.1 EffectofHeatTreatmentonProperties. . . . . . . . . . . . . . . . . . . 9
2.4.2TheCauseof InsufficientStrength Gain afterHeat’lleatment 13
2.5Comment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..14
Chapter 3 — Secondary Ettringite Formation:Fundamental Research 16
3.1 Overview of the Cement Hydration Process. . . . . . . . . . . . . . . . . . 16
3.2 Early Formation of Sulphoaluminates . . . . . . . . . . . . . . . . . . . . 16
3.3 Crystal Structure and Composition of Ettringite . . . . . . . . . . . . . . . 17
3.4 The Role of Ettringite in the Retardation of the Calcium Aluminates . . . . 18
3.5 The Chemical Stability of Sulphoa.luminates. . . . . . . . . . . . . . . . . 20
3.5.1 Stability in Solution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , 20
Page iv
3.5,2 Stability and Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.5,3 Post-Decomposition Behaviour., . . . . . . . . . . . . . . . . . . . . . . 22
3.6 lhe Effect of Alkalis on Hydration and Formation of Ettringite . . . . . . 25
3.7 The Roleof C02and Carbonates . . . . ., . ., . . . . . . . . . . . ...25
3.7.1 Formation of ‘i%aumasite . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.7.2 Formation of Carboahuninates.. . . . . . . . . . . . . . . . . . . . . . . 26
3.8 Formation of Chloroaiuminates . . . . . . . . . . . . . . . . . . . . ...28
3.9 Morphology of Ettringite ...,..... . . . . . . . . . . . . . . . ...28
3,10 Mechanism of Expansion Associated with Ettringite Formation . ,
3.10.1 Crystal Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.10.2 Swelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.11 Mechanism of Secondary Ettringite Formation . . , . . . . . . , .
3012Comment, , . . . . . . .. o... . . . . . . . . . . . . . . . .
. . . 29
. . . . . 29
. . . . . 30. . . 32. . . 3s
Chapter 4 — Secondary Ettringite Deposition 36
4.1 The Importance of Porosity to Ettringite Formation and Expansion. . . . . 36
4.2 The Importance of the Aggregate/Paste and Steei/Paste Interface . . . . . . 36403Comments ... ,,. o,.,..,,. . ., .,, , . . . . . . . . . . ...43
Cha ter 5 —r
Key Examinations of SecondaryEttr ngite Formation 44
5.11980 —Ghorabetal [106] .,,..,.,, , . . . . . . . . . . . . . ...44
5.21984 — Research Institute of the Cement Industry [268] . . . . . . . . . . 45
5.31987 —Heinzand Ludwig [132] ., .,,,.,,.,...,.,,.,,. 47
5.41988 —Sylla[297] . . . . . . . . . . . . . . . . . . . . . . . . . . . ...52
5.51989 —Heinz, Ludwig &Rudiger[133] , . . , , , . . . . . . . . . . . , 54
5,61990 -Lawrence etal[177], .,... ,, ..,,..,.,,......,55
5.7 Comment..,,,,......,,. , . . . . . . . . . . . . . . . . ...59
Page v
Chapter 6 — Secondary Ettringite Formationand the Chemistry of Cement 60
6.1 Effect of Gypsum Content of Cement on Strength and Volume Stability . . 60
6.1.1 Test Results of A1-Rawi [6]. . . . . . . . . . . . . . . . . . . . . . . . . . . 60
6.2 The Practical Significance of the S03/AIZOs Ratio . . . . . . . . . . . . . 62
6.3 Specifications for Heat Treatment of Concrete. . . . . . . . . . . . . . . . 64
6.4 Comment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...67
Chapter 7 —Rapid Test Method for SecondaryEttrmgite Formation 69
7.17he Duggan Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...69
7.1.1 Significance of Duggan-Test exults . . . . . . . . . . . . . . . . . . . . 71
7.1.2 Research of Gillott, et al . . . . . . . . . . . . . . . . . . . . .$, . . . . . . 71
7.1.3 Research of Attiogbe and Wells et al [12, 317] . . . . . . . . . . . 75
7.1.4 Correlations Between Gillott’s and Attiogbe’s Results . . . . . 78
7.2 Interpretation of Duggan-Test Results . . . . . . . . . . . . . . . . . . . . 79
7.2.1 The Zero Strain Datum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
Chapter 8 — General Discussion 83
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
References 85
References Not Found 106
Subject Index 114
Pwzevi
List of Figures
2.1 Carbonation of Bridge Structures . . . . . . . . . . . . . . . . . . . . . ...5
2.2 Chloride Penetration into Concrete. . . . . . . . . . . . . . . . . . . . . . . 5
2.3 Relative Strengths of Heat Treated Concretes . . , . . . . . . . . . . . , . 11
2.4 Effect of Heat Curing and Drying on Relative Permeability . . . , . . . . . 12
2.5 Effect of Post Treatment Water Curing on Strength Gain . . . . , . . . . . 14
3,1 Critical Values for S03/AlzOs Ratio,. . . . . . . . . . . . . . . . . . . . 17
3.2 Sulphate-Ettringite Crystal Structure . . . . . . . . . . . . , . . . . . . . . 19
3.3 Effect of Curing Temperature on Sulphoaluminate Phases . . . . . . . . . 23
3.4 Variation in Ettringite Peak Intensity with Curing Temperature. . . . , . . 24
3.5 Correlation between Volume Increase and Ettringite , . , , . , . . . , . . 31
3.6 Formation of Ettringite from Sulphates Supplied by C-S-H , . , , . , , . . 32
3.7 Changes of Sulphate Ion Concentration with Heat lleatment . . . . . . . . 34
3.8 Change in S04 and OH Ion Concentration During Storage . . . . . . . . . 34
4,1 The Paste/Aggregate Interface at 30 Minutes and 3 Days , , . . . . . . . . 38
4,2 Variation in Ettringite Peak Intensity from Interface, , . . . . . . . . . . . 39
4.3 Compounds and Orientation in the Transition Zone , , . . . . . . , . . , . 40
4.4 Orientation of Calcium Hydroxide Crystals in the Transition Zone , , . , , 41
4.5 Schematic of Changes in the Transition Zone , . , . , . . . . . . . . . . . 42
5.1 Expansion of Various Mortars after Heat Treatment. , . . , . . , , , . . , 44
5.2 Expansion of Paste Disks aftm Heat Treatment . , . . . . , . . . . . . . . 46
5.3 Changes in Expansion, Weight and Resonant Frequency with Storage . . . 48
5.4 Correlation Between Expansion and Weight Gain . . , . . , , . . . . . . . 49
5.5 Correlation Between Expansion and Resonant Frequency , . . . . . . . . . 49
5.6 Series II Experiments, Expansion vs. Storage . . , . . . , , . . . . , . . . 50
5.7 Effect of Humidity Change on Expmsion . , , , . , , , , , , . , , , . . , 51
5.8 Effect of Water/Cement Ratio and Air Entrainment on Expansion , . . . . 52
5.9 Variation in X-Ray Peak Intensity with Curing Temperature , , . . . , . . 53
5.10 Changes in X-Ray Peak Intensities with Soaking Time , . . , , . . , . . 54
5.11 Scattergram of?/ARatio vs. Expwsion . . . . . , . . , . . . . . . . . . 56
5.12 Typical Expansion vs. Time Curves . . . . . . , . . . . , . . . . . . . . 57
5.13 Expansion vs Time Curves Showing Effect of Delay Period . . . , . . . , 57
Page W
6.1 Dependence of 28-Day Strengths on S03 Content ~d Cting Temp. . . ● 61
6.2 Dependence of 182-Day Strengths on S03 Content and Curing Temp. . . . 61
6.3 Effect of S03 Content on Expansion of Normal Fog-Cured Specimens. . . 626.4 Effect of S03 Content on Exp@on tier 48 W=k of Sotig . . . . . “ 63
6.5 Scattergram of Expansion at 14 Dais vs Expansion at 5 Years . . . . . . . 64
6.6 Scattergram of Expansion vsMgOContent . . . . . . . . . . . . . . . . . 65
6.7 Scattergrsm of S03/A120s Ratio vs Expansion . . . . . . . . . . . . . . . 65
7.1 Scattergram of Accelerated vs. Field Expansion . . . . . . . . . . . . . . 70
7.2 Effect of Alkali Content and Cement Type on Expansion . . . . . . . . . . 72
7.3 Expansion of Hardened Cement Pastes and Concretes . . . . . . . . . . . 737.4 Scattergramof Concrete Expansion vs. Paste Expansion . . . . . . . . . . 74
7.5 Observations of a Pessimum SOJM203 Ratio ~ V~OM Ages . . . . ● . 76
7.6 Temperature Testing R6gimes, Duggan and Attiogbe T~* . . . . . . . . . 77
7.7 Expansion of Paste Cores in Attiogbe Test. . . . . . . . . . . . . . . . . . 78
7.8 Scattergram of Strsins Measured by Gillott and Attiogbe . . . . . . . . . . 79
7.9 Comparison of 20 Day Expansions Using Three Zero Definitions . . . . . 81
Page viii
Lkt of Tables
5.1 Effect of Heat Treatment on Cracking and Phase Formation . . . . . . . . 46
5.2 Expansion vs. Sulphate Content . . . . . . . . . . . . . . . . . . ...4 . S8
6.1 Compound Composition of Cfinlws used in A1-Rawi Study . . . . . . . . @
6.2 SpecillcationsforHeat-TreatedCo ncre* . . . . . . . . . . . . . . . . . . 66
7.1 Analysis of Strsins During Duggan TeSt . . . . . . . . . . . . . . . . . . . 81
Page ix
Notation
Where appropriate, cement chemists’ notation has been used. his notation as itapplies to the present nqmrt is:
C = CaO
s = !N02s= S03
A = /d@s
H = H20
Chapter 1 Page 1
Chapter 1Introduction
1.1 Objectives
At the March 15, 1991 meeting of a ‘Task Group on Secondary Ettringite Formation” — a groupstruck by the Canadian Standards Association Committee AS (Hydraulic Cement) Executive —the following recommendation was made:
‘Whereas the subcommittee is not aware of any documented cases of secondary ettringite failuresin Canada, it is recommended that a detailed review and analysis of the literature (and especiallythe literature horn Europe) should be undertaken to:
= attempt to identify significant factors which have caused documented cases of secondaryettringite failures in Europe
~ attempt to determine which of the factors may be relevant to North America — i.e. isthere, or is there likely to be, a potential secondary ettringite formation problem in NorthAmerica’?”
At the May 3, 1991 meeting of the CSA AS Committee, the Committee passed the following mo-tion in support of the Task Group’s recommendation:
“The AS Hydraulic Cements Committee supports, in principle, the need for a detailed review andanalysis of the literature relevant to seconduy ettringite formation”. The Portland Cement Association agreed to provide the funding for such a study; this report is the result.
As well as the above objectives, the report centres upon an attempt to provide an adequate answerto a number of important questions concerning secondary ettringite formation:
Is the occurrence of secondary ettringite and its effect adequatel y documented?
What are the key properties of the constituents of concrete, the key properties of the con-crete itself, and the key environmental parameters that determine whether secondary et-tringite formation is likely to occur?
What can be done in the cement and concrete industries in Canada to minimize the possi-bility that future construction will be prone to damage by secondary ettringite formation?
Isa test available that can predict whether secondary ettringite formation for a given ce-ment or concrete is like]y to lead to durability problems?
The net result is a fair]y comprehensive report. A database search indicated there were 440 refer-ences that may contain information relevant to the topic. Many of these references had obsmueorigins; although the interlibrary loans office of the University tried their hardestj only 327 refer-ences were reviewed. These 327 publications are listed at the end of this document, The 113 docu-ments that could not be obtained rue also listed. After review of these references not all weredeemd to be di~ctly relevant in the report proper, approximately 160 of the 327 references arecited.
1.2 Outline of Report
The report starts with an examination of various case studies relevant to the formation of secon-dary etlringite in concrete. Many of the problems that have occurred centre upon the precast con-
Chapter 1 Page 2
Crete industry; therefore, in Chapter 2 a summary of precast practices and relevant issues aregiven.
Chapter 3 looks at the fundamental processes associated with secondary ettringite formation. Thechemistry and stabiiity of sulphoaluminates are reviewed. The morphology and mechanisms offormation and expansion are studied in detail. Fhmily, chemicai issues related to the formation ofsecondary ettringite are noted.
Chapter 4 is an examination of the physicrd processes associated with the deposition of ettringitewhich includes a review of the importance of the aggregate/cement-paste and steel/cement-pasteinterfaces.
Chapter 5 is an in-depth examination of the handful of key publications that directly relate to sec-ondary ettringite formation and its consequences.
In Chapkx 6 practical issues are discussed concerning the importance of gypsum and the chemis-try of cement to the type and quantity of ettringite that is deposited. Aiso in Chapter 6 the types ofheat-treatment are discussed; emphasis is placed upon specifications and guidelines that are h-tended to prevent “extreme” thermai treatments.
One of the principai conclusions of this report is that secondary ettringite formation could have amajor effect in determining the long-term durability of some types of concrete, Accordingly,Chapter 7 concentrates upon an analysis of the Duggan test as a potentiai method to indicate thevulnerability of cements, aggregates and concretes to long-term deterioration mechanisms,
In Chapta 8 the major findings of the report are summarized and discussion centres upon thesteps that might be taken by the construction industry to Educe the potentiai problem of secon-dary ettringite formation.
1.3 Terminology — “Secondary Ettringite”
One of the frostquestions that arose during the initial stages of writing this report was: what to callthe phenomenon by which concrete, normally at ages of a few years or more, is darnaged by thegradual formation of “ettringite” within the microstructure of the material. Lacking the imagina-tion to invent a new term, the choice for the “appropriate” terminology was made among (1) de-layed ettringite, (2) late ettringite and (3) secondary ettringite, formation.
Study of the literature concerning fundamental mechanisms that cause the phenomenon leads tothe conclusion that the term “delayed ettringite formation” is not accurate. lTis term implies thatconditions within the microstructure might be suitable for the formation of ettringite but that et-tringite does not form; this is not true. It also implies that the delayed ettringite that forms is thesame ettringite that did not form during the early hydration of the cement; this is also not the case.
“Late ettringite formation” is certainly more accurate in the practicai sense, since the “problem”normally surfaces several years after the concrete is case but it is not precise. Late can have manymeanings and, as study of this report will show, the process by which ettringite formation causesthe problem occurs over a long time span. It is only the effect of the ettringite formation thatcomes to light “late”.
The winner, then, is “secondary ettringite formation”. It is accurate because materials engineersand chemists normaily think of primary ettringite formation as the sulphoaluminate that formsshortly after gauging and that tends to disappear when sulphate ions in the pore solution becomedepleted. There can be a period for many concretes, and especially after heat treatment, where no
,,.
Chapter 1 Page 3
ettringite can be detected by X-ray analysis. ‘1’bus,the gradual growth of ettringite peaks on X-raytraces at later ages can naturally be thought of as the formation of secondary ettringite. l%e termalso agrees with the first two definitions of “secondary” found in the Random House Dictionary:(1) next after the fist in order, rx or tim~ (2) not primary or original, but not with the third: (3)of minor or lesser importance.
Unfortunately, the decision on terminology will offend some. In geology, a “secondary mineral”is usually formed after the primary mineral has formed and then dissolved. In concrete this proc-ess may, in fact occur — in which case the terminology that has been chosen is accurate. Otherswould argue, however, that “the secondary formation of ettringite never did any concrete anyharm”l. As review of this report will show, the precise mechanism involved in the late expansionand cracking of concrete due to ettringite formation is far from being clearly defined.
The choice of the terminology “secondary ettringite” should not be taken as an implication thatthe author, at the outset, has taken sides concerning the fundamental mechanisms involved.
1 see, forexample,a letter fromB. Matherto C.R. DuggatIof Feb 27, 1990(aisocopiedto ASTMcommitteeC09.0202)
chapter2 Page 4
Chapter 2
Damage due to Secondary Ettringite Formationin Ordinary and Precast Concrete
2.1 Combinations of Attack Mechanisms
In 1965 Kennerley [161] was one of the first to report possible problems with delayed ettringiteformation. He examined exudations at a cold-joint in the Roxburgh Dam, Otago, New Zealand.Analysis indicated that the white deposit was ettringite. Normally,, when ettringite forms there isexpansion, but he postulated that this expansion was avoided when constituents passed into solu-tion, reacted and precipitated in the voids in the mass. If the solution was low in lime the volubil-ity of both ettringite and calcium aluminates rose. Ettringite and calcium aluminates maytherefore dissolve and be transported through permeable channels to areas high in lime; thesecompounds will then precipitate as ettringite in the lime-rich regions.
Pettifer and Nixon in 1980 [252] described several case studies where secondary ettringite mayhave played a part in the deterioration process.
= Concrete bases of some substations in the English midlands showed deterioration, Al-though there was alkali aggregate reaction, the researchers also observed pores and voidsfilled with ettringite and ettringite coating aggregate particles. This was surprising sincethere were only trace amounts of sulphates in the soil.
= Similar attack was noted in other sub-stations in Western England and South Wales. Thechert-containing limestone reacted, but cores also showed much ettringite co-existingwith the gel reactant from the alkali aggregate reaction.
= Forty year old concrete blocks were examined which exhibited severe alkali-aggregate re-action and much leaching, These blocks were manufactured with unwashed beach graveland were gauged with sea water. Upon petrographic examination, large amounts of cal-cite, gel, ettringite and “secondary portlandite” were observed, (Presumably by “secon-dary portlandite” the researchers mean CH deposited in the pores of the microstructure),
= The Pirow Street Bridge in Cape town, South Africa showed cracking only 4 years aftercompletion, and remedial repairs were necessary after 9 years. Potentially reactive aggre-gates were used, but a low alkali cement was also used. A significant feature of the pet-rogmphic examination was a yellowish gel accompanied by moderate amounts ofettringite.
Around the same time, Volkwein [314] examined 12 to 80 year old concrete bridges for carbona-tion, chloride penetration, deteriomtion and corrosion, As Figure 2.1 shows, the depth of carbona-tion was highly variable (the low carbonation in the 80 year old concrete was because it wasmade with Roman cement). The carbonation depth depended greatly on the moisture condition ofthe structure during its life. Carbonation assisted the penetration of ions into the concrete since itwas found that carbonated concrete conducted vapour “about twice as well” as non-carbonatedconcrete. On bridges, penetration of chloride into the top of decks was negligible; most Cl pene-tmted via seeping, splashing and wind-borne Cl carried to the concrete understructure, Such proc-esses can result in the chloride ion penetmting up to 50mm in sound concrete and up to 90mm infrost &maged concrete. Figure 2.2 shows examples of the penetration depth and Cl- ion concen-trations for various ages and qualities of concrete.
Volkwein found that in these deteriorated concretes, highly contaminated by Cl, “remarkable” ac-cumulations of needle-shaped crystals were found, “particularly in cracks, pores and around ag-
chapter2 Page 5
CARBONATION DEllliS OF DRY CONCR13E FROM BRIDGE
WRUCTURES
❑ Minimum Observed ■ Lower Range of ■ Upper Range of ❑ Maximum ObsewedAverage Depth Average Depth
Figure 2.1Carbonation of Bridge Structures [ data from ref. 314]
Chloride Penetration into Concrete Close to Road - Due to Splashing
\
m....’ ‘....,,..’ . .. .
‘.. .J3.,. ‘“” ‘%,,~..,
‘. ... “q
\.... .,
... .\,..,.,..
---------- 35Ycaold,4SMPa,poorconsoli&tion
__~..___ 21~~~ old,70MPa,good
consolidation
~ 21yearnold,100MPa,goodconsolidation
. .................. ... .......... ......-.-.-.-.-.,,..................................,-‘-”-”’”-’-*-.--.--------.*-.-.--.-.-. -.-.-.m
1
0 10 20 30 40 50 60 70 80 90PENETIU4TION DEPTH (mm)
Figure 2.2Chloride Penetration into Concrete due to Splashing Action [data from ref. 314]
chapter 2 Page 6
gregates”, These crystals were analyzed and were found to be ettringite, with some thaumasite,Volkwein postulated that since the sulphate content had not changed,then the “chloridesdidcause the formationof ettringite” fromthe sulphate in the cement.Additionalreactionsoccurother than the formationof Friedel’s(monochloroaluminate)salt1due to the penetrationof chlo-rides. Wetting and dryingalso helps to transport solublecement compoundsto preferred loca-tions, Volkweinnoted that “it cannotbe said whetherthese ettringitecrystals had caused thedeteriorationof concrete or had grownup in alreadyopencracks”,
Volkwein’sresults are most interestingbecauseettringiteappem to have formed in a chloriderich environment.Contrastthis with the laboratoryresultsof Attiogbeet al [12] who found thatduring an accelerated exposuretest, secondaryettringite wouldnot form while concreteprismswere soaked in a sodium-chloridesatwated solution,Other researchconfiis that ettringite de-composes in sodiumchloride solution [243-245].
Jones & Poole in 1987[154] lookedat the effects of alkali-silica reaction on 3 structures in theUnited Kingdom.They examined20mmthick disks from concretecores taken from the stmc-tures, The disks were stored in sealedcontainemat constanttemperaturesof 1, 15,20,25 and38°C and relative humiditiesof 65,75,85 and 100%,Expansionswere measured;since the con-cretes were taken from structures,all expansionsnoted were relativeto the unknown deformationof the structures.
These researchersfound that initial expansionproceeded fater at elevatedtemperaturebut theMe declined more rapidly,The final expansionswere roughly inverselyproportional to storagetemperature,Petrogmphicexaminationdid not reveal the classicalkali-silica (asr) reaction rimsthat are commonly observed,Crackswere often observedaround“aggregatemargins”whichm-diate into the paste matrix,Asr product was rarely seen inflllingthe cracks,but was most oftenseen at the centre of reactivepatticles, In one structure, large quantitiesof ettringitewere ob-sexved,This material was coarselycrystallineand was seen inflllingthe microcracksand “liningvoids”, Ettringite oftenappeared to have replacedpreviouslydepositedsilica gel,
Jones & Poole noted the researchof ~reening [120]who showedthat monosulphatebecomesme-tastable in the presence of CaCOSand reverts to ettringite2,The reseamhersproposedthat thepresence of limestoneaggregateprovidedthe calcite which renderedettringitethe stable sulphatephase, Ettringite, initially dispersedthroughoutthe microstructurerecrystallizesinto a coarselycrystalline form, The process is helpedby increasedpermeabilitycausedby other accompanyingprocesses such as asr cracking,The formationof ettringite in the voids is thus due to a through so-lutionprocess through the pore fluid,The “preferentialrecrystallization”of ettringite in “gel-filled” microcracksmay contribute to the overall deteriorationprocess,
E1-Sayed[96] did a post-mortemon 7 deteriomtedreinforcedconcrete structures in a marine en-vironment in Egypt, Ettringite was observedin 3 of the 7 concretes,He concludedthat a combina-tion of factorsultimately contributeto premature deterioration,
Most experimental researchnecessarilyattempts to isolateone or two variablesand examinesthe effects of controlledvariationson behaviour,As E1-Sayednotes, the real situation is far differ-ent, It is hard to imaginea practical situationwhere,over the course of severalyears, only onemechanism is responsiblefor concrete deteriomtion,
I See Section3.8 for more informationon the role of chloroaluminates
2 The role of carbonatesin the formationand effectof secondaryettringiteis given in Section3,7,2
chapter 2 Page 7
2.2 Petrographic Observations
One of the common observations of damaged structures is the occurrence of localized formationsof ettringite. For example, Chandra and Bemtsson [48] examined damage in the back-lining ofswimming pools in the south of Sweden. ‘Ihe concrete lining was 15 years old. They observedboth translucent and opaque fibres and “white particles” on surfaces; in spots there was a whiteprecipitate. Analysis confiied asr, but ettringite was also presen~ along with calcium hydroxide,calcite and gypsum which had been leached to the surface, The diagnosis was that deteriorationwas “caused by a combination of interacting factors” including alkali silica reaction, aggressive m-reactionof sulphates, and alteration of the fekispar aggregate [55],
Mackod et al [245] examined damage of a concrete bridge in Strathclyde, Scotland They ob-seaved the presence of “white spheres” in the microstructure, ranging in diameter of 1 to 4mm,which had grown preferential y within the spherical voids of the concrete (air-entrained voids). X-ray analysis identified this material as ettringite, with a small amount of gypsum and “an unidenti-fied chlorine-rich phase”. This material was clearly a secondary alteration product.
Ozol [249] performed petrographic examinations of concrete attacked by alkali-silica reactionwhich revealed “desiccated gel balls” up to 7mm in diameter adjacent to aggregate particles. Finecrystals of ettringite were incorporated into these balls. Ettringite was also found to be present incracks and in coarse-aggregate sockets; sometimes it was found by itself and other times it wasfound “in association with” dried asr gel-reaction product.
Neck [237] noted a common feature of damaged concrete that has been extensively heat pre-treated and exposed to weathting is the “secondary phases” that seem to form at the contact zonebetweenh matrix and the aggregate. ‘I%esephases appear to be ettringite or thaumasite or mixedC7ystals.
Sylla [2971 performed microscopic examinations of samples of cracked concrete. He found thecracks to be almost completely filled with needle-shaped reaction products. The needles were allorientated vertically to either the aggregate or crack surface. It appeared from the observationsthat the crystals grew in a uniform front into the crack (suggesting a topochemical ~action mecha-nism)l. Once the crack is full rhen further transport of ions and growth may result in an increasein crystallization pressure and further damage. Sylla postulates that the initial cracks are “pre-ex-isting” as a result of cracking during thermal treatment.
23 Problems with Precast Concrete and Ties
In a Research Institute document [268], problems prior to 1984 are reported to have occurred inGermany with heat-treated prefabricated concrete elements, and especially railway ties and clad-ding panels. The problem initiates as crack formation at corners and edges which then tend to ex-tend into the interior. More severe separation of aggregate from the paste matrix then follows.Petrographic examination of the cracks almost always shows intilling with thaumasite or thau-masitdettringite mixed crystals. It appears that the thaumasitdettringite mixture forms in alreadyexisting cracks, The report notes that heat-treatment has two important effects to initiate theseproblems:
I See Section3.10 fora descriptionof reactionand expansionmechanismsof ettringite
chapter2 Paga8
= an inadequatepre-treatmentallows internaldamage through debondingbetweenaggre-gate particlesand the paste matrix, and throughcracking,
= heat treatment interruptsthe normal formationof ettringite; this formationcontinues laterin the hardenedconcrete, withundesirableresults,
It was noted that the solution to the problem is to providean adequateperiod for precuring,
Tepponen [303] reportedthe Scandinavianexperiencewith problemsof productionof railwaysleepers (ties) that have occurredsince the 1960s,Ties were manufacturedin the 60s and 70swith high early strengthcement (approximately4% S03 and 8% C3A). The fineness of cementswas in the range 460-480 m2/kg, The cement content of the concrete mix was approximately 400kg/m3and the water cement ratio varied from0,36 to 0,39,Ties were manufacturedwith either a1or 2 hour delayperiod] and a maximumtemperature in the mnge 75-80”C,The soakingperiodwas 2,5 or 4 hours. One-daystrengthof the 1965ties was 56 MP~ and was 43 MPa in the 1971ties.
Tepponen noted that the ties showedvisible damageafter 15years,Thin-sectionanalysis showedpartial filling of the microcmcksin the ties that showedcracks,The more deterioratedties had allthe pores and some of the largercracks filled with “microcrystalline ettringite” which“had accu-mulated in the free space”, In the more severelydamagedconcrete the bond betweenpaste andaggregatewas poor, Further studies in 1970confirmedthat the ~ reason for deteriorationwas,however,the poor ilost resistanceof the concrete,To solve this problem air content was in-creased and the maximumheat-treatmenttemperaturewas reducedto 60°C.
New studiesperformed in 1980revealedthat “microcracks,,,arethe primarycause of deteriora-tion” [303],These cracks occur due to host, load,prematureheating, improperheat treatment,too severe or rapid heat treatment, etc, If temperature is also greater than 70°C duringcuring,then “metastablemonosulphate”is producedwhich formsettringite when (a) the temperaturedropsback to normaland (b) sufficientwater h supplied,
New productionmethodswere adopted in the 1980swhich employednew Germanguidelinesfor heat treatment.These consist of a prestomgeperiod of 3 hours, a mte of temperature rise be-tween 10=15°C/hrand a maximumtemperaturebetween6$70°C, Little changes in the cementcompositionwere made and, in fact, the newcement that was used had a higher aluminateandthus C3Acontent (12Y0in 1980VS,8?40in 1965), One possibly significant change was the mduc=tion in the maximum size aggregate from 32 to 16mm, The new concrete strengthat 1day was68MPa, Tepponen reportsthat the ties have been in service for 5 years and no expansionhasbeen observedeven though microcracksdue to stressare present [303],
Heinz and Ludwigs’sexperiments[132],discussedin detaii in Sections5,3 and 5,5, werepromptedby practicalprobiemsthat surfacedwithprecastunits — specifhxily claddingpaneisand precast units, These high-strengthstructuralelementswere heat-treatedduringproduction,Damage surfhcedafter severalyears’ exposureto “open-airweathering”where there was lle-quent saturation.Crockswere first observedaroundedges, foliowedby furthercracking and lossof bond betweenthe paste matrixand coarseaggregateparticles,
In another publication,Heinz and Ludwig [133] noted occurrencesof damage in Europe of high-strengthprecast units which used Type 55 high-early-strengthPortlandcement, These memberswere heat-treated in production.Damagealways occurredon units exposedto the weatherand
1 also called“pre-storage”or “preset”— the time betweencastingand the startof thermaltreatment
2 See Section6,3 for a discussionof precastspecifications
chapter 2 Page 9
subjected to frequent moisture satumtion. The damage was attributed to the reformation of et-tringite following heat-treatment.
Testing at the British Cement Association[177] to examine the importance of secondary et-tringite formation was prompted by problems with site concretes in Germanyand Finland,andparticularly with deterioration of rail ties. In the U.K. there have been a few cases of damage ofparticular prestressed and reinforced concrete bridge beams that have cracked at a late age due to“ettringite crystallization”.
In particular, 15 prestressed and reinforced concrete beams were examined that were cast be-tween 1963 and 1969 [177]. These beams cracked due to “unexplained causes”. All were cast us-ing high early strength cement, and all were probably steam-cured. The cement content wasgreater than 450 kg/m3 in all concretes. The cracks took as long as 15 years to appear. The prob-lem was initially thought to be due to alkali-silica nxwtion, but examination of thin sectionsshowed dense bands of ettringite, 25 pm thick, around coarse aggregate grains. In untracked sec-tions there were thinner layers of ettringite around aggregate particles.
Ettringite has been observed in normallyast structures which, in most cases, have been dam-aged by a combination of fhctors. In such structures ettringite appears capable of being a contribu-tory cause of destruction, but unless an ample source of external sulphates is provided does notappear to be able to instigate substantial destruction. On the other hand, case studies which haveexamined deteriomtion of precast concrete, and many examinations to be described in followingsections, strongly indicate that secondary ettringite formation can be ti main cause of destruc-tion once the heat-treatment process has established the nucleation sites for ettringite to form. Ac-cording y, it is appropriate at this point to examine the precast concrete process.
2.4 Precast Concrete
2.4.1 Effect of Heat Treatment on Properties
It is clear flom the literature that the main reason accelerated-curing r6gimes were devised forprecast concrete was to accelerate the strength sufficiently that production of structural elementscould be performed on a one day cycle. Economic pressures provided encouragement to obtain astrength sufficient to allow release of prestress; this value varied for different applications, but ingeneral the 18 hour to 1 day target strengths were in the range 27-31 MPa [129, 184,218]. Untilthe 1980’s, strength development and the result of accelerated curing on both early and later-agestrengths were the prime objectives of the research.
In 1951, Saul [278] noted that from 1921 to the time of his writing considerable controversy andconflicting research had occurred concerning steam-curing. He noted that there had been littleagreement on optimum curing temperature, duration of treatment, delay period, etc.
Saul found with his own research that the most important factor influencing the ultimate strengthof the concrete was the rate of temperature rise. He reported that good results could be achievedif the concrete temperature did not reach 50°C until 2 hours after casting, and 10O°Cuntil 6 hoursafter casting. Saul criticized some commercial operations because they used too rapid tempera-ture rises, but noted that this is satisfactory if a suitable delay (preset) is employed prior to thestart of heating. Saul noted that very rapid temperature rise can be used if very high earlystrengths are needed, but 7 and 28 day strengths may suffer by as much as 1/3 (when comparedto a normally cured control); later, others came to similar conclusions [e.g. 321]. The effect ofheat-treatment, and especially 100”C treatment, on durability of concrete is not a theme that isconsidered by Saul — or by any of the early research on precast concrete. There is no indication
Clwpter2 Page 10
that concretes manufactured with the aggregates and cements available at the time showed anylong-term distress.
Chamberlain[47] was another early researcher of precast concrete. He observed that steam-curingbetween temperatures of 85 and 93°C caused severe damage to the concrete when delay periodswere too short. Furthermore, subsequent strength gain was small. On the other hand, concretestreated at 74°C were as strong as the control concretes at 28 days. Based on these results, it wasconcluded that a delay period of “several hours” is required after casting, and a maximum tem-perature rise of less than 22°C/hr is necessary if it is desired to prevent damage to concretessteam-cured at temperatures greater than 74°C.
Plowman [256] confirmed that the low strengths of heat-cured concrete at later age was a well-es-tablished phenomenon; ukimate strength is influenced by the rate of early hydration. It only takesa high temperature for the first few hours of hydration to reduce ultimate strengths significantly.
The work of Klieger in the late 50’s and early 60’s [164-166] fmly established the pros andcons of the heat-treatment procedure. Fairly moderate curing temperatures from 4.4 to 49°C wereused by Klieger in his research. The strength at 1 day of the 49°C cured concrete was 178°/0thatof the control concrete cured at 23°C, Later-age strengths were somewhat reduced but “no retro-gression in strength occurs with age”, Klieger suggested that an acceptable heat-curing cycle con-sisted of 3-6 hours delay period, a 16 hour heating period and then gmdual cooling over 3 hoursto avoid excessive drying,
Klieger also found that elevated curing temperatures affected flexural strength in a similar man-ner to compressive strength; although initial strengths were higher the concretes that were testedexhibited considerably lower strengths at 3 months and 1 year [165].
In one of the few examinations of durability, Klieger observed that curing at elevated tempera-ture of concrete made with high-early-strength cement does not hinder freeze-thaw durability ifthe concrete is dried prior to exposure, Klieger suggested that a few days of drying in the yard isenough to accomplish sufficient drying. He also found that concretes cured at elevated tempera-ture in a 24 hour cycle show lessdryingcreep and shrinkageunder sustainedstress than thosecured at normal temperatures[166].
Hanson [128] also noted that accelerated curing causes significant reductions in creep and shrink-age, This has important influences on the magnitude of prestress loss which can be reduced by asmuch as 40% if heat-treatment is used, The author suggestedthat a heat-treatmentrbgimeconsist-ing of 4-6 hours delay period followedby a 17°C/hour temperature rise to 66°C and a soaking pe-riod of 13 hours is about optimum with respect to strength development,
In a subsequent paper, Hanson examined the properties of lightweight concrete [127], The opti-mum conditions for steam-curing lightweight concrete were not substantially different than thosefor normal-weight concrete, this was confirmed by other research [185]. It was also found thatlightweight concrete is less susceptible to damage if severe heat-treatments are used (too hightempetuture, too rapid tempemture increase, insufficient delay period). This is probably due tothe high elasticity of lightweight concrete enabling it to absorb better the internal strain that oc-curs upon heating.
Merritt and Johnson [218] determined the strength of various heat-treated concretes. Soakingtime and soaking temperature were the main variables. A summary of the results is given in Fig-ure 2,3, For all conditions heat-treatment resulted in a strength immediately after treatment thatwas greater than the fog-cured concrete. The 7 and 28-day strengths were always less than con-trol. Note, however, that the delay period for all casts was very short (0-0,5 hours), At this shortpreset the optimum curing r6gime, both mechanically and financially, appears to be a soakingtime of 18 hours within a temperature range of 52-66°C.
chapter 2 Page 11
% of Strength Compared to Fog-Cured Control
180 I ❑ After Heat TreatmentI I
L DelayPeriodforAllCasts=0-0.5hrs.
160 --------- ~-’--------------------‘1
❑ 7 Days
140
120
100
80
60
40
20
0
---- ---- - ~[Q28D.ys
II::::::--;-------.--------::::~
Sosklime (hrs) 18 42 66 18 42 66 18 42 66 18 42 66 18 42 66
~ 38°C 52°C 66°C 79°C 93°C
Figure 2.3Relative Strengths of Heat-treated Concretes:
Dependence on Soaking Time and Temperature [data from ref. 218]
Australian experience indicated that a premature application of steam can reduce the compressivestrength at later ages by up to 200A.An optimum delay period between 3-5 hours is appropriatefor temperature gradients from 10-40°C/hour. If an adequate delay period is used, then sub-sequent strength development is directly related to the maturity. However, with a proper delayand moderate temperature rise the 28 day strengths of heat-cured concretes can exceed those ofstandard-cured concretes [184].
Farslq’ noted that after a plastic strength of 1 to 1.5 MPa is achieved in the concrete the speed ofheating does not influence strength. The period of delay prior to heating is, however, a decisiveinfluence on later strength development. He proposed that a reasonable period of delay would bethe time to initial set of the cement [100].
Some researchers have suggested that tailoring the cement chemistry or the use of chemical ad-mixtures can help to reduce the problem with low ultimate strengths. Saul [278] suggested thatthe adverse effect on strength due to a rapid temperature rise during pretreatment can be compen-sated for by additions of calcium sulphate or alkali to the mix.
It was also proposed that the use of a superplasticizer can permit as much as a 25?40reduction inthe water content of precast, heat-treated concrete. As a result, the normally observed reductionin long-term strengths can be avoided. Concrete cast with w/c=O.3 through the use of a supeqhs-ticizer resulted in a 43% greater strength at 28 days than a normally-cured, superplasticizer free,equivalent Type I concrete [254].
Chupter2 Page 12
Pfeifer and Marusin [255] observed that the compressive strength of heatared concrete wasgreatly affected by the chemical composition of the cement. With respect to optimizing 1 daystrength, they proposed that the best chemical composition is one with: (a) C3S:C2S = 3: 1; (b)C3A:C4AF = 2: 1; (c) C3A = 10-15%; (d) S03 = 5-7%. This is similar to a standard Type 30 ce-ment except for the higher S03 content. For optimum high early strength “the S03 contentshould be as high as allowed”; current ASTM specifications allow a maximum of4.5% S03, but“early-age strengths can be dramatically increased during heat curing by using somewhat higherS03 contents than currently allowed by ASTM C150”l.
It is important to note that although strength has been the primary property investigated, long-term strength is not the only property that suffers by inadequate heat-curing. For example, a shortsteam-cure at 54°C followed by drying resulted in a 4000% increase in permeability when com-pared to a water-cured control specimen [135]. This is shown in Figure 2.4. All steam-cured con-crete resulted in an increase in permeability, Fog-curing afier heat-treatment is beneficial since itresults in a dectease of permeability meas~ed at 28 days,
6 HR.STEAM@54°C,28 DAYDRY
24 HR.STEAM@54°C,28 DAYDRY
24HR,STEAM@64°C,7 DAYFOG,21 DAYDRY
SHR.STEAM@71°C,28 DAYDRY
8 HR,STEAM@71°C,7 DAYFOQ,21 DAYDRY
24 HR,STEAM@71°C,28 DAYDRY
24 HR,STEAM@71°C,7 DAYFOG,21 DAYDRY
1
I
1
1
1
I
1
1
I
1
1
1
1 I I t 1 1 I 1 t1 I
o 5 10 15 20 25 go 35 40 45PERMEABILITY OF SPECIMEN/ PERMEABILITY OF 28 DAY CONTROL SPECIMEN
Figure 2.4Effect of Heat-Curing and Drying on Relative Permeability [ data from ref. 135]
1 Note that there is no mentionof long-termpropertiesin this document,The proposalby theresearchersto use as much S03 as possible is, in this author’sopinion,indicativeof the acceptanceofstrengthas the universalqualityindicatorthat predominatedin the industryuntil recently,Fortunatelywe arenow learningvery quickly that adherenceto that philosophyoften leadsto infkriorconcreteinthe longterm-with resultantcostlyrepairs.
chapter 2 Page 13
2,4.2 The Cause of Insufficient Strength Gain after Heat Treatment
Hansom in 1963, noted that microstructuml damage is caused during a short pre-steaming perio~especially when the heating rate is high. Cracks that occur are due to the action of tensile stressescaused by a rapid heating rate and resultant stress distributions that occur across the specimen orstructural member. The greatest cause of these stresses is probably the much larger expansion co-efficient of free water when compared to the solid ingredients of the concrete. To avoid this dam-age, Hanson suggests an optimum curing period consists of 5 hours delay, a temperature rise at22°C/hr, and a maximum constant temperature of 66°C held until steam shutoff at 18 hours; thisallows 6 hours for form stripping and readying for the next run [129].
Alexanderson [7] proposed other techniques that could be used to minimiie the effects of dam-age associated with expansions and cracking:
~ nxisting pore pressures-through the use of closed forms, or by letting the concrete at-tain a minimum strength before the stati of heating;
~ eliminating pore pressure-by heating the concrete ingredients before casting or by elimi-nating air voids;
~ letting the concrete crack-and repairing it, perhaps by mwibration after heating.
Others have also attributed damage to the principal mechanism of differential thermal expan-sions. Sylla [297] stated that differential thermal expansion appears to be the primary cause ofcracking during thermal treatment. Water is a key component in this regard because it has an ex-pansion coefllcient about 10 times greater than any of the solids. Sylla hypothesized that prema-ture heating causes large expansions in the thin layer of water that coats the aggregate particles atcasting. If heating is delayed somewhat then some of the water in this layer is removed due to hy-dration; also the concrete has some stability so that the overall damage is less.
Soroka et al [292] listed three reasons to explain why heat-curing causes a reduction in later-agestrengths when compared to a normally-cured control.
The first hypothesis was proposed by Verbeck and Helmuth [3 12]. Due to heat-treat-ment, the paste microstructure is more heterogeneous because of the very rapid rate of in-itial hydration. Ample time is not allowed for hydration products to dlfl%seaway andprecipitate in the space between cement grains. A dense hydmte forms near the cementgrains and a weaker gel and more interstitial space occur away from the grains. Cementhydration at later ages is also retarded because of the denser layer of hydrate surroundingthe unhydrated grains
Temperature appears to change the morphology of C-S-H gel particles. Heat treatment re-duces the relative amount of long C-S-H particles to short particles. It is not entirely clearwhy this should have a profound eff=t on strength development.
Rapid heating results in differential thermal expansions and cracking —thus, damage isproduced.
To examine this reduction in strength, concrete was steam cured after a 30 minute delay period.The treatment temperatures were either 60 or 80°C and were held for periods ranging from 2hours to 4 hours:40 minutes (Figure 2.5). For some specimens (Cure 3) no fiuther moisture wasprovided (65’Mor.h.) after heat-treatment until specimens were tested at 28 days. In Cure 2, speci-mens were water cured after heat-treatment until 7 days and then placed at 65% r.h. until 28 days.Control specimens (Cure 1) were treated like Cure 2 specimens except there was no heat-treat-ment. The results given in Figure 2.5 indicate that lower strengths may not be due to damage dur-ing heat treatment, but may be due to inadequate curing after the heat treatment.
Chapter 2 Page 14
Cure 1 = Water cure at 20 “C for 7 days, then at 65% rh until test at 28 daysCure 2 = Heat cure at given T atler 30 minutes delay, followed by water cure at20”C until 7 days, 65% rh to 28 days , 1Cure 3 = Heat Cure of Cure 2, no water cure, ❑ CURE 2 ❑ CURE 3
Strength as Y. of Standard Cure Y. OF CURE 1 ‘YoOF CURE 11
100- -
.80- -
60- -
40-
20- -
ol-
---- ..-
+
---- ---
---- ----
---- ----
---- ----
---- ----
---- ---- ---
80-2.00 80-2.45 80-3.35 60-2.30 60-3.40 60-4.40
MAXIMUM TEMPERATURE ‘C -- TIME AT MAXIMUM (HRS)
Figure 2,5Effect of Post-Treatment Water-Curing on Strength Gain [data from ref. 292]
Most mxearchers blame the presence of water and its large thermal expansion coefficient as themason why damage occurs. However, Pfeifer & Marusin [255] propose that one must be espe-cially careful when air-entrainment is present. Moist air expands in the pores during heating, soeven if the air content is as low as lYo,cracking can develop under certain conditions. Above43°C a delay period is not necessary as long as temperature rise is gradual. At 60”C, the delay pe-riod should be 4.5 hours, and at a 90°C heat-treatment temperature, the delay period should be ap-proximately 6 hours. me delay period can be reduced if the initial temperature of the rawmaterials is increased, thus increasing early tensile strength gain and reducing the volume expan-sion of the various components.
2.5 Comment
The heat-treatment figimes for the precast industry appear to have evolved with one over-ridingconsideration — to obtain the highest strength possible in the shortest time. When engineers de-vised rt$gimes to do this they quickly tealized that later age strengths could be substantially lessthan that of a normal-cured concrete. If later age strength was not importan~ then a certainamount of cracking during the heat-treatment process was accepted. There appears to have beenlittle concern or, perhaps, appreciation that long-term durability of the cracked material could bean issue. The chemistry and physical properties of cement in the 50’s and 60’s were significantly
Chapter 2 Page 1S
different than now; it maybe that during this period strength MS a much stronger indicator ofoverall quality than it is at present.
Most concrete manufacturers appear to have searched for a compromise to develop a heat-treat-ment r6gime that would still produce high early strengths but which would not result in large com-parative strength reductions at later ages. To do this it was realized that the rate of temperaturerise had to be low and/or there had to be a significant delay period between the time of castingand the start of heathg. As will be shown in the following chapters, there is a third important fac-tor that must be considered if both acceptable early- and late-strengths and long-term durabilityare to be ensured — the maximum curing temperature.
The deterioration of concrete in the field will almost always be a combination of influences, in-cluding freezdthaw, corrosion, alkali-aggregate reaction, stress cracking, sulphate attack carbona-tion, secondary ettringite formation and a number of other possible processes. The mechanismsby which the overall deterioration process advances is extremely complex and probably beyondany model that could be tested in the laboratory or on a computer.
Nevertheless, particular processes can be recognized and we can ensure that the influence of thoseprocesses in the overall deterioration scheme are minimized. Corrosion and sulphate attack mtwo examples where simple steps have been taken to ensure, at least on paper, that their effectswill be minimized.
Recently, several researchers in Europe (see Chapter 5 as well as this chapter) have providedstrong evidence that under the right conditions secondary ettringite formation maybe a significantfactor in influencing the long-term durability of some types of concrete. Like corrosion and sul-phate attack steps can be taken to help ensure that secondary ettringite formation is either an in-significant or only a minor influence; some of these steps can be discerned from reading thematerial to follow.
At the moment, secondary ettringite formation is different in one key aspect when compared tocorrosion and sulphate attack it is by no means as widespread in occurrence. Nevertheless, it maybecome a very important consideration if its potential is ignored. It should, however, be kept inmind that at present the proportion of damaged concrete where damage has been directly attrib-uted to secondary ettringite formation is a very small proportion of all precast concrete cast inEurope and North America.
chapter 3 Page 16
Chapter 3SECONDARY ETTRINGITE FORMATION:
FUNDAMENTAL RESEARCH
3.1 Overview of the Cement Hydration Process.
This report assumes that the reader has a working knowledge of the chemistry of cement and ce-ment hydration; therefore, no attempt is made to perform a detailed review. The reader is referredto the excellent reference book by Taylor [302] for comprehensive information on all aspects ofthe properties of cement.
The formation and consequences of formation of ettringite are the principal topics of this report.In a normal modern cement, ettringite forms due to the reactionbetween gypsumand calciumalu-minate. X-raypeaks that are associatedwith ettringiteare detectablewithin a few hours and thequantity increasesduring the first day. After this time the ettringitepeaks normallyweaken,but,dependingupon the chemistryof the cementand the environmentalconditions, ettringitemaypersist indefinitely,It is a general belief that if all of the gypsum is consumedthrough the et-tringite reaction, ettringiteconvertsto monosulphoahuninate.In an average cement this conver-sion process starts at about 1day and can be monitoredas a reduction in the size of the ettringiteX-raypeaks. Much of the monosulphoaluminatephase that forms instead is poorly crystalline[302].
3.2 Early Formation of Sulphoalumlnates
The commonlyheld chemical formula for ettringite is:
3Ca0,AlzOy3CaSOq.32Hz0 or in chemists’ notation is: C6AS3Hn
and for monosulphateis:
3Ca0.Alz03,CaS04, 12Hz0 0!’: c4A~12
The reaction that occurs between C3A and gypsum can be found innumerous texts:
C3A + sC~Hz + HZ6+ C6A~3H32
Except for the unusualcombinationof very low C3Acontentsand high gypsumcontents, the gyp-sum will be used up by this reactionbeforethe C3A,When this occurs, the remainingC3Awilltake part in a reaction wherebyettringitedecomposesto monosulphoaluminate:
2C3A + c(jA~3H3z+ H4+ 3C4ASHU
Thus, in the net reaction,3 moles of CSAand 3 molesof gypsumwill produce 3 moles of mono-sulphoaluminate.Use of the Bogue equationand performingstoichiometriccalculations fkomtheformulaeabove, one can cdcuiate the S03/Al@ massor molar mtios (TVAratios) required toconsumeall of the C3Aand convert it to monosulphoaluminate,Figure 3,1 showsthe depend-ence of this mtio on the C3Acontent of the cement (givenan assumedFe20s content of 3%), Ifthe WAmtio is greater than the value shown,C3Awill still be available after all of the ettringitehas been consumet the remainingC3Awill react withcalciumhydroxideand water to produceC4AH13[302],The above analysis is highly idealized.It is importantto recognize,however,thatthe conditionsunder whichettringite forms,and the stmcture of ettringite itself are highly com-plex,
chapter3 Page 17
S03/A1203 Ratio Required to React with AU TricalciumAluminate to Form Monosulphoaluminate
0.7 I I
/
0.6 ------------------
/“A0.5 -------------- &------------ ------
..-
cl--
---- ./0-”
.
-//n- 7-Molar Ratio
/.
u“
/’-”7Mass Ratio
S031A1203 0.4 - -------. -~ ’------- ‘ ----------------------RATIO #
(mass or /’molar) 03 T~------ ‘ --------------------- -----------
t
0.2 ‘ - - - - - - - - - - - - - -”------- ----------------
1-0.1 --------------------------- “-----------
OLH—H—HJ2 3 4 5 6 7 8 9 10
C3A PERCENTAGE
Figure 3.1Critical Values for S03/AlZ03 Ratio
3.3 Crystal Structure and Composition of Ettringite
The use of the term “ettringite” without qualification normally refers to “sulphate ettringite”,which has the formula C6A~3H32.However, it should be noted that “e~ingite” is) in f~ts a gen-eral term used to denote a number of minerals, all with very similar crystal structures. McConnell& Murdoch [202, 203], for example, noted in 1962 that ettringite should be represented by thegeneral formula
16[A4(X04)34(H20)12]
where “A” represents six-fold coordinated atoms (Ca, N% Al), and “X” are four-fold coordinatedatoms (S, Si, I-Uand also some Al), The formula is written to make clear the interchangeability ofthe various ions.
Moore & Taylor [231] wrote the formula for ettnngite in a different way, again in an attempt toclari~ all of the various substitution of ions that can occur, but also to illustrate the crystal struc-ture of this class of material:
@[ Al(oH)6]2.(so4) 3”26H20
chapter3 Page 18
The researchers noted that the crystal structure is based on columns with composition
[ti3[Al(OH)(i]- 12HzO]3+
which run parallel to the c, or needle, axis. The sulphate and xemaining water molecules lie be-tween the columns.
Taylor [300] provided a very clear picture of the ettringhe-group crystal structure which is char-acterized by four positively charged columns in a trigonal structure, between which occur chan-nels which contain anions and sometimes water molecules. Figure 3.2 shows thesulphate-ettringite structure. The a and b lattice dimensions are 11.23A, while the repeat dimen-sion in the c direction is 10.7~.
Two other minerals which closely parallel the ettringite structure, and are of interest to concreteresearchers are thaumasite and jouravskite. The parallel structures can be clearly seen in thechemical formulae:
~ ettringite: {Ca~Al(OH)6~24H20}(SOA)s2H@
= thaumasite: {Ca~Si(0H)6]224H20}@C)4)y(C03]2
@ jouravskite: {Ca~Mn(0H)6~24H@}@C)A)y(C03)2
In thaumasite, silicon teplaces ahuninium in the columns (Figure 3.2), while in jouravskite man-ganese replaces aluminhum For both thaumasite and jouravskite water molecules contained be-tween the columns of the ettringite structure are replaced by carbonate,
In fact, Taylor [300] notes that partial or complete replacement of sulphate ions can occur withC032-, Cr042-, Cl-, and 103-”The aluminium, on the other hand can be replaced by Ti, Cr, Mn,Fe, or Ga, The calcium can be replaced by strontium, Other researchers [196, 326] have con-firmed the wide variety of ions that can replace aluminium and/or sulphate, More recent reseamh[130, 171] has also shown that oxyanionssuch as arsenate,borate, chromate,molybdate,selenateand vanadate may substitute for sulphate in the ettringite structure,
Taylor [300], expandingon earlier work re-definedthe compositionof the columnper half unit
cell in sulphate-ettringiteas {Ca~Al(OH)G~24H20~+,while the material in the ettringitechan-
[ 16-
IRh iS (SOA)3,2H20 to make the entire structure electrically neutral, He noted that each col-umn is of nearly cylindrical shape and its surfhce is comprised entirely of water molecules, “ormore accurately the H atoms that these contain”.
It has also been shown that various ions can replace sulphate in monosulphoaluminate. The widevariety of chemical compositions of both compound types has prompted the use of the terms (a)“AFt” for Ahuninate-Ferrite-trisubstituted — the “needle-like hexagonal hydration productswhich crystallize with 3 molecules of a calcium salt per molecule of C3A”; and (b) “AFm” nota-tion to stand for ahminate-ferrite-monosubstituted compounds, which are hexagonal or pseudo-hexagonal plates which contain one molecule of a calcium salt per molecule of C3A [163].
3.4 The Role of Ettringite in the Retardation of the Caicium Acuminates
Gypsum is added to Portland cement clinker to @ard the rapid hydration of the calcium ahuni-nates. Lerch [180] outlined the mechanism of retardation of aluminates by sulphates. When waterfirst contacts cement there is an initial rapid dissolution of anhydrous aluminates and rapid crys-tallization of hydrated calcium aluminates. This occurs before the solution becomes saturated
chapter3 Page 19
.
10.7
i
Blank Circles =Water Molecules
Structure of Columns in Ettringite Crystal
I Part of Single Column ProjectionBlank Circles are Water Molecules I
Plan View, Showing Columns and Channels
/s\ /l\ \/
Lattice a dimension=11~ A x
S04> and Water MoleculesLocated in Channels
Between Columns
o 2A1
Figure 3.2
Sulphate-Ettrlngite Crystal Structure [extracted from Taylor [300] and annotated]
chapter 3 Page 20
with lime and gypsum and corresponds to the fust peak in the heat of hydration curve. Since limeand gypsum in solution decrease aluminate volubility, the rate of formation of aluminate de-creases as more of these materials are dissolved (lime alone is not sufficient to retard the hydra-tion of aluminates), Once a saturated Iirnehulphate solution is achieved, aluminates continue tohydrate more slowly. Calcium silicates dissolve and form C-S-H. Sulphoaiuminates start to form.Because of the formation of sulphoaluminates, the gypsum eventually becomes depleted and theconcentration of sulphates in solution starts to drop. The volubility of alumina again rises and arapid aluminate reaction begins again,
Another theory attributes retardation to the direct formation of a coating of ettringite on the sur-face of calcium aluminate grains [69]. The retardation mechanism of gypsum is more effective inthe presence of CH since ettringite crystals are more fine-grained and can therefore coat the sur-face better. Some later research, however, has questioned whether a sufficient amount of etlringitecan deposit in a short time to have the pronounced retardation effect that is observed [206].
Although there has been no convincing explanation of the retardation mechanism, fairly convinc-ing evidence does exist that ettringite forms by a through-solution mechanism. There has beenmuch controversy about this over the years, and various schools of thought have formed aboutwhether formation is through solution [e.g. 134] or topo-chemicai, There is also some suggestionthat formation may be due to a combination of mechanisms: the first is a through-solution mecha-nism. The second is a process by which some ettringite crystals sinter together by a solid-solid re-action where crystals grow together and sinter [149].
The most convincing march also relates dkctl y to the secondary ettringite issue. It has been ob-served that the transition zone at the aggregate and steel interface in concrete has a higher propor-tion of ettringite than elsewhere in the bulk matrix [225]. ‘l’hisfinding supports the throughsolution mechanism of formation, since constituents must dissolve and dtffuse towards thesteei/aggregate surface where ettringite is precipitated, In another series of experiments Monteiro& Mehta [228] showed a distinct gradient in ettringite concentration in the interracial zone be-tween aggregate and paste, Much larger quantities of ettringite are present immediately next to theaggregate surface. They concluded that this could only be explained by a “through solution”mechanism and not by a “topochemicai” or “solid-state” theory whereby compounds do not dis-solve, The importance of the “transition” zone to secondary ettringite formation is discussed in de-tdl in Section 4,2,
3s The Chemical Stabiiity of Suiphoaiuminates
3.5,1 Stability in Solution
The earliest studies (1929, [181]) on the stability of ettringite showed that it is more stable in solu-tions of calcium sulphate and calcium hydroxide than in distilled water. On the other hand, mono-sulphate in solutions of calcium sulphate, sodium sulphate, calcium chloride and sodium chloridetend to change to the high sulphate (ettringite) form. Lerch concluded that only the high sulphateform of sulphoaluminate can be stable under the conditions expected in concrete.
The instability of the monosulphate form was noted by IQdousek [156]. The AFm phase is a me-tastable compound this is also confirmed by research that shows monosulphoahuninate is metas-table below 70°C [204 reported in 132]. Once monosulphate forms it is metastable but maypersist for indefinite periods under the right conditions; its metastability dictates that it will finallyconvert tQthe trisulphate form (ettringite) — this is confirmed by other, more recent research[163].
llie formation of monosulphate requires nearly saturated solutions of both calcium hydroxide andgypsum. If excess sulphate is not present the monosulphate may persist for weeks but will finallyconvert to the stable trisulphate. The stability conditions in solution for monosulphate involvehigh CaO, low CaSOAand high temperature [1521. Mehta found that monosulph~ hydr~ formsin cement pastes along with ettringite “but persists only in contact with solutions of much lowersulphate concentration than required for stabilization of ettringite” [207].
At the nornul temperatures experienced by concrete, Kelly found that ettringite is more stabletin monosulphate or C3AHG.Monosulphate readily forms at higher temperatures, however,which is due to its “greater crystallizability” as opposed to its higher stability [160].
ettrhwite 1.O*1040monosulphoaluminate 1.7*10-28
carboaluminate 1.4*10-30chloroaluminate 1.O*10-30
calcite 8.7*10-W
The stability of ettringite in relation to othercompounds can be explained by the “rule ofvolubility product” [326]. The compound withthe lowest volubility product is the most sta-ble. The table on the left shows the volubilityproducts for four major aluminates that amknown to form in concrete. Calcite, a soluble
compouncLis included for comparison. Based upon this table, the following transitions can occw
calcium aluminate + calcium hydroxide+ gypsum+ ettringite
monosulphoaluminate + gypsum + etlringite
ettringite + NaCl + chloroaluminate
monosulphate + CaC12+ ettringite + chloroaluminate
monosulphate + CaCOs + ettringite + carboaluminate
This analysis may bean oversimplification of the issue. Hampson & Bailey [124] noted that et-tringite does not exhibit a constant volubility product-it varies 5 orders of magnitude as the prod-uct of hydroxide and sulphate activity increases. This leads them to postulate that ettringite maylose its characteristic fibrous morphology at high pH and forma coherent coating over other parti-cles. They note two possibly distinct forms of ettringite, depending upon the pH of the pore solu-tion:
= at pH= 11.5-11.8 — crystalline ettringite precipitates from solution
~ atpH=12.5-12.8 — the result is a “topotactic” formation of a non-crystalline material thalhas a similar composition to ettringite.
The one special situation that the volubility product method does not predict is the formation ofmonosulphate from ettringite. Monosulphate has a volubility product at 25°C of 1.7*10-28,whereas ettringite has a solubilit y product of 1.1*1040. Normally, the most insoluble sak et-tringite, will prefemntiall y be formed. However, when the sulphate in the pore solution is de-pleted, ettringite will convert to monosulphate. The extent of conversion depends entirely uponthe amount of excess C3A present after the initial formation of ettringite
3.5.2 Stability and Temperature
For curing at 25°C ettringiteis detectableby X-ray analysisaf~ a few hours [302].When hydra-tion occurs at 60”C,however, ettringite X-ray peaks begin to form at 15 minutes and there is al-ready appreciable ettringite at 6 hours. The ettringite crystals formed at the higher temperature areappreciably larger than those developed at 25°C [1341.
Chupter3 Page 22
At temperatures above 60”C, a different scenario develops, At approximately 70”C ettringiteloses much of its water. For example, Daerr et al [79] note that the water content drops from 32to 10 molecules at 70.5°C. Also, when water is lost between temperature of 70-85°C all ettringitecompounds lose the molecular water in the channels and become amorphous [260]. Skoblinskayaet al agree; as ettringite loses water its structure remains intact only during the first-stage redu-ctionfrom 30 to 18 water molecules. However, when water drops from 18 to 6 molecules duringthe second stage of desiccation the crystals become “roentgenoamorphous”. Withdrawal of the re-maining 6 water molecules causes transverse rupture and the crystals disintegrate [290,291].
There is a considerable diversity of research that quotes various tempemtures at which ettringitedecomposes. Abo-El-Enein et al [1] found that ettringite, when hydrated at 25”C, loses about 20water molecules at 58°C in a dry atmosphere. If it is hydrated at 60”C, however, it loses 18 watermolecules at 58°C; thus, formation at elevated temperature makes ettringite more thermally sta-ble. Similarly, pressure also tends to stabilize ettringite against thermal decomposition-et-tringite pellets were manufactured at two different pressures; the pellets made at the higherpressure were more stable when heated [2].
Some research has concentmted upon examination of ettringite decomposition under very severeconditions. Although in pure water ettnngite is stable at 60”C at a pH of 11.2, at 100”C 2.25moles of sulphate are split from the ettringite crystal, and monosulphate forms. If temperature re-mains at 100”C the monosulphate decomposes to gypsum after 11 days of boiling. At lower tem-peratures (30”C), monosulphate is unstable and decomposes partially to ettringite after 6 hours.At intermediate temperature (60°C) monosulphate enters into a solid solution with calcium alumi-nate hydrate in 6 hours, which decomposes to ettringite after a day [104]. Ghorab et al [106] re-port that the structure of ettringite is destroyed (a) at 18°C in a vacuum of 10-6torq (b) at 74°Cunder normal atmospheric pressure; and (c) at 82.5°C under wetted N2. Re-storage at 90V0r.h.leads to dehydration [106].
It has been postulated that up to approximately 70°C ettringite is the stable compound in cementand concrete, while above this temperature it is the monosulphate that is stable [ref. 143 notes thework of Mchedlov-Petroysan [204]). There is some research, however that shows that ettringitecan exist in equilibrium with its aqueous solution up to 90-93°C [277]. The apparent instabilityof ettringite at higher temperatures may be related to its higher volubility [106, 107],
Mehta reports that the short-prism type of ettringite found in the paste-matrix of concrete is sta-ble at 65°C in a dry atmosphere, but decomposes partially at 93°C. In a moist environment, how-ever, ettringite shows no significant decomposition at 93”C. However, if ettringite is exposed tosaturated steam at 149°C it’will decompose to the monosulphate. The fme-gmined type of et-tringite found in concretes and pastes is more thermally stable than the long slender needles of et-tringite that form from dilute solution [214].
3.5.3 Post-Decomposition Behaviour
The combination of the S03 content of the cement and the heat-treatment r6gime determines thecompounds that are present in a cement paste. Kalousek [157] tested pastes with 0.4 to 4’%S03which were cured at 24, 54 and 82”C. The pastes were put through a 19 hour heat-curing cycle,dried and analyzed. At 54°C curing, Kalousek noted that the most notable change over 24°C wasthe decrease in the amount of ettringite and increase in monosulphate. For curing’at 82°C et-tringite was absent except for specimens where S03 content was greater than 3V0.There is anamount of “missing S03” which is larger in the 82°C cured specimen than in the 54°C specimen.
Kalousek proposed that heat curing results in an amount of S03 that is neither bound in the mono-sulphate nor in the ettringite. It was suggested that this missing ettringite becomes a “lattice sub-
chapter3 Page 23
stituent” in tobermorite (C-S-H) gel; the S042- ions occupy sites of the Si044- tetmhedrons.Other substitutions of ions such as A~ A120s, FezOS, MgO and Cl- ions are well known andtherefore there is no reason why S04 - cannot substitute also. Figure 3.3 shows the distributionof phases at the various treatment tempemtures. As treatment temperature rises the amount ofS03 missing (i.e. in “Phase X“) increases. The presence of ettringite shifts to higher sulphate con-tents.
4.51 .
4.0
1’
24 “CTREATMENT3.5
3.0 ‘/AMOUNT S03 IN
DIFFERENT PHASES, 0/0
2.5
2.01.5
1.0
0.50.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
TOTAL S03 IN CEMENT, %
4.5 ~
4.0- -
Y
54 “C TREATMENT3.5- -
3.0 “-AMOUNT S03 IN
DIFFERENT PHASES, % 2.5- - ~2.01.5! .00.5O.q
Unknown hydrated 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4,5calciumaluminate TOTAL S03 IN CEMENT, %
4.5 ~
4.0- - 82 “C TREATMENT3.5- -
3.0- -
AMOUNT S03 IN 2.5- -
DIFFERENT f’HASES, ~0 2.0--1.5- -
1.o- -
0.5-“ETTRINQITE 1
I
5 2.0 2.5 3.0 3.5 4.0 4.5calcium aluminata
TOTAL S03 IN CEMENT, %
Figure 3.3Effect of Curing Temperature on Sulphoaluminate Phases [data from ref. 157]
Type 30 cement. DTA analysis performed at age of 19 hours
In 1965 Richards [269] produced some interesting xesults that relate to the stability of ettringite.He soaked mortars in various sulphate solutiow, the temperatures of the sulphate solutions rangedfkom 20 to 80°C. He found that soaking at temperatures greater than 40°C resulted in a dramaticreduction in expansions. Measurements were takenup to 24 months.Richardsconcludedthat theexpansionprocess that is normallyvery destructivedoes not take place above 20°C. It is unfortu-nate that he did not continueto measureexpansionsafter specimenswere cooled.
Odler [242] examined the hydration reactions of pastes containing cement with Oto 20% C3A,3% S03, and a Blaine fineness of 3000 cm2/g, Curing temperatures ranged from 5 to 95”C. Thenxearchers found that at 5°C gypsum was consumed within 3 days and the quantity of ettringitewas large. At 25 and 50”C the conversion process from gypsum to ettringite was accelerated; both ‘AFt and AFm phases were present at 28 days. At 75°C curing AFt was only present during thefirst few hours, while AFm was observed at all times up to 28 days. At 95°C no AFt was detectedat all; AFm formed instead but disappeared after 3 days. Similar results were obtained for all ofthe cements examined. Odler concluded that as temperature rises there is an enhanced incorpora-tion of Al-, Few and S042- into the C-S-H lattice.
‘l%edisappearance of ettringite with elevated temperature was well documented by Chino &Kawamura [56]. Hardened cement pastes (w/c=O.5) were cured for 14 days at temperatures rang-ing from 20 to 90°C. Ettringite was determined by X-ray diffraction. As Figure 3.4 shows, thequantity of ettringite in the paste dramatically decreases at curing temperatures above approxi-mately 70°C.
Stadelman[294]useddifferentialthermalanalysisto analyzethe stabilityof ett.ringitecontent inhardenedcementpastes when treated at temperatures ranging from 20 to 100”C. When ettringite
1.20Cement Paste Cured for 14 Days
❑at Given Temperature
1.00 -- ❑
●
0.80 --
RELATIVEINTENSITY 0.60 --
X-RAY PEAK+
0.40 --
0.20 --
0.00
~ mean
❑ high
● low
o 20 40 60 80 100
CURING TEMPERATURE (C)
Figure 3.4Variation in Ettringite Peak Intensity with Curing Temperature [data from ref. 56J
chapter 3 Page 25
was treated at temperatures above 60”C it no longer appeared on DTA tmces; instea~ monosul-phate appeared. During storage afler treatment, ettringite peaks reformed which Stadelman con-clude~ “must be seen as a cause of concrete damage”.
3.6 The Effect of Alkalis on Hydration and Formation of Ettringite
Alkalis are normally present in the clinker as the neutral sulphates Na2S04, KXS04 or the mixedsalt (N~K)2SOd [71, 252]. These compounds are highly soluble. Afier contact with water thepore solution contains almost entirely N% K and OH ions with “very low” concentrations of QS04 and Cl. The pH of the pore solution is in the range 13-14, depending upon the alkali level. Ifno alludi is present then pH=12.5 [71].
Alkalis accelerate the hydration process through the alteration of the CqA-C~Hz-CH system[105]. The main action of the alkalis is to increase ettringite volubility and decrease lime volubil-ity [79]. This is confirmed by Wang et al [315] for example: The molarity of pore solution whenlow-alkali cements are used is approximately 0.2M with respect to alkalis; normal cements pro-duce a molarity of greater than 0.7M. The pH in the pore solution might nmch 12.9 within min-utes and 13.7 or higher after 28 days. Gypsum does not affect this pH value. The OHconcentmtion due to alkali is on the order of 15 times greater than the concentration of saturatedcalcium hydroxide solution. Therefore, the presence of alkalis decreases the solubililty of CH andaccelerates the formation of ettringite; these findings are supported by similar conclusions ofDaerr [79]. Experiments also show that ettringite forms even at a pH of 13.3.
An alternative mechanism for ettringite formation in alkali solution is given by Chino& Kawa-mura [56]. They hypothesize that in the presence ofNaOH, ettringite formation is suppressed be-cause S042- is more stable in NaX30d than in ettringite. The retardation of ettringite formationtwults in an acceleration of early hydmtion of C3A. Through the reaction of NaOH, ettringite isdecomposed as:
3Ca0.Al@[email protected] lHzO + 6NaOH + 3NazSOq + 4ca(OH)z + 2A1(OH)3
Way and Shayan [316] looked at the hydration of Portland cement in 0.5 and 1.OMsodium hy-droxide solution. They found that the nature and appearance of all hydrates were the same as inwater. The alldi did accelerate the time for depletion of sulphate ions by the formation of et-tringite.
In alkaline solution at 30”C (0.08M NaOH), Wang et al [315] found that the stability of ettringiteis the same as in water. However, at higher temperatures (60”C, 0.08M NaOH) ettringite paxtiallydecomposes to the low sulphate form. Both phases can exist for long exposure times under theseconditions. Higher alkali concentrations (0.2 to 1.OM)decompose the ettringite very quickly,while the stability of monosulphate hydrate rises in the more concentrated alkaline solutions[104].
3.7 The Roie of C02 and Carbonates
Grounds et al [122] reported research where samples of lab-manufiwtured ettringite were storedin sealed containers at 25-95°C and 100% relative humidity (over water). At all tempemtures et-tringite showed complete decomposition to other phases. The time of decomposition was 55 daysfor 25 and 50”C storage, and 15-20 days for 75 and 95°C storage. The decomposition was attrib-uted to the action of vey small quantities of C02 in the system. The decomposition products
chapter3 Page 26
were identified as (a) gypsum, (b) calcite, (c) aluminium hydroxide, and (d) water according tothe equation:
[email protected]~04.32H20+3 ~04.2H20 + Scacos + 2A1(OH)3+ 23H20
Ghomb et al [106] found that in real pastes and in the presence of C02, ettringite decomposes togypsum, bayerite and aragonite. Carbonation also produces a much more porous microstructure[106].
3.7.1 Formation of Thaumasite
Despite the fact that Grounds tested an “ideal” system, his research indicates the potential forcarbon dioxide to alter ettringite. In practice, carbon dioxide has a role in the formation of “thau-masite” from ettringite. Both ettringite and thaumasite formation have been found to be the causeof damage to plaster [179].
It is postulated that the formation of thaumasite involves expansion similar to that due to the for-mation of ettringite [142, 173]. The formation of ettringite usually precedes that of thaumasite,but both compounds can exist together [68]. Both ettringite and thaumasite are produced morerapidly at cold ambient tempemtures (O-lO°C). Their chemical formulae are [74]:
~ Ettringite: {Ca6[Al(OH)6]z.24Hz0}[(SOA)s.2H20]
e Thaumasite: {Ca~[Si(OH)@24HzO)[(SO@][(COs)z]
The similarity of the two chemical formulae reflects the fact that both thaumasite and ettringiteare AFt phases with similar crystal structures (see Section 3.3) [203]. Hunter noted that thau-masite forms a solid-solution series with ettringite. At temperatures greater than 15°C ettringiteprobably forms first and then later converts to thaumasite [142].
Taylor [302] stated that the combination of sulphate attack and carbonation (which occurs oftenin practice) can result in the formation of thaumasite; this can cause severe softening or cracking.“It can easily be misidentified as ettringite”. Thaumasite’s formation requires a constant high rela-tive humidity in a cold environment (4”C), an adequate supply of S042- and C032- and the pres-ence of reactive aluminate. Thaumasite does not form directly, but requires ettringite to form firstwhich acts as a nucleating agent. Whereas the quantity of ettringite formed is limited by the avail-able Alz03, the amount of thaumasite formed is only limited by the lime and silica contents,since alumina is not part of the compound. A continuous source of sulphate ions would allowthaumasite formation to effixtively destroy a mortar or concrete through a softening reaction.
Ludwig &Mehr[193] examined deterioration of historical buildings. They found that at tempera-tures between 2 to 40”C, damage was due to ettringite formation. However, at later ages and attempemtures less than 20°C, ettringite decomposed to thaumasite in the presence of unreactedgypsum and calcite. No evidence was gathered that the formation of thaumasite caused expan-sion. Ettringite and thaumasite form a mixture, but do not participate in a solid solution.
Sylla [297] noted that thaumasite may have a role in influencing expansion after heat treatment.He stated that when conditions are right then both thaumasite and ettringite can form with time af-ter heat treatment. For example: a paste sample was heat-treated at 80°C for 5 hours, then storedat 5°C with C02 present; analysis showed a mixture of ettringite and thaumasite present with ahigh proportion of thaumasite. Sylla believes that both thaumasite and ettringite formation afterheat treatment can cause damage; the sulphate is supplied from where it is weakly bound to thecalcium silicate hydrate during heat treatment.
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3.7.2 Formation of Carboaluminates
The addition of limestone to cement, or the presence of limestone aggregate in concrete, can re-sult in the formation of carboaluminates as early as 7 days after the start of hydration. In a systemwith Portland cement clinker, 2°/0gypsum and 6°/0limestone calcium carbonate reacts with C3Ato form calcium carboaluminate hydrate. There appears to be both a high and low form of car-boahuninate, similar to a high and low form of sulphoaluminate. Although carboaluminatesform, ettringite formation proceeds normally in the presence of limestone.
Klemm and Adams [163] outlined three fiumtions that limestone fhlfills when it is present in con-crete: (1) it acts as a micro-filler, thus improving strength; it may also act as a nucleation agent toaccelerate cement hydration; (2) carbonates can accelerate C3S hydmtion and the carbonate canbe incorporated into the C-S-H phase; (3) With fme limestone the principal intemction is to reactwith aluminates and ferntes to form calcium carboahuninate (femite) hydmtes. This interactioncan contribute as much as 10% to the compressive strength between 2 and 28 days.
The carboaluminates also form AFt and AFm phases. These are:
~ AFt: [email protected]+ needles
= AFm: CsA.CaCOs. 11H20+ hexagonal plates
The AFt phase is much less stable than the Al?m phase, and thus is unlikely to form in cementpaste. When carbonate is present it slowly dissolves and will react with any monosulphate pre-sent to form the carboaluminate which is the more stable phase because it has a lower volubilityproduct than monosulphate. The chemical formula is:
3C3ACaS04. 12Hz0 + 2CaCOJ + 18Hz0 + 2C3A.@C03. 11H20 + CsA.3Cas04#32Hz0
In other words, monosulphate decomposes in the presence of carbonate and water to form both acarboaluminate iiwl ettringite.Both the carboaluminate and the ettringite are very stable and canco-exist. This is a slow process; the presence of limestone (either in the aggregate or due to car-bonation) can ~sult in a later-age ettringite formation. The tests that were performed by Klemmand Adams with 5’XOlimestone addition showed the start of carboahuninate formation at an ageof 91 days. Note that the sulphate ion can also react with monocarboaluminate, as well as mono-sulphoaluminate, to form ettringite. The carbonate ion will not react directly with ettringite be-cause the result would be more soluble (less stable) products.
Kuzel andStrohbauch[172] note that expansive damage can occur due to ettringite formation asa result of “carbonation mctions of the calcium aluminate hydrates crystallizing in the laminarmode”. Also, if C02 is present in an environment where pH is greater than 12, the C4A$JOs.aqgroups in the interlayer of the monosulphate are displaced by C032- ions. This reaction leads tothe formation of a hemi-carbonate (C4A.~zC@@ and gypsum. The gypsum then reacts withthe unchanged fmction of monosulphate to form ettringite. In a strong alkaline medium the hemi-carbonate reacts as follows:
C4A.lACOz.aq -+ monocarbonate + CaCOJ + A1(OH)3.
Although this process of reaction is explained differently, the formation of ettringite from the re-action between monosulphate and carbonate is essentially the same as proposed by Klemm andAdams [163],
Calcite has one other fhnction. The reaction between C3A and gypsum to form ettringite, and theconversion of ettringite to monosulphate are accelerated by the addition of CaC03. The calcite re-acts with aluminate to produce carboahuninates. Under certain conditions all of ettringite, mono-sulphate and carboaluminate can co-exist. As well as awboaluminate, a solid solution of
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hexagonal calcium aluminate hydrate, monocarboaluminate and sulphoaluminate hydrates mayalso exist [265].
3.8 Formation of Chioroaiuminates
Richartz [270] stated that chloride can combine with all phases in a cement clinker. In solutionswhere the chloride concentrations are up to 10g Cl- litre, C3A and aluminoferrite can formFriedel’s salt, which has the formula 3Ca0,A1203CaC12, 10H2O (a monochloroalurninate), Athigher Cl- concentrations, a tri-chloroahuninate phase that resembles ettringite can form; the for-mula is: 3Ca0.A120s.CaC12 .32H20,
If gypsum is also present, Richartz notes that at normal temperatures ettringite always forms fimt;the chloride containing compounds form afler the gypsum is consumed. At elevated temperature(40-80”C) Friedel’s salt forms instead of either ettringite or the chloroaluminate phase, evenwhen gypsum is present. Friedel’s salt is shown to be stable in water and in saturated CH up to90°c.
This analysis [270] suggests that heat treatment may be beneficial for reinforced concrete wheresubstantial amounts of chloride are present. Friedel’s salt is preferred over ettringite formationand therefore the chloride is tied up and unavailable for the corrosion reaction. Note, however,that if Friedel’s salt is present at 20”C, sulphate acts on it to form ettringite while the chloridegoes into solution; this would be a slow process that could result in later expansions,
Richartz’ analysis is supported by Worthington et al [324]. They found that chloroah.uninates donot form at the expense of ettringite, but do form preferentially over monosulphate. Ettringitemay reappear due to the breakdown of monosuiphate to form chloroaluminate, Since the amountof chloroaluminate formed is related to the alumina content, blended cements can form more thannormal cements. Chloroaluminates appear to be stable; the trichloroaluminate is the more stableform at lower temperatures [283],
If Cl- ions are present, chloroaluminates can form from the fmt stages of hydration. Chloridespromote the formation of a porous C-S-H, This effect subsequently promotes the penetration ofmagnesium ions (if present) into the hydration product and “conversion of C-S-H to the non-hy-draulic M-S-H” may occur [102],
When exposedto seawater,Type I cements showboth chloroaluminateand ettringite formation.At early ages these products deposit in the voidsand exert little expansion [158],If temperaturesare low the presence ofNaCl causesettringiteto decomposeto form Friedel’s salt. At lowertem-peratures in a chloride environmentthe Cl ion can be incorporatedinto the ettringite structure;this results in a “less-expansivephase” [243-245]
3.9 Morphology of Ettringite
The controversy over whether ettringite forms by through-solution mechanism [220] or by topo-chemical means [222] has also resulted in two hypotheses concerning the morphology of et-tringite in the hardened cement-paste matrix, Mehta [209] noted that ettringite has two types ofmorphology, depending upon the conditions under which it forms:
= Type I: are large lath-like c~stals 10-100pm long and several pm thick. These form insolution under condhions of low hydroxyl ion concentmtion. These crystals are not ex-pansive. These are the type of crystals that have been produced in numerous lab-based re-
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search projects and of which thousands of scanning electron microgmphs have beentaken.
= Type II: are small rods l-2~m long and 0.1-0.2km thick. These form when the hydroxylion concentration is high (i.e. the normal situation for concrete). Ettringite formed withthis structure can be a source of strength, or can cause disruption when, in a confinedspace, agglomemtions of these crystals absorb water and expand. It is also possible toprepare this type of ettringite in the laboratory, but special techniques are required [216].
Mehta [207] notes that in concretes the calcium-sulphate hydrates form in a supersaturated envi-ronment. Under these circumstances the cystal structure of both monosulphate and ettringite areextremely fine-gmined. The ettringite crystals are short prisms with a thickness:length ratio of1:3.
This does not mean that the larger crystals do not form, because there is ample evidence of theiroccurrence in concrete. However, it is conjectured that Type I morphology only appears whensufficient space is available, such as in high w/c pastes, in large cavities, or during early hydra-tion. In more restricted spaces, Type II ettringite occurs as “short prismatic crystals”; under theseconditions ettringite appears to be able to form “anywhere and everywhere” in the system [213].
As will be shown below, the presence of the smaller “colloidal” size ettringite provides the cor-nerstone of a hypothesis concerning the expansion mechanism. Accordingly, in support of theType II morphology, Mehta [205] noted that the presence of lime in Portland cement pastes andconcretes affects the formation of ettringite such that it has a colloidal morphology, and does notform as long lath-like crystals. The colloidal nature of the ettringite results in the attraction oflarge numbem of water molecules which cause interpaxticle repulsion and expansion [213].
3.10 Mechanism of Expansion Associated with Ettringite Formation
As mentioned above, there is controvemy surrounding the fundamental mechanisms associatedwith the formation of ettringite and subsequent expansion. Cohen [60] has categorized the hy-potheses into two schools — crystal growth and swelling.
3.10.1 Crystal Growth
In the “crystal growth” school expansion is caused by the formation of ettringite at the surface ofreactant grains. The growth of this inner layer pushes other particles out and thus causes expan-sion.The mode of formation also appears to depend upon whether CH is present or not — if it isnot present then c~stal growth is topochemical, if CH is absent then ettringite forms from solu-tion and does not produce expansion.
Research on sulpho-aluminate based expansive cements (Type K) [61] shows that expansion iscaused by growth of ettringite crystals on the surface of expanding particles; a topochemical IWW-tion between the particle and the surrounding solution is responsible. The magnitude of expan-sion does not depend upon the quantity of ettringite formed but on the size of crystals produced.A few long crystals at a small number of sites can produce large expansions and microcracks. Al-ternatively, formation of small crystals at a large number of sites can produce a network ofsmaller microcracks
Chatterji and Jeffery [52] have hypothesizedthat expansiondue to the presence of sulphates iscaused by the solid state conversion of C4AHI3 to monosulphate hydrate in the presence of CH.This hypothesis has been researched in detail by others. Meh@ for example, was not able to re-produce Chatterji’s results and could not detect the presence of C4AH13 [208]. It is now com-
chapter 3 Page 30
monly held that expansion is due to the formation of ettringite and not monosulphate. Even ifChatterji’s theory about monosulphate formation was correct, Mehta [207] has shown that mono-sulphate formation in a confined space should not result in expansion because the plate-likemonosulphate crystals can orient themselves like “leaves of a book”. On the other hand, ettringitecrystals do not re-orient and therefore are more likely to be able to exert pressure.
The hypothesis that pressure can be exerted from individual crystals is contrary to the findings ofa number of researchers. Dron et al [88] noted that ettringite’s expansive forces can only developin a confined space. A uniaxial force of only 1 MPa is sufficient to inhibit expansion in that direc-tion; ettringite will then proceed to grow in another direction. The destructiveness of ettringiteformation depends upon where in the matrix it forms. If it forms near an aggregate particle thendestruction is maximum. If it forms near a cavity, then the cavity can act as an expansion cham-ber to relieve potential stress build-up.
Very recently, Ping & Beaudoin [256,257] postulated that formation of ettringite is by two proc-esses, nucleation and crystal growth, The production of “crystallization pressures” requires a con-fined space in which the cxystals grow, The lack of a confined space explains why ettringitegrowth does not cause pressures during early stages of hydration, Most of the ettringite may formbefore expansion occurs; it is only the formation in conflmed space that produces expansion. Inhot, Beaudoin & Ping note that any solid product will produce crystallization pressure ifl
= crystalgrowth is in a confined space
= the volubilityproduct ratio is greater than 1,0(i.e. the products are less soluble than thereactants)
3.10.2 Swelling
Cohen [62] summarizesthe “swellingschool”of thought: ettringite formsby a through-solutionmechanism,In a saturated CH environmentettringitecrystalsare gel-like and colloidal in size,The high surfhcearea results in adsorptionof significantquantitiesof waterand strongswellingpressuresdevelop,
The theory was f~st proposedby Mehta [215,217]; the expansionmechanismfor ettringite issimilar to that of some clays which expandbecauseof “polar orientedelectrostaticattraction ofwater molecules”.Small ettringitecrystals of colloidal size, formed in an alkaline environmentpossess a negative surfwe charge,The high surfhcearea combinedwith the negativecharge re-sult in an attraction for large quantitiesof water.Tests on disks of pure ettringiteconfirmthe hy-pothesis; the disks adsorba large quantityof waterand large swellingstrainsoccur, A check ofthe XRD pattern of the ettringitebeforeand after swellingshowsno substantialchange.
Early fundamental research by Seligmann & Greening [285], as well as other research [295] tendto support Mehta. Seligman & Greening found that when C3A pastes hydrate they undergo largevolume increases due to imbibation of large quantities of water. As an example, pastes were pre-pared with WA mass ratios from 1/8 to 1.0. The water/cement ratio was 0.4 to provide just suffi-cient water for ettringite formation. Nevertheless, during soaking for 180 hours after casting,much larger quantities of water were taken up and the fresh pastes showed as much as 57°/0ex-pansion. When sodium or potassium hydroxide was added, all reactions accelemted; there wereno observable changes in the nature of the reaction products.
Sagrera’s work [273] is also supportive, He mixed paste cylinders containing either calcium sul-phate or sodium sulphate additions. Volume change was measured and the quantity of ettringitein the pastes was determined by X-ray diffraction. Figure 3.5 clearly shows the positive correla-tion that exists between ettringite content and 0/0 volume increase.
Chapter 3 Page 31
180■
160 +
VOLUME 100
I
❑ %nINCREASE ❑
% 80 9■
60
/
•1 ❑❑
■ ■
40 ■u
20t
■1 1 1 1 Io 1
1 1 1
0 50 100 150 200 250 300 350 400
ETTRINGITE CONTENT (arbitrary units)
Figure 3.5Correlation Between Volume Increase and Ettringite Content [data from ref. 273]
However, also note that in Figure 3.5a significant amount of ettringite forms before there is anyvolume hwrease-Le. there is an X-axis intercept that is greater than zero. Sagrega’s results couldaeeanmodate both the swelling theory and the “crystal growth in contlned space” theory as de-seribed by Ping & Beaudoin [256,257].
Like many other observed processes that occur within concrete, there is undoubtedly no singlemeehardsm that determines behaviour. Both mechanisms — swelling and crystal-growth — areactive; environmental conditions at any particular time determine which, if any, is predominant.
Mather [199], in tis discussion of Cohen’s work [62], summarizes the issue from an engineeringviewpoint. He argues that the details of the formation of ettringite are of less importance than thefundamental meehanism involved in the process. The fact that the morphology of ettringite is crys-talline m not is “of minor importance”. The important factor is that the products attempt to oc-cupy a huger volume than is available for their formation. Mather quotes from ACI Committee223 that “thermodynamics shows that there exists a very large amount of free energy, about 55kcal/mol (formed from C3A) which equals about 169,000 ft-lb of work”. Thus, ettringite forma-tion in a eonllned space k capable of producing very high forces. Clearly, these forces cannot de-velop unless there is a rigid framework to resist the forces.
Mather continues: the mechanism of development of expansive forces involves the alteration, insitu, of a quantity of aluminate reaetant to form ettringite “by a process that delivers water andother xe.actantsin solution to the naction she, and leaves elsewhere in the structure some spaceformerly oecupkd by solution that R no longer filled with solution”. From a review of crystalgrowth theory, Mather concludes that “when you put more than one unit volume of anything in arestrained three-dimensional space having, hitiall y, a size of one unit volume something has to
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give; either the quantity added must reduce itself in amount (some leaks out) or in volume (it iscompressed) or the space enlarges either elastically or inelastically without rupture or it crocks”.The most important fhct is that the volume that ettringite forms per unit volume of C3A used is8:1; “the f=t that the ettringite is crystalline or colloidal is irrelevant”,
3,11 Mechanism of Secondary Ettringite Formation
Ettringite needs sulphateto be in solutionto form. Its formationduring sulphateattack is easilyexplained, since sulphate ionspenetrate the concretemicrostructurefrom outside.However, in
I IYI’A TRACES I
100 mTEMPERATU~ “C
J
CSS with 24% gypsum belore hydration
As IlbOV~but wtth a further 1,S%RYPSUmadded whereCSAadded
Figure 3.6Formation of Ettringite from Sulphates Supplied by C-S-HDTA results taken directly from Odler [240] and annotated
, ,,
chapter3 Page 33
this repoti we are considering mechanisms by which ettringite forms at later ages without assis-tance from outside sources.
The key to explaining this mechanism lies in the research of Lerch [180, 181] who found that dur-ing hydmtion some sulphate goes “missing” and form “Phase X“ (Fig. 3.3). By 1980, these ideashad been clarified by Odler [240]: Odler examined the bonding of sulphate within C3S pastes inorder to confkm the initial findings of Copeland et al [72] that C-S-H can bind and then releasesulphates. Odler’s DTA results are summarized in Figure 3.6. An alite cement with 2.5% gypsumwas hydrated for 28 days, ground to cement freeness and then blended with C3A in a molar mtioC3A/CaS04=l :3. The new cement was mixed with additional water to form a paste and allowedto hydrate. DTA and X-ray diffraction analyses were performed at various ages after drying at35”C. The experiment was performed for alite cements with various sulphate additions and simi-lar results to those shown in Figure 3.6 were obtained. X-ray diffraction analysis confirmed theDTA results.
The formation of the ettringite peak during the second hydmtion period and the absence of a gyp-sum peak after the start of hydration confirm that the sulphate is bound within the C-S-H struc-ture. The “gypsum is bound by the C3S hydration products in a way that makes it undetectable bythe [DTA and X-ray] methods employed”. The maximum amount of bound gypsum correspondsto 9.8 g S03 for 100 g of CSS.
Another test showed that the sulphate in the C-S-His readily extracted by satumted calcium hy-droxide solution. Thus, when a source of aluminate is added to the ground paste containing C-S-H with bound gypsum, the ettringite peak quickly forms and reaches its final intensity within 3hours after hydration.
Lachowski’s findings are in support of Odler [174]. He examined the composition of C-S-H forpastes hydrated up to 900 days and concluded that overtime S042- ions leave the C-S-Has thepastes age. Most of this loss, however, occurs during the first 28 days.
.In Neck’s research [237], concrete was manufactured with 400 kg/mj of PZ55 high-early-strength German cement, at a w/c=O.36. Strength tests at 7 days gave 60-70 MPa and 70-80 MPaat 28 days. Behaviour after 2 heat treatments was compared: In treatment “A” there was no pre-cure, a temperature rise to 80”C at 60°C/hour and 80”C was held for 4 hours; in treatment “B”, 3hours pre-cure was used followed by a 20°C/hr rise to 65”C, which was held for 2 hours. Fortreatment A Neck postulated that some of the calcium sulphate remains mobile and is availablefor secondary phase formation when moisture is introduced. Free sulphate is a requirement forthe expansion process, while moisture must be present to dissolve and transport the sulphate.Also, “crevices and disturbances” must exist for pathways for water and transport of ions. Thesealso provide space for crystallization of the secondmy phases.
Wicker etal[319,320] summarize the entire process nicely. During heat treatment the thermal de-composition of “primary” ettringite results in an increase of sulphate ion in solution and a de-crease in OH- ion concentration. Figure 3.7 shows the sulphate ion concentrations in the poresolution immediately after heat treatment for pastes with various WA ratios and alkali contents.On the other hand, after heat treatment the pore solution changes with time. Figure 3.8 shows thatthe S042- concentration reduces steadily over 90 days for both a 60”C treated and 90°C treatedpaste.
Wicker et al propose that during heat treatment the ettringite formed during early stages of hydra-tion is decomposed. The process is accelerated by akalis. In the temperature mnge 50-80”C the re-versible formula is:
chp16Jr 3 Page 34
500
400
CWCEN- 300TRATlON
200
100
■ 20 ❑ 50 ❑ 60 ■ 70 ❑ 80 ❑ 90 ❑ 100I
ITREATMENT TEMPERATURE I 1
SIA4,6 SIA=O.68 SfA=O.63 SIA=O.66 SIA=O.86 SIABI.06 SIA=O.92ALK=O.62 ALK-O.68 ALK-O.87 ALK=O.66 ALK=l.03 ALK=l.13 ALK=l.24
Figure 3.7Changes of Sulphate Ion Concentration with Heat Treatment
S/A= S03/A1203 Ratio; ALK=% equlv. alkali (data from ref. 319)
90060 C TREATMENT
800
700
600
CONCEN- mTRATION
In?noln400
300
200
100
1---—---.—---
““””G90 C TREATMENT/
/“.-
.“/-
/“
i~
4 60 C TREATMENT
o I I
19h 26d 90CI
STORAGE TIME AT 20C AFrER HEAT TREATMENT
Figure 3.8Change in S04 and OH Ion Concentration During Storage
(data from ref. 320)
Ckpk!Y3 Puge3s
At temperatures greater than 70*C monosulphate also decomposes according to the mwersible for-muhx
3Ca0.AlZ03.CaS04. 12Hz0 + 2NaOH 7k~~O~+ 3CaO”AlzOs”6Hz0+ NazSOq + Ca(OH)2
3.12 Comment
‘l’heAFm phase in Concrete is very complex both chemically and morphologically. One of itsastonishing features is the wide variety of ions that can substitute in ettringite for Al and sulphateions, while the basic crystal morphology remains the same. ‘llw complexity makes ettringite veryuseful for a wide variety of industrial applications, but it also causes problems when one wishes todetermine the influence of the trisulphate phase on the engineering properties of concrete.
From an engineering viewpoint one important research finding is that ettringite does not appear tobe stable in concrete at temperatures above approximately 60-70”C. When pastes, mortars or con-cretes are cured at elevated temperature ettringite disappears and some of the sulphate goes “miss-ing”. Various ideas have been put forward but the most accepted view is that at elevatedtemperature sulphate can easily be incorporated into the C-S-H gel structure. Various solid solu-tions may form. Further transformation of ettringite to monosulphate may also play a key role inthiS regard
Whatever the exact mechanism, it appears that a sufficiently high heat treatment nxults in the sul-phate being unusually bound. ‘l%ebond is such as to allow a later slow release of sulphate ion intothe pore solution and combination with aluminates to produce ettringite. It also appears that othercompounds such as carbonates, chlorides and carboaluminates may influence the formation of ex-pansive compounds at later ages.
The mechanism by which ettringite produces stms and strain is also not precisely knowILMather’s viewpoint is therefore preferred; from the practical viewpoint we are interested in know-ing under what conditions compounds form which try to occupy more space than is availablewithin the pore structure of the material.
chapter4 Page 36
Chapter 4Secondary Ettringite Deposition
4.1 The Importance of Porosity to Ettringite Formation and Expansion
Post-mortem studies of deteriorated concretes tell us a considerable amount about how ettringiteand other crystalline materials form. Many studies show crystalline deposits within the capillaryand air-entrained pore structure.
For example, Gillott andRitchie[112] examined cement pastes that were 50-70 years old. Theyobserved various crystals forming in the small voids in the pastes. These were identified as: (1)thin plates, frequently extending from one side of the void to the other, which were identified asCH crystals; (2) radiating clusters of needles or spherulites; these were identified as ettringitecrystals.
Murat [233] examined the fracture surface of asbestos fibre-reinforced cement paste used tomanufacture hot-pressed pasteboard. He discovered a number of large voids that were filled with“flower-like” crystallization product. Mumt noted that calcium sulphate becomes concentmted invoids; “ettringite grows in flowers from a seed”.
Specimens that had undergone freeze/thaw attack show crystals of portlandite and ettringite inthe air voids [236]. It was hypothesized that a reduction in the lime concentmtion in solution(which can be brought about by a number of different effects) results in an increase in the volubil-ity of hydrated calcium aluminates. The fkezing action in the pore structure results in pore solu-tion being expelled into the voids where conditions are right for ettringite to form. Formation ofettringite in the pores does not result in expansive pressures.
St. John [293] noted that ettringite crystals are often found in the pores and crocks of concretethat has deteriorated. These crystals are deposited in the voids as a result of movement of fluidsthrough the pore structure. The presence of ettringite in this form does not appear to be harmfhlto the concrete. However, there are “sub-microscopic particles of sulphoaluminates dispersedthrough the concrete” that may be “remobilized into solution” as the concrete deteriorates. Re-crystallization can then occur “as visible needles at the nearest interfile”. This phenomenon hasbeen observed even in young concretes that have cracked due to overstress and have fluid flow-ing through the pore structure.
Observations of ettringite in the larger voids and cracks, including air-entraining voids, appearcommonplace. At the same time, it seems that such formations are not directly responsible for“secon(kuy ettringite” attack. It maybe just the reverse: ettringite within these spaces maybe in-dicative of a process that might have led to expansion and cracking, but which was relievedthrough the transport of the reactants to “free space” where the products form in a stress-flee en-vironment. Conversely, micro-porosity, concentration of key compounds, microcracks and aweakened microstructure around the paste/aggregate and paste/steel interface appear to be nuclea-tion sites where secondary ettringite can grow and produce damage; this is the subject of the nextsection.
4.2 The Importance of the Aggregate/Paste and Steel/Paste Interface
At the heart of the issue with respect to ~ of secondary ettringite formation is the aggre-gate/paste-matrix interface, normally called the “transition zone”. Most petrographic examina-
Chapk?r4 Page 37
tions have shown that if one wishes to find ettringite in a deteriorated concrete, the first place oneshould look is at the transition zone. This section will explain the reasons for this.
A macroscopic indication of the existence of the transition zone in some concretes can be ob-tained simply by visual observation. For example, Hoshino [140] observed that the porosity ofpaste near the lower boundary of coarse aggregate is higher than elsewhere in the matrix. At thelower layer the boundary was “compamtively whitish” and contained a large number of CH crys-tals. At the upper boundary the amount of CH and the crystal size was much smaller. Using amore sophisticated procedure, Kayyali [159] noted that if one compares the porosity of concrete,aggregate and cement paste by mercury porosimetry, one finds a discrepancy in the porosities.This discrepancy indicates the existence of a higher porosity, or “interfiwial layef’ at the surface “of the aggregates.
The early fimdamental work of Hadley [123] and, later, Barnes [17] produced a clear picture ofthe transition zone which has since been substantiated by a large volume of research. In his PhDthesis, Hadley outlined the steps in the formation of the paste/aggregate interface
after consolidation of the concrete there exists a “considerable volume” of water-filledspace immediately around the aggregate
Very quickly a film of lime, l/4pm thick deposits directly onto the aggregate surfaces.This lime is “perfectly oriented”, with the c-axes normal to tr,e aggregate surface
a layer of C-S-H deposits on top of the lime film
after 4-8 hours “large and randomly-oriented” lime crystals form in the “interracialvoid”. These crystals grow to encapsulate hydrating cement grains.
after 1-3 days “ettringite needles” and “booklets” of “large platy hexagonal crystals” alsodeposit in the interfhcial region, The “booklets” were confirmed not to be lime or mono-sulphate; instead, Hadley postulates that they are “a complex solid solution of C% Si02,Alz03 and sulphur.
Observation of fracture surfaces shows that fracture occurs mainly through the interlayer voidspace. As the concrete ages, fracture occurs through the lime portion of the interfuial film, orthrough aggregate particles if weak cleavage planes are evident. Fracture never occurs at the truepaste-aggregate interfme. Hadley noted that some degree of orientation of CH occurs in the re-gion up to 150-200wn from the interface. Figures 4. la and 4. lb show Hadley’s schematic of theinterracial region at 30 minutes and 3 days after the start of hydration.
The research of Monteiro and co-workers [225, 226,229, 84] has provided valuable informationabout the transition zone. The aggregate/paste-matrix interfhce is a weak zone in the concretewith higher porosity than the bulk matrix [226]. Calcium hydroxide is deposited with preferredorientation in the region within 30~m from the surfiwe. Both coarse aggregate and sand groinsshow a calcium hydroxide coating with the c-axis perpendicular to the aggregate surface. The et-tnngite content in the transition zone is approximately 2.5 times that in the bulk matrix at lpmfrom the surfhce and only reaches the bulk matrix levels 20Lm from the surface. The thickness ofthe transition zone is larger for larger aggregates.
Monteiro et al [225] have examined the transition zone at the steel-paste interhce. This zone isvery similar to that occurring at the paste-aggregate interface. The CH that deposits in the transi-tion zone has preferred orientation; further tests show that pronouncedorientationstill exists after420 days in a 50pm region near the interface.The transitionzone has high porosity and the crys-tals are larger than in the bulk matrix.There is a higherconcentrationof ettringitecrystals in thetransitionzone, but the ettringitecrystalshave no preferentialorientation (Figure4.2). The domi-nant effect as aging occurs is the densification of sttucture behind the CH film. This densification
chapter4 Page 38
I CEMENT GRAIN ON JNTERFACE AT 30 MIN. 1
/ Smail ‘%picule#’ I
I Interface I
INTERFACE AT 3 DAYS
RI
C-S-H
nRandomly \Oriented Y
Massive CH “~.. .3 *crysta@ ■ ● ‘
Encapsulating “ S \+4
\~~
,.- ,.r ●
..0
. ● :
. . . . “... . . .“ m!liw
I I
Figure 4.1The Paste/Aggregate Interface at 30 Minutes and 3 Days
(drawings scanned from Hadiey [123] and annotated)
chapter4 Page 39
180 ~ Alldata for Plain, 5% Silica Fume and 16% Silica Fume Pastes
180
t
t
—.A
X-RAY 100INTENSITY
~ ¤~A ~
20
tA
. 1 DAYi
❑ 7 DAYS
I A 30 DAYS
coUl,
❑o ❑.
OWOA @Aw
❑m
❑BA
1 1 1 1 1 1 Io I1 1
0 5 10 15 20 25 30
DISTANCE FROM THE STEEL INTERFACE, pm
Figure 4.2Variation in Ettringite Peak Intensity from Interface (data from ref.
35 40
225)
process is assisted by the presence of silica lime. The steel/paste transition zone is not signifi-cantly different than the aggregate/paste transition zone.
In 1988 the issue of whether CH was truly orientated in the transition zone was clarified. Det-wiler and Monteiro [84] addressed the criticism that the standard X-ray diffraction technique todetermine CH orientation near the aggregate interfhce was inaccurate; the argument was that dur-ing analysis crystals which are not oriented with either a 0001 or T(IT1plane am missed. Instead,the researchers used a pole-figure goniometer which eliminates this presumed bias. They foundessentially the same results as standard X-ray analysis— namely, that CH is oriented near aggre-gate surfizes.
The type of aggregate at the interface, however, does have an important role to play. The transi-tion zone near carbonate aggregate is different than that for other aggregates, because a “basic cal-cium carbonate hydrate” forms which substitutes for the large and highly oriented CH crystals[229]. The carbonate-hydrate has smaller crystals and therefore there is a strengthening of thetransition zone, The formation of this material occured with both opc and C3S pastes; carboalumi-nates in the transition zone do not appear, therefore, to be the reason for the increase in mechani-cal strength.
The superiority of limestone aggregate in this regard is confirmed by Saito and Kawamura [275].They examined the transition zone for ordinary cement pastes cast against two types of polishedaggregate-limestone and granite. Fly ash and slag paste/aggregate interfaces were also exam-ined. The researchers found that both ash and slag substantially Educe the formation of CH inthe interracial zone, but their analysis of plain pastes at an age of 36 days also showed some inter-
chapter4 Page 40
LIMESTONE AGGREGATE/PASTE INTERFACE, AGE=36 DAYS
45
40
35
X-RAY 30PEAK
HEIGHT 25FOR
ETTRINGITE 20AND
CARBOALUMINATE15
10
5
0
m‘\-\
“’......-‘\
\
LafY
ETTRINQITE‘k
CARBOALUMINATE
1 10 100
DISTANCE FROM INTERFACE (yin) (log 8oale)
GRANITE AGGREGATE PASTE INTERFACE, AGE=36 DAYS
3,5
3
2.5
ORIENTATION “p INDEXFOR
CALCIUMHYDROXIDE
1,5
1
0.5
45
40
35
X-RAYPEAK30HEIGHTFORETTRINGITE25
Ca/f%TIO 20
15
10
5
0
T +
t
1 \l MONOSULPHOALUMINATEII
1 L ,!
I
1 10 100
DISTANCE FROM INTERFACE (pm)(logscale)
3,5
3
2.5
ORIENTATION
2INDEXFORCALCIUM
HYDROXIDE
1.5
1
0.5
Figure 4.3Compounds and Orientation in the Transition Zone — Limestone and Granite Aggregate
(data from ref. 275)
,,,
chapter4 Page 41
esting results (Figures 4.3). The intetiace normally extends from 50 to 100pm into the matrix. Et-tringite and/or monosulphate exist in high proportions very close to the aggregate surface, but thequantity drops off rapidly and no X-ray peaks for either compound were observed i%rtherthan 5-10p,mfrom the surface. Both types of aggregate/paste interfaces showed similar results except forthe significant finding that carboaluminates appear also to preferentially form near the lime-stone/paste interfhce (see Figure 4.3). In other tests it was found that the presence of fly ash andslag resulted in an @rease of carboaluminate in the interracial zone.
La.rbi& Bijen [176] report recent results which fhrther help to establish that calcium hydroxideorientation occurs. The orientation was revealed through an Orientation index which, if crystalsare not oriented should be unity. Figure 4.4 shows test results from X-ray analysis near a paste-polypropylene interfue; polypropylene was used to simulate a flat aggregate surface. The orien-tation index, and thus orientation, is maximum at the interfme; some degree of orientation occursup to 80pm ffom the aggregate surface. The introduction of fly ash has a pronounced effect in re-ducing the thickness of the transition zone from 60#m to 15~ at one month. As shown in Figure4.4, the degree of orientation of calcium hydroxide is also significantly affected.
Figure 4.4 also illustrates the beneficial effects of fly ash; CH orientation produces weak planes:if orientation can be reduced then the transition zone is strengthened. Saito & Kawamura (above)noted similar improvements [275]. The use of pozzolanas is one method to improve the transitionzone. Goldman & Bentur[113], for example, illustrated the beneficial effects of silica fume. Thetransition zone at the aggregate/matrix interface is a highly porous ~gion. The solids containedin this zone have a heterogeneous microstructure. A “rim” of calcium hydroxide is frequently ob-served in contact with the aggregate surfme. The “porous pockets” that are interspersed through-out the region contain a needle-like material that is “probably ettringite”. The interface region isshown in the first two drawings of Figure 4.5. When silica fbme is present the transition zone is
ORIENTATIONOFCALCIUMHYDROXIDECRYSTALS,AGE=28DAYS
ORIEN
6
5
4
ATIONINDEX 3
l=RANDOM
2
1
—m—
.
ORDINARYPORTLANDCEMENT(OPC)
OPCwith QUARTZFLOUR
OPCwith FLYASH#1
OPCwith FLYASH#2
o I 1 1 1 1 I1
0 20 40 60 80 100 120
DISTANCEFROMINTERFACE(pm)
Figure 4.4Orientation of Calcium Hydroxide Crystals in the Transition Zone (data from ref. 176)
chapter4 Page 42
PC=Portland cemenfi SF=sUica fume; CH=Ca(OH)2; CSH=calcium silicate hvdrate ett=ettrtiite“
–=–
I
I
Fresh Concretewithout Fume
I
Filling of transitionFresh Concrete Transition zone in
with fine silica fume mature fumemne with Ca(OH)2,C-S-H, and particles concrete
ettringlte. Note ~~porous pockets,some filled with ett
I I
Figure 4.5Schematic of Changes in the Transition Zone With and Without Silica Fume
(drawing scanned from ref. 113 and annotated)
entirely altered (see third and fourth dmwings of Figure 4,5). The interface region is lower in po-rosity, is more homogeneous, and there are no porous pockets or CH rim around the aggregate.
Zimbelmann [327] suggested a method by which the aggregate/paste and steel/paste interface canbe improved directly. In one series of tests, aggregate is coated with a normal commercial wash-ing agent, The effect is to make the transition zone thinner. Zimbelmann found that the contactzone reduced horn its normal 2-3pm to 0.5- l~m. This resulted in an increase in bond strength of100-180Y0.In another series of experiments aggregate surfaces were coated with a suspension ofpumice, detergent and water glass (sodium silicate); the objective was to replace the contact layerof CH with a layer of calcium silicates. Theoretically there should be a high “physical affinity”between the silicate contact layer and the aggregate and between the silicate contact layer and thehydrates. Tests showed that this method increased the bond strength by 200-270% at 100 days,,and in some cases the bond strength exceeded the tensile strength of the hardened cement paste.
chapter4 Page 43
4.3 Comment
There is little doubt that the transition zone in normally-cast concrete is the weak link. It hashigher porosity than the bulk paste, it has oriented CH crystals, it contains other crystals of un-known origin, and it is a region where sulphates are concentrated.
It is also a region where materials of distinctly different properties meet. The most important dif-ference is the thermal expansion coefficient between aggregate and hardened cement paste. Oneintuitively expects, therefore, that if heat-treatment of concrete is going to produce damage, thedamage is going to occur at the transition zone — behveen aggregate and paste (including bothcoame and fine aggregate) and between steel and paste,
Given the properties of the transition zone, it is therefore not at all surprising that Heinz & Lud-wig [132] conclude that for mortars showing substantial expansion due to secondary ettringiteformatio~ SEM indicates that ettringite forms at the contact zone between paste and aggregateand that this is the “origin of the degradation”.
Chauter5 Page 44
Chapter 5Key Examinations of Secondary Ettringite Formation
5.1 1980 — Ghorab et al [106]
Expansion tests were performed on mortars manufactured with a typical German high-early-strength cement (with 4,0% S03) and a sulphate resistant cement. The heat-curing temperaturevaried fkom 60- 100”C with a duration from 12 to 72 hours. Expansion tests were performed in ac-cordance with DIN 1164 and strains were measured to an age of 300 days, Mortar bars wem40x40x 160mm bans made with a water/cement=O.5 and cement/sand=O.33,
The experiments indicated no significant late expansions occured for curing temperatures lessthan 80”C, regardless of the period of heating. For treatments above 80”C expansions started af-ter soaking in water for 70-80 days, Expansion was usually complete after 130 days soaking, ex-cept for the mortar cured at 90”C for 18 hours; this specimen showed continuously increasingexpansion over the period 70 to 300 days.
In a second series of experiments specimens were subjected to a 1 day delay period followed by1,5 days at 80”C and then a 6 hour temperature rise to 100”C; cooling followed immediately thespecimens reached the maximumtemperature,Figure 5,1 showsthe expansionof bars manufac-tured with various blendsof sulphate-resistant(SR) and high-early-strength(HES) cements,Blends which contained more than 25% HES cement showed substantial expansions, again start-ing after about 70 days after the start of soaking, In all cases, expansions were accompanied bylarge reductions in resonant frequencies that were measured concomitant with deformation.
In a third series of tests the effect of repeated heat treatments was examined using the susceptiblePZ55 cement. Set I specimens were heat treated at an early age while Set II specimens were not,Set I showed accelerating expansions after 50 days of soa.ldng; this expansion (approx, 1,2%’0)
CEMENT TYP~ 4SR= SULPHATERESISTANT
H13s= HIGHEARLYSTRENGTH............r..=z.-.r.*z.=...*
‘--------- l~z sR
‘------ 75%sR + 25%H~
‘––+--- so%sR + 50XH=
~ 25%SR+ 75%HlN
---- 100ZH=-,-u-.-.-u
,-,-...-,a.-u”-’-’-~-’-””-”-’-”-’-~-”-”-”..-,,...-..-..-..-..-..r....... .. .—
o 50 100 150 200 250 300
STORAGE PERIOD (DAYS)
Figure 5,1Expansion of Various Mortars after Heat Treatment, DIN1 164 Test Method
(data from ref. 106)
chapter5 Page 45
was finished after about 330 days. During this time, Set II specimens showed no appreciable ex-pansion. At 330 days Set I specimens were subjected to a second heat-treatment and Set II speci-mens were given their first heat-treatment. Approximately 40 to 50 days afier the treatment at330 days Set I specimens started expanding even more and reached a total expansion of 2.5% atabout 500 days. Set II specimens also started expanding about the same time (380 days) andreached a maximum expansion of about 1.7’Yo,200 days after their fwst heat-treatment occurredat an age of 330 days. The heat treatment appears to be a crucial component of subsequent expan-sion. Furthermore, it appears that the time at which the heat-treatment is applied is not very im-potint; if anything, expansions are larger when the heat-treatment is delayed.
A fourth series of experiments examined the effect of pozzolans on the expansion mechanism.Various cement/pozzolan blends (containing the same amount of S03 as the PZ55 cement) wereused to prepare mortars; these mortars were tested as above. It was found that the pozzokm sub-stantially reduced expansions:
e 0°/0pozzolan: 100°/0PZ55 HES cement Expansion = 1.00%
I= 15°/0trass: 85°/0 HES cement Expansion = 0.70!40
o= 15°/0 ash: 85°/0 HES cement Expansion = O.15%
@ 30°/0 trass: 70°/0 HES cement Expansion =0.05’%
e 30°/0 ash: 70°/0HES cement Expansion = 0.02%
The authors suggest that the expansions are due to the formation of stable ettringite with time, de-rived from the monosulphate and solid solutions which contain tetracalcium aluminate hydrate.X-ray analysis clearly shows the development with time of the ettringite X-ray peak in the heat-cured sample, but not in the sample continuously cured at 20°C. Where calcium carbonate is pre-sent or carbonation is significant, Ghorab proposes that these can increase the damage caused bythe gradual transformation to ettringite.
5.2 1984 — Research Institute of the Cement Industry [268]
Test disks of cement paste with dimensions 50 mm diameter x 10 mm thick were manufacturedwith various cements. Normal consistency was used. After various delay periods, specimenswere heated over water at 100”C for 2 hours. The cements used had a C3A content of either 7.0or 13.1’XO,3% S03, and 3500 cm2/g Blaine. In some cements C3A activity was enhanced by add-ing K20. Furthermore, the sulphate type that was used was varied-either gypsum or 1:1 anhy-drite:hemihydrate.
Figure 5.2 shows the expansions of the disks measured at a constant time after the start of expo-sure, Clearly, expansion increases as the delay period decreases. Expansion was the largest at thelower C3A content of 7.0%. The addition of potassium oxide did not have a substantial effect onthe results. In a parallel series of tests which used a 3 hour pre-storage, low expansions were alsoobserved and were independent of the other parametem. It appears that the extent of prestorage isthe critical factor which governs whether expansion will take place.
In a second series of tests the behaviour of concrete made ftom cements with higher S03 con-tents was examined.Concreteprisms,4x4x16cm, w/c=O.38,were manufacturedwith one of twoclinkers in which sulphatewas added to give eithera 4’%o or an 8’%S03 content. After a delay pe-riod of either Oor 3 hours, specimens were heat treated at 40 or 80”C and a relative humidity of60 or 95% for a period of 2 hours. After cooling, specimens were stored in water for 3 years ateither 5 or 40”C.
chapter5 Page 46
45
40
35
<30EE 25c
“$ 20z~ 15
10
5
0
--“’””””””r■ OHR.DELAY """""""`""""""""""""""`""""""""""""`.n............................
~L.___J..❑ ~HR DELAY ,, ., ....................... ... ...................................... ......................................
..
■ 3HR DELAY.,,,,,-,-......,,,,..,.-,,,,-,,,,...,.,,.. -,,.,,,,,.,.,,,.........”,,,.,..,,,,.-,,,.,,,,,,-
-.. .................. ..,.,,.,,,,---,,,,”,-,,,,,,,, -.,,,,..,,,,,
Figure 5.2Expansion of Paste Disks After Heat Treatment (data from ref. 268)Note that 1Pexpansion was >12 mm/m (1,2%) cracks were observed
Table 5,1Effect of Heat Treatment on Cracking and Phase Formation (data from ref. 268)
IHeat Treatment Temperature
40 c8 80 C9
Cllnke ‘r&S03
Storage % by Relative Humidity Durtng Heat Treatment
(hrs) mass 60% 95 /6o 60 /’so 95 /’so
Subsequent Storage Temperature for 3 Years
Dark Shading - designatesotrongcraokformationaooompanladby obviouonew “phaseformation”
LightShading - designatesslightcfaok formationwith “insignlfioant’now phase formationNo Shading- Indisatssno eubstanflalvisualohanges to prismsduring3 YOWOstorage
The qualitative results are presented in Table 5,1. Heat treatment at 40”C did not result in signifi-
cant cracking for any combination of parametem, except for the very high sulphate cementswhich were not allowed a prestorage period. In most cases, heat treatment at 80”C led to cmcking
Chaph5 Page 47
and new phase formation. The 3 hour delay period had an important effect on subsequent behav-iour.
5.3 1987 — Heinz and Ludwig [132]
This paper reports on continuation of experiments started by Ghorab et al [106]. Again, experi-ments were performed on mortar prisms in accordance with German specification DIN 1164. Thehigh early strength cement had 12% C3A and 3.8% S03 (denoted PZ55 cement).
In series I experiments mortar bars were heat-treated in the moulds after a delay of 2 hours. Thetreatment temperature varied from 20- 100°C. The duration of heating was controlled in order thatall samples had the equivalent treatment of 72 hours at 20°C. Bars were heat-treated in a waterbath. After cooling, specimens were stored at 20”C in water during which time, length, weight,and resonant frequency measurements were taken. Measurements were taken to 1000 days ofsoaking.
Figure 5.3 shows the expansion, increase in weight and reduction in resonant frequency that oc-curred in the prisms that were heat-treated at different temperatures. There is clear indication thatlarge expansions can occur if specimens are treated at temperatures above 75°C (all specimenstreated at temperatures below 75°C [not shown] showed no significant changes with time of soak-ing). There is some indication that the 75°C cured prisms could expand significantly at agesgreater than 1000 days (note that the rate of expansion starts to increase after 1 year) — also notethe abrupt change in resonant frequency and increase in weight of the 75°C samples at 800 days.
Figure 5.4 shows that a positive correlation exists between weight gain and expansion afier 28days. This correlation is consistent with Mehta’s hypothesis [205] that expansion due to ettringiteformation is the result of imbibation of water and swelling of ettringite clusters. Figure 5.5 is ascattergyam of expansion vs. change in resonant frequency; the correlation is not as good; thechanges in resonant frequent y, however, could be related to microcracking within the mortar as aresult of expansive stresses.
X-ray analysis of specimens immediatelyafler heat-treatmentindicatedthat the amount of et-tnngite decreasedas treatment temperature increased.Immediatelyafter a 70”Ctreatment X-raypeaks correspondingto the monosulphate(AFm) phase were observed and, as treatment tempera-ture increased, became the only sulphoaluminate present.
Heinz and Ludwig postulate that as treatment temperature increases the aluminate and sulphateform hydrates that have decreased definition. Most significantly, both the aluminates and sul-phates are better incorpomted into the C-S-H stmcture. Thus, at high temperatures ettringite de-composes, not necessarily to monosulphate, but so that the sulphates and aluminates areincorporated into the C-S-H structure.
The sulphate contained in the C-S-H apparently becomes mobile with time, because after 3 yearsstorage the samples treated at 80- 100”C contain “practically nothing but ettringite as calcium alu-minate sulphate hydrate”. In comparison, a sample stored continuously at 20”C shows both AFtand AFm phases.
In Series 11experiments Heinz and Ludwig examined the influence of the WA molar ratio on per-formance during soaking. In these tests anhydrite was added to the PZ55 cement to produce S03contents within the range 2-8.60/o;thus, the WA ratio varied between 0.44 and 2.1. All bars wereheat treated at 90”C for 12 hours in order to examine behaviour after a pretreatment period that isknown to produce damage; a 2 hour delay period preceded the heat treatment.
chapter 5 Page 48
RESONANT FREQUENCY (kHz)13.0- ! ,
~ji
! I! I
I 1
12.5- i~!~ l-~~11~
,*4 -,_...__+ ~ ~.T- .. .. ...n.~.-.- . .. .. ..n-.F.-”-”-~-’~ ”-”-~ -’-F-_..&._..~,.,,.,, ;,,.,;, !- ~-
--- i
1
- . . . . . . i7’++ + ‘------- 0----+ ‘--- \ --~.-Q-~_-. ‘o_+& - ‘“n.,., I ~11,5., .,...,_._.&...&.-.....- .“-..-..-.....-..-+.-..-..-.. ..-..+-. . . . II
“b....
11.o- “< : -$ L.------Q-. .-$s-: ~ ~ + = !@& . ,
I ‘...,*,.: ; jl10.5- “
I ‘ II%,.,
II
<r-.+------- .-*”------+--------.-0-...’---’ .& .+..+.,,
10.0 ~ !1 )10 100 1000
WEIGHT GAIN, %4.0 ‘
3.5 ‘“ )3.0 ““
2.5 “‘
2.0. / R ,/ ,....4----4P,#...- -
1.5 “‘ / A.....’- -..”’-0’
A * /J~.1,0 J 4
0,5- -
0,01
12
10
8
6
4
2
0
10
EXPANSION, mtim
100 1000
.
/
,.:
I/ T
I------,,/
o’~.-0“
,//,’ I
100 1000STORAGE PERIOD (days)
~ 75C,16hr ---0--- fjoC,15hr ‘----------- 85C, 13hr ‘------ WC, 12 hr ~ 100 C,llhr
Figure 5.3Changes in Expansion, Weight & Resonant Frequency with Storage (data from ref. 132)
chapter5 Page 49
3.00
2.50
2.00
WEIGHT GAIN%, AFTER 28d 1“50
1.00
0.50
0.00
0.00
■ s4
h
b’ 9:■
■
● ?“
1
I1 1 1 1 1
2.00 4.00 6.00 8.00 10.00
0.40
0.20
0.00
(CHANGEINRESONANT ‘0”20
FREQUENCY
qrzzad -0.40
-0,60
-0.80
-1.00
EXPANSION mm/m, AFTER 28 DAYS
Figure 5.4Correlation Between Expansion & Weight Gain (data from ref. 132)
12.00
~i 1 b 1 1
0t 2“00 d.oo 6.00 8,00 10.00 1;
●
m#-*
■ m
■
EXPANSION mtim, AFTER 28 DAYS
Figure 5.5Correlation Between Expansion and Resonant Frequency (data from ref. 132)
chapter5 Page 50
EXPANSION (mm/m).-
10 --
8- -
6- -
4- -
2- -
—+——— &A=,44-,66
..----0“-- ~lA=.67
-----+-- ~/A=,79
--- -+ --- 51A=.88
~ ~/A=l.5
‘--.& --- &A=2,j
TTT10
—
3
--l
s
100 1000
STORAGE PERIOD (days)
Figure 5,6Series II Experiments, Expansion vs. Storage (data from ref. 132)
The expansion results are summarized in Figure 5,6, The figure clearly shows that if WA is lessthan 0,66 there is no expansion and no indication that expansion might occur afler 1000 days, Atan WA =0.67 there is a slight indication that expansion has started after about 800 days. AboveWA=O.67,however, large expansionsof bars occur,Furthermore,at intermediatevalues of WA(values that are entirely practical for Canadiancements) the rate of expansioncan be dormantforas much as 1year and then “suddenly”accelerate.At the very high WA mtios (1.5 and 2.1) themte of expansion is appreciable fromthe start of storageand reaches0,6% at 500 days.
It is important to note that times to the stati of expansionare dependentupon the size of speci-men, In a parallel series of experiments,Heinz and Ludwignoted that a change in specimenge-ometry from 40x40x160mmprisms to 10x40x1601nxnprismsproduced expansionsmuch earlier,
In Series III experimentsthe effect of relativehumidity of storage was examined,The researchersfound that below 95% rh no expansionor reductionin resonant frequencyoccurred,However, ifspecimenswere stored at 60% relative humidity for an extendedperiod (120days)and thenplaced in a saturatedatmosphere,the subsequentdamagingnmtion and expansionwas intensi-fied (Figure 5.7).
Series IV experimentslookedat the effect of water/cementratio and air-entrainmenton the ex-pansion process (Figure 5.8), A finding significantto the Canadianenvironmentwas that air-en-trainment suppressesexpansion,At the same time other experimentsshowedthat freeze/thawcycles magnifi the problem; microcracksoccur during f/t cycling whichprovide sites for forma-tion of ettringite.
WEIGHT GAII
m
%4
3
2
1
0
-1
!
J-=
,,R.
- j. ❑ .. ...-Q. -.-c?”
/1()0 ,/,/,/
/f
!
.-i.+ ----~
b
i
.+.
Qt
—
—
...
(,’
I
.
-,
..—
—
L.......-1
/-.”’
. .. ...,..-..,
Do
-2
-3
I
- .-..<.,-4 -+..---..,.+,_
-........_..,4
-5
....... ..
Humidity Change 60% to 100%
ntimXPANSION14
12
10
8
6
4
2
0
—
/
——
/
,/.’/.”’
f+---r
I ]60% to 100?4
/
10
I
100
STORAGE PERIOD (days)
~ WATER SOAK ‘--n--- 100% R.H,--.+...-.. 604 oo~eR.H.
I
Figure 5.7
Effect of Humidity Change on Expansion (data from ref. 132)2 hr. prestorage, 12 hr. treatment at 90”C, PZ55 HES mortar bars
Chapter S Page 52
10
9- -
8- -
7- -
6- -
5 ‘-
4 ‘-
3 --
2 “-
1
Expansion (mm/m)
~ w/c=o,4 &w/@0,5 with airentrainment
---cl----- w/*o,5
----+--- w/~o,45
---*--- WIC=O.7
TTT A
9’/1’1’
/ ‘“/’
f/
k$“””
d
T/
/
/,
,6I
3.
4
I
I
100 1000STORAGE PERIOD (days)
Figure S$Effect of Water/Cement Ratio and Air=Entrainment on Expansion (data from ref 132)
delay perlod=2 hrs,, treatment for 12 hoursat 90*C,mork bars with HEScement -
5.4 1988- Sylla [297]
Sylla reporteduponEuropeanexperiencewith secondaryettringite formation,As temperaturerlsa in concrete the reactivity and speedof dissolutionof C3Aincreases,while at the same timethe availablesulphatein solutiondecreases,Therefore, there is an increased tendencyto formmonosulphaterather than ettringite,l%e importanceof the treatment temperatureand heat-treat-ment period on the quantityof ettringitepresentare clearly shown in Figure S,9, Results are re-ported from pastes made with two cements:bothcementshad approximatelythe same sulphatecontent,but cement B had 12,1%C3A.CementA had 8.7%C3A.The extra aluminatein cementB results in monosulphateformingin some pasteswhen treated at 60 and 80°C,The high C3Ace-ment also shows a more rapid reductionin ettrh@e content,
Sylla notes that the formationof monosulphatealonedoes not account for all the sulphatere-leased whenettringitedisappears,He postulatesthat the predominantpotion of sulphatejs “in-creasinglyattached”to the C-S-Hat high temperatures.In fact this hypothesisis confirmedbyparallel “rnicrofluorescence” and thermal analysis studies, l%e results given in Figure 5.9 are forpastes that had no delay period, Sylla notes that for tests performed with a delay of 3 hours the et-tringite formed during the delay period remained, even @r 5 hours of treatment at 80°C,
Sylla also performedexperimentsto determinewhethernew phasesform during storage after heatlreatment.Paste specimenswere storedat 20°C and 100%relative humidityafter 5 hours heat-treatment at 40,60 and 80°C.There was no delay time. Resultsare given in Figure 5,10.
For 40 and 60°Ctreatmentthere are no large changesin ettingite content as sorddngtime pro-ceeds; In fact for cement B ettringitecontentdecreaseswith soakingtime. After en 80”Ctreat-ment however,there is a very large increasein ettringiteduringthe first 28 days of soaldngand a
chapter5 Page 53
X-RAY PEAK INTENSITY
20 –20C
18- -
16- - 40C
12
10
8- -
6- -
4- -
2- -
0 t 1 m y,
1 2 3 4 5
HEAT-TREATMENT PERIOD (hrs)
X-RAY PEAK INTENSITY30 –
=:
25- - ~Zoc
15- -
10: Lw
5- - .. &-- - O---;’----*-- ------1--- .--- .
0+--- .A 1* w I
1 2 3 4 5
HEAT TREATMENT PERIOD (hrs)
Figure 5.9
EEttringite and mono-sulphate peak measuredimmediately after heat-treatment
\
/
Cement A:3.55% S03, 8.7% C3A
Cement B:3.36% S03, 12.1% C3A
Treated without pre-storage at giventemperature for periodsfrom 1 to 5 hours.
Variation in X-Ray Peak Intensity with Curing Temperature (data from ref. 297)
I
!
chapter 5 Page 54
~’1 Note Iarga Inmass in eltrkrgks with tires for 80C curing
EITRINGITE PEAKS
Y
WC ~ ,,,,
CEMENT A\::::,,;,,,,
.: :.:
\
40c w ,:::,,,,:::.:/..::{60C ::.!,:;:;
MONOSULPHATE PEAKS M C “’.
CEMENT B
?
28 SOAKINGo TIME, days
AT 20C.
Figure 5.10Changes in X-Ray Peak Intensities with Soaking Time (data from ref. 297)
small increase thereafter. Afler 56 days soaking, all ettringite peak heights are the same value fora given cement. Note also that for cement B the monosulphate content also increased with timefor all specimens.
No cmcking was observed in the pastes and Sylla hypothesizes that the newly formed ettringite isfinely distributed over the entire sample. If microcracks were present, then transport processesmay result in large local accumulations of ettringite, with subsequent destruction of a longer pe-riod.
5.5 1989 — Heinz, Ludwig & Rudiger [1331
In this paper the authors summarize the findings of a number of previous publications [125, 157,159, 160, 161].
Chqxkr 5 Page 55
Mortars or concrete prisms were cast and heat-treated in water at temperatures between 50 and100”C. Various heat-treatment temperatures and r6gimes were used, with all r6gimes designed togive the same 3-day strength for all specimens. A wide vaxiety of cements were tested, includingthose with slag, fly ash, fume and limestone. Storage after treatment was normally in water at20”C, but other storage media were also tried.
In general, it was found that to avoiddarnagethe maximumtreatmenttemperatureshouldbe keptto less than 70°C.There is no clear indicationof the extentof delay periodrequired;prestorage(delay) for 1day to 1year prior to heat treatmentmay lead to more damagebecausethe structuralmatrix is stiffer and thus is less able to withstandthe build-upof pressures.When the delay periodis within the first 24 hours then a longer durationresults in less damage.
The cement type and sulphate content of the cement is important. For the same treatment, thehigh-early-strength cement is damaged, while the sulphate resistant cement is not (even at l(N)”Cmaximum temperature and when 490 additional sulphate is added to the sulphate-resistant ce-ment).
Use of 8% ground limestone to HES cement neither diminishes nor increases the extent of theproblem. On the other hard trass, ash and slag when used to partially replace some cement (30%to 50%) substantially improved performance; no darnage was observed after 10 years storage.Five to ten percent mass replacement by fume also resulted in a clear reduction in expansion. .
Heinz and Ludwig postulate that the composition of cement is very important in determiningwhether expansions will occur. In particular, the main influence derives fmm thesO~~203 ~~oof the cement. However, the sulphate proportion appxirs to have a higher weight in determiningbehaviouq therefore, the authors suggest that the ratio (S03)2/A1203 is a parameter that shows thestrongest correlation to subsequent effects of secondary ettringite formation. In this ratio the alu-mina content is that contained in C3A only — i.e. the so-called “active” alumina.
Figure 5.11 is a scattergram of expansionvs ~/A ratio. All data for various cements are plotted.me authors suggest a “Safe Ratio” of 2.0 — cements with a smailer ratio than 2.0 are not suscep-
tible to secondary ettringite attack. Note that at higher @/A ratios expansion is again lower. Inother words, there appears to be a pessimum ratio where expansion and darnage are a maximum.
One other important findingfrom these authorsis that inclusionof 14%air-entrainmentinto themortiu “drasticallyy“ reduces expansion; ettringite formation occurs, but takes place in the air-voidsystem rather than within the confhmi space of the pore structure. On the other hand, if a poten-tial exists for secondary ettringite formation, a material that undergoes fxeezdthaw cycles is likelyto be damaged to a greater extent. After such damage Heinz and Ludwig observed AFt crystals onaggregate particles. Cracks also occur in the matrix; in these cracks ettringite crystallizes at rightangles to the crack edges.
5.6 1990 — Lawrence et al [177]
‘l%etest programme at the British Cement Association was performed primarily in an attempt tocontlrm German observations of secondaryettringiteformation;U.K. cements were used
Prisms, 40x40x 160mm, were cast in accordance with RILEM standard. The mortar compositionwas 1:3:0.5cement/sanct/water. Twenty U.K. cements, mostly commercial rapid-hardening ce-ments were employed. A pre-cuxing period of 2 hours in the moist room was used. Specimenswere then placed in a water bath and the temperature was raised to 100”C over the course of twohours. ‘he boiling environment was maintained for another 3 hours and specimens were then al-
chapter 5 Page S6
EXPANSION, mtimExpanslon=31 mm/m
416
14
12
10
8
6
4
2
00
A‘A Suggested “Safe Ratio” ofI
2.0. Cements with ratio less1,
■❑ than 2,0 not susceptible to
I seoondary ettringite attack.1 ■ Mo
,● 0
0
I
I ■
+
#a
I
1
,
:0 ■
o1
1
1■ ■
I otI
Ii1
O%**
14 ❑
@*& ,
A-W+J3&ia
IIr
I I I I
2 4 8 10 12 14 16 la
@/A RATIO
Illgure 5.11
Scattergramof ?/A ratiovs. Expansion(datafrom ref. 133)
lowedtocool naturallyuntil 16 hours when they were storedin water at 20°C,The aevemtreat-ment was chosen to reveal damagein the shortestamountof tlmq the regime chosen is more “ex=treme” than is likely in practice,
Examplesof expansiveand non-exansiveresults are shownMFigure 5,12; other results are givenin Figure5.13, Lawrenceet al were able to classify the types of expansivebehavlourinto 7 cate-gories:
~ (a) zero expansion— e.g. cementES3579,Figure5.12
* (b) sm~l continuousexpansion
~ (c)small acceleratingexpansion
= (d) l$rge=elerating expansion— e.g. cementES3594,Figwe 5,13
~ (e) @el~ating or limitingexpansion
= (f) large S-shapedexpansionIesdhg to a definitemaximum— e.g. cementsES3595(Fig.5.12) and ES3572 (Fig.5,13),
.
chapter5 Page 57
1,00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0,00
1,00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
EXPANSION, Yo
t
---- ---- ---- ---- ---- ---- ---- ---- ---- ----
t-
---- ---- ---- ---- ---- ---- ---- ---- ---- ---
t---- ---- ---- ---- ---- ---- ---- ---- ---- ----
—---- ---- ---- ---- ---- ---- ---- ---- ---- -- Exampleof=pansive
CementES3595---- ---- ---- ---- ---- ---- ---- ---- ---- ---
---- ---- ---- ---- ---- ---- ---- -,-#_#”f -
---- ---- ---- ---- ---- --- ---fl ------------
J---- ---- ---- ---- ---- --- ---- ---- ---- Exampleof Nonexpansive
w=’Cement ES3579
---- ---- ---- ---- -
1
0 5 10 15 20 25
SQUAREROOTOF TIME(days)
Figure5.12Typical Expansion vs. Time Curves (data from ref. 177)
~ES3572,3HRS
— Cl— ES3572,16HR
— x- –ES35943HRS
— +— ES3594,16HR
~ ES3740,3HRS
— * – ES374016HR
EXPANSION, %
*--------: X2X’------------ --’---- ---- ---- --
0 5 10 15 20 25
SQUARE ROOT OF TIME (days)
Figure 5.13Expansion vs Time Curves Showing Effect of Delay Period (data from ref. 177)
chapter5 Page 58
Table 5,2 The addition of 1’%0S03 to cementExpansion vs. Sulphate Content (ref. 177) ES3578 changed its behaviour flom
a class (e) to a class (b) category—
4.00 1.00 0.704.64 1.64 1.105.27 2.27 1,50
the addition of sulphate removed thetendency to reach a limiting expan-sion and resulted in a more gradualexpansion process,
The addition of sodiumsulphatetosome cements did not change thetime at which expansionstarted,butthe magnitudeof the maximumex-pansion increasedas sodiumsul-phate content increased.Table 5,2showsthis effect.
Figure 5.13 illustrates the influence of duration of heat treatment on subsequent expansion. Speci-mens that were heat-treated for 16 hours, mther than 3 hours showeda reduction in the time tothe start of expansion,The effect is pronouncedfor cementES3740; the cement that showedverylittle expansionwhen treated for 3 hours showedthe maximumexpansionwhen treated for 16hours.
Petrography indicated that for specimens which crocked,the crockswere filled with ettringite,Et-tringite also formed ridges around surfacecracks.Expansiondid not necessarilymean that thespecimenscracked, however;some bars showedas much as 1.2%expansionwithoutcracking,
Analysisof thin sections showedrims of ettringitearound sand grains.An “acicular” ettringitemorphologyexisted within these rims,
Lawrenceputs forwarda number of points to explainsecondaryettringite formation:
Any ettringite that formsduring the delay period decomposesduring heating at 75°C orabove
Water storageat roomtemperatureafter heat treatment results in the m-formationof et-tringite without significantchanges in the quantity of monosulphate.For ettringite toform in preference to monosulphatethe pore solution must be high in sulphatecontent,Lawrenceproposes that this is the result of modificationof the calcium aluminate hy-drates— hydrogarnet like materials— duringheat treatment; the modifiedaluminatecompoundscannot rapidlyabsorb sulphatesand allow free sulphate ions, such as thoseslowly releasedby C-S-Hto persist in solution,
The naturalprocess of slow releaseof aluminateswith time then results in ettringitecrys-tallization from solution,
A less severetemperature regime doesnot modi~ all of the aluminates. Somecan mp-idly absorb S04 immediatelyon cooling.Monosulphatethen forms insteadof ettringitebecause there is insufficientsulphate in solution,
Ettringite formationoccursat nucleationsites: on the surfhceof voids, in microcmcksand at the cement-aggregateinterfhce,
The air-voidsor flaws must become filled in order for internal stressand concretedisrup-tion to occur,
chapter5 Page 59
5.7 Comment
The first important works directly related to seconda~ ettringite formation were performed byGhorab et al in 1980. They found delayed expansions when specimens were cured at tempera-tures greater than 80”C. Heat treatment was found to be an essential component if expansion dur-ing soaking was to occur, but the time at which the treatment occurred was found not to be veryimportant. In many respects the work done at the Cement Industry Research Institute [268] sup-ported these initial findings.
The work of Heinz& Ludwig, reported in 1987 and later in 1989 firmly established the phenome-non of secondary ettringite formation. They found several crucial factors influencing the poten-tial formation of secondary ettringite:
the critical heat-treatment temperature above which damage could potentially occur is inthe range 70-75°C
the longer the delay period within the first 24 hours, the lower is subsequent expansion,However, delay periods of several days or weeks, followed by heat treatment, could alsoproduce sufilcient cracking to provide the nucleation sites necessary for secondary et-tringite effects
the cement type and particularly the ratio of sulphates to aluminates are important for de-termining the potential for secondary ettringite formation. In 1987 Heinz& Ludwig es-tablished that an WAratio of 0.67 appeared to be a “critical” ratio above whichsubstantial expansion and cracking had the potential to occur. In 1989 Heinz et al estab-
lished another ratio, ~/A (where A is the “active” alumina contained in C3A); the safevalue for this ratio was determined to be 2.0.
the time at which significant expansion starts and the rate of expansion is a function ofspecimen size. Although the research was fairly limited in this respect, it appears that thelarger the specimen (or member), the longer is the time to start of expansion, and theslower is the mte of expansion. If Heinz and Ludwig’s interpretation is correct, it ex-plains why expansions and cracking in the field have been observed after several years,while laboratory experiments, on smaller samples, show large expansions after the firsttwo or three years of soaking.
air-entminment of the mortar phase dmstically reduces the expansions that are observedwhen comparison is made to a non-entrained mortar. This phenomenon has been ob-served by several researchers and indicates that one simple method of deterring damagedue to potential secondary ettringite formation maybe to ensure that adequately en-trained concrete is cast.
Chapter 6 Page 60
Chapter 6Secondary Ettringite Formationand the Chemistry of Cement
6.1 Effect of Gypsum Content of Cement on Strength and Volume Stablllty
Retardation of set and early strengh requirements are both important practical considerationsthat call for the production of cement with an “optimum” gypsurn content. The fineness of the ce-ment plays an important role in this respect and affects both retar&tion and early strength gain;the amount of gypsum required to produce proper retardation is higher the higher the freeness[180]. In 1917 ASTM C19-17 limited S03 content to 2%; considering that current Type 30 (andoccasionally Type 10) cements are ground above 600 m2/kg Blaine, the 2% S03 limit is clearlynot acceptable. Current CSA standards allow 4.5°/0S03 or more,
A perusal of the Chapter 5, and some fbrther test resultsgiven below, indicatethat what is “opti-mum” for strengthand retardation is not necessarily(and perhaps not normally)optimum with re-spect to long-termstability of concrete.
6.1.1 Test Results of Ai-Rawl [6]
The most significant work in this area was performed by AMtawi in 1977 [6], The rate of forma-tion of sulphoahuninates is accelerated by an increase in temperature, Therefore, one might ex-pect that the optimum S03 content for maximum strength to increase as temperature increases.Brown [34], for example, noted that maximum strength was achieved in a heat-cured concretewith 7.1’%S03. However, such concretes exhibited high expansions and he noted that it maybenecessary to limit S03 to 5-6% to keep expansions to a tolerable level,
A1-Rawi examined the strength and expansion of concretes made with two different clinkers,Both clinkers (Table 6.1) had high C3A and C3S contents which are believed to be beneficial tosteam-curing.
The clinkers were blendedwith gypsumto give S03 contents of2,0, 3,8, 5.7 and 7,5%. Concretecubes and prisms were cast with w/c=O,5, and a/c=5. Specimenswere subjectedto an 18hour cur-ing cycle consistingof 4 hours delay, a 12°C/hourheating rate to either 50, 70 or 90”C,followedby 6 hours of curing, Strengthawere determinedimmediatelyafter cooling, Other specimenswere fog cured for ages up to 182days.Deformationreadingson prisms were taken to an age of48 weeks,
Strengthsat ages of 28 and 182daysare shown in Figures6,1 and 6,2 (the notation C1-70Ameans concrete made with cement 1heat-curedat 70°C),Both figures indicatethat the cementsshould have between 3.8 and 5.7% S03 content to produce optimumstrengthsof heat-cured
Table 6.1Compound Composltlon of Clinkers used in A1.Rawl Study [6]
..
Chapter 6 Page 61
65.0
60.0
55.0
50.0
STRENGTH 450MPa “
40.0
35,0
30.0
25.0
— C1-20A
~ C1-50A
~ C1-70A
~ C1-90A
- - C2-20A
~ C2-70A
, ,20.04 I , r r , , T , {
1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8
S03 CONTENT%
Figure 6.1Dependence of 28-day Strengths on S03 Content and Curing Temp. (data from ref. 6)
Figure 6.2
— C1-20A
~ C1-50A
~ C1-70A
~ C1-90A
Dependence of 182-day Strengths on S03 Content and Curing Temp. (data from ref. 6)
Chapter 6 Page 62
specimens.On the other hand room-curedconcretesshow a clear regressionin strengthas S03content hicreases above 2%. Generalconclusions,however,shouldnot be made upon the basisofonly two high C3Scontent cements.
optimum S03 contents for strengthare not necessarilycompatiblewith optimumcontentsfor vol-ume stability.Figure 6,3 showsthat S03 contentsaboveabout4% result in large expansionswithin the first 20 weeks of water-curing.On the other hand,results shownin Figure 6.4 indicatethat an 18hour heat-treatmentcan reduce expansionduring48 weeksof water-soaking,
me results given in Figure 6.4 are contraryto the commonly-heldviewthat heat-treatmentpro-duces an increase in expansiondue to secondaryettringiteformation.Damagedue to SEF hasbeen connectedto the productionof microcracksduringheat-treatment.Suchcracks are reduced,or eliminated,by provkiingan adequatepre-curingperiod.Note that A1-Rawiemployeda 4 hourpre-cure prior to heat-curingto as high as 90°C.
6.2 The Practical Significance of the S03/A1203 Ratio
FaI Type 30, high early stnmgth cemenc CSA A5-M88 allows a maximum of 3,5% S03 whenthe C3A content is less than or equal to 7.5%, and a maximum of 4,5% if C3A content is greaterthan 7.5%. The American Society for Testing and Materials Standard C150-89 also allows asmuch as 4.5% S03, but the C3A limit above which this is allowed is 8%, rather than the 7.5%found in the CSA standard.ManyotherCountriesin the world followthe Americanstandar~
1 while others set a single S03 limit varyingfrom 3.()to 4.5%. [Cembureau,Cement Standardsof! the World,46].
0.14
0.12
0.1
Expansion0.08
(%) o ~.
0.04
0.02
0
0 5 10 15 20 25 30 35 40 45 50
Curing Time, Weeks
Figure 6.3Effect of S03 Content on Expansion of Normal Fog-Cured Specimens (data from ref.6)
Chapter 6 Page 63
0.2
0.18
0.16
0,14
0,06
0,04
0,02
0
2 3.8 5.7 7.5
S03 Content (%)
Figure 6.4
Effect of S03 Content on Expansion after 48 Weeks of Soaking (data from ref. 6)
Canadz Colombiaand Hungaryaxethe oniy three countriesnotedin the Cembureaureport that al-low 4.5% S03 in cements whichcontain as little as 7.5%C3A(Colombiaand Hungaryset a 4.5%S03 maximumwhich is independentof C3Acontent).Canada and thosecountries that use ASTMC150-89 also permit S03 contentsgreater than 4.5%if it can be shownthat mortarbars preparedin a standard manner and soaked in water for 14 days expand less than 0.02%. Thus, a low expan-sion value in a 14 day testis used to assess the long-term volume stability of cements with highsulphate contents. The question that arises is: is a 14-day test an adequate method to predict long-term stability?
Lerch and Ford in 1948[183] testedprismsmade from 27 differentcements that were moist-cured for 1day and then storedcontinuouslyin water at 21“Cfor 5 years.The S03 contentsof thecements were fairly low, but the results, shownin Figure 6.5 point out the lack of correlationbe-tween expansionsat 14days and expansionsat 5 years.Severalcements showless than 0.02%ex-pansion at 14days, but show appreciableexpansionsat 5 years.The test resultshelp to accentuatethat predictionof long-termbehaviourfrom short-termtests is seldomreliable. In the case ofLerch’s results, the reliabilityis low.
Aithough the WA ratio appears to be the most reiiable “indicator” of whether secondary ettringiteformation could be importan~note that other factors also ~ntribute to the ~~~~tion ofwhethera given cement is like]y to be stable in concrete.Wells et al, for exampleproposedthat itis not just the WAratio that determineswhetherexpansionoccurs after heat treatment;both a lowratio and a low alkali content are necessaryto avoidlarge expansions[3171.
Chen and Grattan-Bellew[55]in their studyof alkali-aggregatenmctionaddedvariousquantitiesof KzSOAand NaX30qto a clinker from a singlecementplant. A suite of 17cements was manu-
Chapter6 Page 64
0,25-
0,2- “ ?“
0,15EXPANSIONAT5 YRS,%
0,1IO“O:b=zaLL-
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
EXPANSIONAT14DAYS,%
Figure 6,5Scattergram OFExpansion at 14 Days vs Expansion at 5 Years (data from ref. 183)
fhcturedwhich containedvariousalkali contents.The cements were intergroundwith an opti-mum quantity of gypsurnto a finenessof 370 m2/kg.Expansionwas then monitoredwith time ofstorage in a moist environment.A good correlation(r=0,91) was foundbetween acid-solubleal-kali content and expansion.However,fbrtheranalysis of the data for the present report alsoshoweda positive correlationbetween expansionand MgO content (Figure6.6) and between rateof expansionand SO#Al*Os ratio (Figure 6,7); both comelationcoefficients(0.73 and 0.80)aresignificantat the 5°Alevel.
Chen & GnMan-Bellew also considered the potential effect of sulphates and looked at the hy-pothesis that “ettringite in cracks and air-voids might be contributing to the late stages of expan-sion”, They found that there was no correlationbetween mte of expansion of the high-expansionbars (those withNa20 equivalent>1.0%)and the amount of ettringitoin the moctarat 6 months.
6.3 Speclficatlons for Heat Treatment of Concrete
Concern overpossibleproblemswith secondaryettringiteformationhas prompted some m-researchersto recommendheat-treatmentguidelines,some authorities to change their specifica-tions, and some authorities to considerchangingtheir specifications,Properheat-treatmentmethods would theoreticallyeliminate,or substantiallyreduce, the potential for problems even ifthe cement and/or aggregate were foundto be inferior,
Neck [237]proposedthree types of heat treatment,dependingupon the proposed durabilityexpo-sures, with the objectiveof avoiding damagedue to secondaryettringite formation:
Type 1:1 hour pre-cure with a maximumsoakingtemperatureof 80”C— for interiorex-posures with no moisture
Type II: 2 hour pre-curewith a maximumsoakingtemperatureof 70”C— for exterior ex-posurewith no contact with the earth
Chapk?r6 Page 65
30
25
20
5
0
5
0
CORRELATION BETWEEN EXPANSION AND MgO CONTENT17 POINTS, CORR. COEFF. = 0.732EXPANSION = .651 + 5.974 * MgO
30
5
0
.5 .6 .7 .8
S03/AL203 RATlO
Figure 6.7Scattergram of SO#Ai@3 Ratio vs Expansion (data frOm ref. 55)
~hapter 6 Page 66
= Type III: 3 hour pre-cure with a matimum soaldng temperature of 60°C — for exteriorexposure wh.hcontact with the earth
The European Committee for Standardization in 1989 has responded to the issue by setting moreslringent specifications for the heat treatment of concrete [98], These are:
~ The concrete temperatureduring the first 3 hoursof curingcannotexceed 30°Cand can-not exceed40°C duringthe first 4 hours
~ The rate of temperaturerise cannot be greater than 20 OC/hr
= The average maximumtemperaturecannotexceed 60°C;individualtemperaturereadingscannotexceed 65°C
~ The codng rate aftexheat-treatmentcannotexceed 10OC/hr
~ Concrete must be protectedagainstmoisture10ss
Also in 1989,The GermanCommitteefor ReinforcedConcrete [103]recognizedthat intensiveheat treatment can affect durabilityof structuralmembers~e, Faults in theconcrete due to too rapid warmingor sharplydiffering expansion are susceptible to “sulphatebonding [that] can occur with a high level of moisture in the concrete”. Accordingly, speciflca-tiona for damp conditions are different for dry concrete, as shownin Table 6,2,The 60”CmaxLmum temperaturefor concrete that will be damp in serviceis clearly a change made to addresspotentialproblems with [email protected],
The Britishhave been slowerto respond,perhapsbecausethey have been waMngfor detailedre-search results from the BritishCementAssociation(e,g, Lawrenceet al [177]),It 1sinterestingthat the Britishcode of practice in 1972CP110:1972noted that damagedue to excessiveheattreatment wouldnot occur if
~ temperaturerise during the first 3 hours 1snot greater than 15°C!/hr
~ thereafter, the rate of increaseor decreaseis not greater than 35°C/hr
= the maximumtemperaturereached by the concreteis not greater than 80°C,
However,the 1985Britishcode of practice BS8110:1985ondtted the above thres guidelines,Other Brltlsh authoritieshave taken steps to 1111the gap, The BritishDepc of TransportSpecifica-tions for HighwayWorks (1986),Part 5, Clause 1709:5(Ii)requh’es(1) the concrete must be leflfor 4 hours withoutaddkionalheating; (2) the concretetemperaturecannotbe raked at a rate
Table 6.2Specifications for Heat=Treated Concrete
German Committee for Reinforced Concrete, 1989 [103]
I Moisture CategoryDry Damp
Minimumholding (delay)period (hrs) 3 4+ or +
Maximumconcretetemperatureduring holding, “C 30 40
Maximum heating rate, OC/hr <20* <20*
Maximumconcretetemperature**,“C 80 60
heating rate should be reduced to 10OC/hrfor lightweightconcrete** individual values may be up to 5°Chigher
Chapter 6 Page 67
greater than 20°C/hr; (3) the maximumconcretetemperaturecannotexceed 70°C; (4) the rate ofcoolingcannot exceed the rate of heating [177].
[n North Americameasuresto controlexcessiveheat treatmentshave been takenon a locaJscale.The results of Merritt& Johnson [218]on concrete strengthdevelopmentafter variousheat treat-ments were incorporatedinto the Iowa State HighwayCommisionspecificationsfor precast con-
‘1’leinitial concrete temperature should not be greater than 38°C for a minimum of 2hours after casting
The rate of temperature increase after a 2 hour period should not be greater than 14°C
‘I%emaximum temperature attained should not be greater than 66°C
The maximum temperature should beheld for a period sufficient to develop the nx@redstrength
‘he rate of temperature decrease should not be less than 11°C/hr.
Units should be kept covered for at least 24 hours after casting.
This is all good, practical, engineering sense; it was not intended to address directly the issue ofprevention of secondary ettringite formation. There is some concern, however, that such stepsshould be taken immediately, and by national authorities.
Hill [136], points out in a letter to ACI committee 517.2R,the issues that ntA to be addressedconcerning guidelines for precast concrete manufacture. He notes that the present guidelines donot mention the current potential problems related to secondary ettringite formation of precastconcrete. l%e three conditions that appear necessary for this problem to surface need to be in-cluded in a warning statement. These conditions are:
= a curing (heat-treatment) temperature greater than 60-71“C
~ concrete that will be submerged or exposed to 100Zorelative humidity
- concrete made with a mment with a high WA ratio and high alkali contents.
Hill was concerned that under the guidelines noted in517.2R a user could face a situationwherethe above three conditions occur. Hill recommended that the current document be withdrawn.
It appears that in Canada a revision of CAN3-A23.4-M78 on precast concrete methods is about totake place. The 1978 standard specifies that the concrete should attain initial set before heat is ap-plied (this is normally 2 to 4 hours but could be as low as 45 minutes). During this time the con-crete must be maintained at at least 10”C. Temperature increase should occur at approximately20°C/hr to an optimum temperature range of approximately 65-70”C, but in no case should thecuring temperature exceed 80”C. Temperature decrease should not occur at a rate exceeding30°C/hr. Proposed changes for the new version of A23.4 include reducing the maximum tempera-ture tlom 80°Cto 70°C.
6.4 Comment
Determination of “optimum” gypsum content involves the determination of the proportion of gyp-sum that will produce optimum strength and a satisfactory setting time. Research performed inEurope and in Canada (see Chapter 7) indicates that the sulphatehlumina ratio of the cement mayplay an importantrole in determininglong-termstabilityof concrete.Thus, “optimum”gypsum
Chap&r 6 Page 68
needsto be redefined; long-termdurabilityof productsmanufacturedwith Portlandcement needsfurtherconsideration,
Clearly, cement is just one ingredientof concrete.Other ingredientscan affect durabilityas canthe mix-design,casting and curing procedures.However,all other factorsbeing satisfactory,it isnecessaryto be able to evaluatecements for long-termdurability.This requires that a satisfactorytest methodis avtdlable,
It is unrealistic to expect a good correlationbetween the expansionof mortar bars soakedh waterfor 14days and expansionador crackingof concreteat 10or 15years,The scattergramof Fig-ure 6.5 showedone exampleof the poor correlationthat exists between short and long-termex-pansions;the cementswere manufacturedat rrdd-century,It is Mghlyunlikely,for a numberofmasons, that the correlationfor moderncementsISbetter,
The viabilityof the 14-daysoakingtest shouldbe *examined in the light of the chemistryandphysicalpropertiesof modem cements. ‘Ms is especiallyimportantgiven (a) secondaryettrlngiteformationmay bean importantdurabilityissue, and (b) both ASTMand CSA standardsallowmore than 4,5% S03 if a cement can be shownto pass the 14-daysulphateexpansiontest.
In CanadL A23.4 specificationsfor precast concreteam presentlyunder revIsIomIt is suggestedthat careful considerationshouldbe paid to the potential secondaryettringlte issue and to the Ger-man specifications(Table6.2) that have addressedtMspotentialproblem,It is especiallytrouble-some that the proposedA23,4still relies onthesetdng time to define the minimumdelay periodprior to treatment this settingtime could be as short as 45 minutesand still satls~ both A23,4and ASstandards,
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Chapter 7Rapid Test Method for Secondary Ettringite Formation
Researchers have been looking for rapid test methods to “predict” long-term behaviour of con-crete for many years. For example, in 1949 Scholer [282] reported on results of a revised acceler-ated performance test which was first developed by Gibson in 1932; Gibson wished to look at themap-cracking of concrete pavements [108].
In Scholer’s test method, saturated concrete specimens (3’’x4”X16”) are dried at 54°C for 8 hours,then immersed in water at 21-27°C for 16 hours. This process is repeated for 6 days a week whileon the seventh day they rested — immersed in water. This accelerated test can be performed foras long as the researcher wishes, with expansions being measured at regular intervals.
To substantiate his test method, Scholer compared field performance of various concretes to ac-celerated performance. Concretes were made with 2 types of aggregate (one good performance,one poor performance) and a wide variety of cements. Prisms (3’’x4”X16“ prisms — the samesize as those used in the accelerated test) were placed on the ground and exposed to 5 years ofnatuml freeze/thaw cycles. The accelerated exposure W* runOver285 ~YS. Figure 7.1 showsthat a Dositive correl~tion between field and a~elerated performance occ~red. ‘Failure of thespecimens occurs by excessive expansion, cracking and loss of strength.
It is interesting to note that as early as 1949 there is some inference that something other than theknown reactiotdexpansion processes (such as aar) may be at work. For example, Scholer recog-nized the importance of the bond between the paste and the aggregate and that after this bondfhils, a “jacking action” tends to develop to increase expansion.
Although Scholer achieved good correlations between laboratory and field performance, and hiswork was heralded as landmark research (by Bryant Mather, for example [see discussion to pa-per]), the “accelemted” exposure lasting 285 days would hardly be considered “accelemted” to-day. In recent times, many other researchers, including the present author, have attempted todevelop laboratory tests that reflect long-term field performance. The primary difficulty in thisdevelopment is that it takes a very longtime to determine whether an accelemted test is accuratebecause it takes a very long time to obtain ~levant field data.
An adequate test must ensure that it “measures” or predicts potential expansion in an unbiasedfashion. Figure 5,13 (Chapter 5) illustrates the influence of duration of heat treatment on sub-sequent expansion. Specimens that were heat-treated for 16 hours, rather than 3 hours showed ateduction in the time to the start of expansion. The effixt is especially pronounced for cementES3740; the cement that showed very little expansion when treated for 3 hours showed the maxi-mum expansion when treated for 16 hours. The development of a rapid test method must takethis into account [177].
Observations by Heinz et al that freeze/thaw cycling exacerbates any potential for secondary et-tringite formation and damage, leads them to the suggestion that freeze/thaw cycling can be usedto accelerate potential deterioration of concrete that has a dense microstructure [133]. However,they did not attempt to develop an accelerated test method based on this principle.
7.1 The Duggan Test
One accelerated test method that has been extensively developed will be the subject of the re-mainder of this chapter, because it is purported to “measure” the effect of secondary ettringite for-mation of various concretes. This is the Duggan test, whose development, results, and
chapter 7 Page 70
0.25
0.2
0.15
0,1
0.06
0
DATA FROH SCHOLER, 194948 eamasl CORR, COEFF. ■ ,707
FIELD EXP, = -,017 + 1.382 * ACCEL. EXP.
,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
0 . ..”!,..! 0 . ..’” !1
!
II -1
0,02 0.04 0.00 0,08 0,1 0,12
ACCCLCRA7E0 CXPANSXON, X
Figure7.1Scattergram ofAccelerated vs. Field Expansions (data from ref.282)
significanceofmsults havebeenreported indetailinDuggan &Scott [91-93]; Scott&Duggan[284]; GillOttetal[110, 111];Jones & Gillott [155];Attiogbeet al [12];and Wells et al [317],
As originally devisedthe Duggan test involvesthe followingprocedure [284]:
Concrete cores (5 minimum)are taken from.a structureor from laboratory-castprismsorcylinders,The coresare 25mm in diameterand at least 65 mm long and are cut to 50mmlengths.The ends are groundsmoothand parallel,
Initial length measurementsare taken with a comparatorjust before the start of heat treat-ment. In later tests by Duggan& Scott and in tests by Gillott et al [110,111]this measure-ment was used as the zero point from whichother strain ~dings were taken.
Coresare soaked for 3 days in distilledwater @21°Cin a closedcontainer
Coresare then placed in a @-air ovenat 82°C for one day
Cores are removedfrom the oven, allowed to cool for 1hour, then placed back in dis-tilled water for one day
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~ A second one-day heating, one-day soaking cycle is performed
= A third cycle is performed, but this time cores are left in the oven at 82°C for 3 days
@ At the end of the third heating cycle, cores are removed from the oven, allowed to coolfor 1 hour and then length measurements are taken relative to a steel standard. In theearly tests by Duggan and Scott this measurement was used as the zero point for furtherstrain readings.
@ Cores are placed in distilled waterat21 “C
= Length measurements are taken relative to the steel standard at intervals of 3-5 days.
7.1.1 Significance of Duggan-Test results
The test was initially thought to be an accelerated means by which potential alkali-aggregate reac-tivity of various cement/aggregate combinations could be assessed. Scott & Duggan set about es-tablishing that the Duggan test successfidly predicted aar of various lab-cast concretes. By takingcores from both sound and deteriomted site concretes, they also established a rough correlationbetween observed behaviour and results of the Duggan test. Presumably due to time/fmnciallimitations, no attempt was made to establish a formal correlation between Duggan results onyoung concretes and the long-term performance of those concretes. Scott& Duggan [284] notedthat the test could be used both to classi@ lab trial mixes, and to evaluate existing structures.
Observations that (a) concrete known to be susceptible to aar and concrete especially preparedwith reactive aggregate showed large expansions in the Duggan test and (b) concrete with smallexpansions in the Duggan test did not contain alkali-reactive aggregate, led the researchers to con-clude that the test could be useful for evaluating potential alkali-aggregate reactivity.
Later research (Duggan & Scott, 93) showed that both the type of aggregate, the type of cementand the alkali content of the cement play a role in determining the 20-day expansion value(which was defined by the researchers as the critical time at which pass/fail criteria could be es-tablished), Figure 7.2 shows some of the results. Duggan & Scott concluded that only concreteshould be ranked by the Duggan test — it is the cement/aggregate combination, and not just thecement alkali content or the extent of reactivity of the aggregate that determines potential expan-sion behaviour, They advocated perforrnace testing, exemplified by the Duggan test, such that“the testing emphasis currently placed upon acceptance or rejection of aggregates should beshifled to acceptance or rejection of concrete”. In this regard, the researchers should be com-mended for espousing a philosophy that is starting to gain wide acceptance; a significant numberof researchers, the present author included, believe that it is a dangerous and potentially costlypractice to acceptor reject the individual material components of concrete, such as aggregate, ce-ment and pozzolan in isolation.
7.1.2 Research of Gillott, et al
The research of Gillott et al [110, 111] and Jones et al [155] provides valuable insight into the sig-nificance of the Duggan test.
In the first series of test, [111], various concretes were manuf~tured with 4 commercial cementswith different alkali contents, 2 alkali active aggregates and 1 inert aggregate. Specimens weresubjected to both the Duggan test and to the standard concrete prism test for Potential Expansiv-ity of Cement/Aggregate Combinations (CSA A23.2-14A).
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EXPANSION,‘/0
0.8
0.7
0,6
0.5
0.4
0,3
0.2
0.1
0
I ❑ AGG. A (INERT) ❑ AGG. B (REACTIVE) ❑ AGG, C (REACTIVE) I
I m
0.95 0.95 1.00 1.00 1,07 1,10 1.17TIO T 30 T 30 T 30 TIO T 30 T 10
CEMENT ALKALI CONTENT, ?’0AND CEMENT TYPE
Figure 7.2Effect of Alkali Content and Cement Type on Expansion (data from ref. 93)
Throughcarefhl experimentationand comparisonsof the responseof the variousconcretes to thetwo types of test, Gillott was able to make the two primary conclusionsthat:
- al~li.aggregate rwtion is not the major cause of expansion measured during the first21days of the Duggan test
= expansion in the Dugga.n-test at ages up to 90 days “correlates with the relative propor-tion of ettringite and frequency of microcracking in the concrete”,
Other more tentative conclusionswere that (a) Duggan-testexpansiondependsprimarily on theproperties of the cement, and (b) the form of the sulphatethat is present (either gypsum or anhy-drite) may affect the extent of microcrackingcausedby the Duggantest, and thus may affect sub-sequent expansions.
In a second set of experiments, reported in detail in Gillott et al [110] both cement pastes and con-cretes were manufwtured from various cements and aggregates. Six different Type 30 Portlandcements were chosen on the basis of variations in S03 content. Two non alkali-expansive aggre-gates were used in the preparation of the concrete. Cement pastes were prepared for each of thesix cements, with a w/c ratio of 0.4. Ten concrete mixes were pre ared with different ~ment/ag-
!lgregate combinations; the nominal cement content was 475 kg/m , and the water/cement ratiowas approximately 0.4 for all concretes. All specimen sizes were 3“x3”X14” and, in an attempt tosimulate precast concrete, specimens were subjected to an accelerated curing regime OR2 hourspre-cure, 2 hours temperature increase to 85°C, 4 hours at 85”C, and slow cooling overnight, The24 hour strengths of the concretes were in the range 27-44 MPa, while the 28 day strengths werein the range 56-64 MPa.
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1,2
1.0
0.8
M.
!!0.6
B
0.4
0.2
0.0
0
EXPANSION OF CONCRETEI
~ CEMENT A
—a-— CEMENT B
– ‘“– CEMENT C
~ CEMENT D
—-&— CEMENT E
..-.
----
----
----- ----- ----- ----- -
/’----------------------------/----- ----- ----- ----- ----- ----
---
./ /---
/--
A
--
-. >4 ------------------ --
10 20 30 40 50 60 70 80 90
SOAKING TIME, DAYS
0.4
0.2
0.0
EXPANSION OF CEMENT PASTE, 1
~ CEMENT A.- ---- ---- ---- ---- ---- ---- ---- ---- -
— U- – CEMENT B
—. ● —.- CEMENT C ----------------------- ----------
~ CEMENT D
-- — -A- — CEMENTE ‘ --------------- ‘ - - - - - - ---”----- -_—- -
---- . . . . ---- ---- --- .F ---- ---- ---- ---- ---- --
“F~/---- -. -R- ---- T.-Y----:----.:-7 ::: ; :: :?::
t=
o 10 20
Expansion of Hardened
30 40 50 60 70 80
SOAKINGTIME,DAYS
Figure7.3CementPastes and Concretes Made with theSame Cements
(data extracted from ref. 110)
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Gillott found that all cement pastes had significant expansion aller being subjected to the Dugganr6gime. All pastes (Figure 7.3) showed a monotonically decreasing expansion over the course of90 &ys of soaking, The initial and final expansion values and the mte of expansion over varioustime periods was different for the different cements,
All ten concretes that were tested also showed significant expansions during the Duggan test. It isunderstandable that after Gillott’s findings were released those in the cement and concrete indus-try became rather concerned about the Duggan test; Five of the six concretes made with the Nel-son aggregate (a dolomitic limestone) fbiled the proposed Duggan limit of 0.05%at21 days,while two of the four concretes made with Exshaw aggregate (a calcitic limestone) tiiled.
The results presented by Gillott provided an opportunity to compare the expansion behaviour ofpastes and concretes, Figure 7.3 shows the expansion of cement pastes and of concretes manufac-tured with the same cements at approximately the same water/cement ratio. The aggregate usedto make the concretes was the Nelson dolomitic limestone.
There is a fair correlation between the expansion of pastes and the expansion of “equivalent con-cretes” at soaking times up to approximately 40 days. Beyond 40 &ys, however, behaviour be-tween the two “types” of material is different. The expansion curves for pastes become concavedownwards — the expansion process is starting to exhaust — while the curves for concrete re-main straight or, in the case of the concrete with Cement D, become concave upwards, The con-crete strains at 90 days for two of the concretes are much greater than those of the correspondingcement pastes, The lack of correlation at higher expansion levels is clearly shown in the scatter-
1,20
1al
la’
i?0,80
k
5& 0.60
b
[ 0.40
kbt
0,20
O,cm
■ 22 DAYS
❑ 48 DAYS
● 90 DAYS
● /’
==%”””/“
0’/“
/“/
/“,
❑ ❑ ’. ‘n/
/’■ ,
❑m ‘/ “+ /’
•1 /’‘/’./ 1 t I I t
Oocm 0.20 0.40 0.60 0.80 1.(x) 1,20
% EXPANSION OF CONCRETE AT GIVEN TIME
Figure 7.4Scattergram of Concrete Expansion vs. Paste Expansion (data from ref. 110)
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gram given in Figure 7.4; this is a plot of expansion of concrete vs. expansion of the equivalentcement paste at a given time.
Based upon an examination of the research noted in this report, it is suggested that the reasons forthe disparity between paste expansion and concrete expansion are that with time ettringite be-comes more concentrated in localized regions such as in microcracks and at cracks at the aggre-gate/paste interface. The total ettringite content in the concrete matrix is the same as in the paste,but in the concrete ettringite is localized and is therefore more efllcient in producing expansions.
By using a suite of cements with various sulphate and aluminate contents Gillott was, in essence,able to confirm the findings of Heinz et al [133] (see Figure 5.11, Chapter 5) who noted a pes-
simum value for ?/A ratio and that cements with high @/A ratios showed smaller expansionsthan those with smaller mtios. Figure 7.5 plots expansions vs WA ratios for both cement pastesand concretes (made with Nelson aggregate). There appears to be a pessimum range for WA mo-lar ratio centred about a value of 1.0.
One important aspect of the test progmmme was that the expansion tests that were performed onconcrete in the Univesity of Calgary laboratories were repeated in the CN laboratories. It is sig-nificant that when one compares both the qualitative behaviour of expansion vs. time and thesizes of expansions observed at given time, there is excellent agreement between the results fromthe two laboratories; the Duggan test on concrete appears to have excellent reproducibility.
Gillott’s mearch resulted in a number of practical conclusions:
Delayed ettringite formation is the major cause of expansion of cements and concretesexposed to the Duggan test
The cement is the main component causing expansion; expansion is highest, all other tic-tors equal, when the WA molar ratio is approximately 1.0 (Note that the present authorhas attempted to correlate oxide compositions and Blaine surfhce areas of cements usedin Gillott’s work and in Attiogbe’s work with expansion. Various correlation methodshave met with failure; there does not appear to be single parameter or group of parame-ters which show significant correlation — other than the 13/Aratio).
The severe heat treatment of the Duggan test causes microcracks in the concrete. Et-tringite can form fwter due to easier penetration of water and an increase in the numberof potential nucleation sites.
The formation of ettringite at nucleation sites effects fbrther microcracking and affectsexpansion mtes.
7.1.3 Research of Attiogbe and Wells et al [12,3171
Attiogbe also examined the use of a severe heat treatment followed by soaking in water and meas-urement of expansion to evaluate the cements and concretes with potential durability problems.The researchers prepared 75x75x380mm prisms of concrete and 150mm cubes of both concreteand cement paste. The water/cement ratio for the pastes was 0.50. Specimens were stored at 23°Cin a moist room for the first 24 hours after casting, then in a fog room until 7 days.
Unlike Gillott’s test progmmme, a simulated precast heat-treatment regime was not used prior tothe start of test. The “Duggan test” performed by Attiogbe normally consisted of two cycles of(a) heating to 80”C for 3 days, followed by (b) 1 hour cooling to room temperature, and then (c)soaking in distilled water for 2 days. The Duggan test and the Attiogbe test are compared in Fig-ure 7.6. Note that Attiogbe defined “zero strain” two days after the end of the last heating period(see Figure 7.6). In other respects the Attiogbe test method was similar to the Duggan test.
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t
t
I
1!20 ‘
1,00- ----------------------- -
ii!
0.80- . - - ” - - - - - - - ” - - - ”----”-”-
B0.60. . - - - ” - - ” - - ” - - - -------”” -J=
K OAO-- ----------------- ------
-- -------v
C3
21 OR22
000 +DAYS
0.8 0,9 1 1,1 1,2 1.3 1.4
MOlARRATt0303/A1203
1,20 ‘
lSKI .- ---------------------- -
i!
0,80. . - - - - - - - - - - - - ” - -”------”
0.60. . - ” ---------------------m
hw 0.40. ----- . . . . . . . . . 04!5OR 48
DAYS
0s)0 -008 049 1 1,1 1)2 1,3 1.4
MOlARRAT10303/A1203
1.20 ‘
i!!8
Osxl ~0.8 0.9 1 1.1 1.2 103 1,4
MOLARRAT10S03/A1203
Figure 7.5Observatlonsofa PessimumS03/A1203 Ratio atVarious Ages (data from ref. 110)
Ptwe 77
DATUM LENGTH MEASUREMENT(A)
m
SATURATED SPECIMEN
ATIIOGBEAlllOGBE et al
REGIME
distilled water
+
at roomtemperature
(b) / mm
rDUGGANREGIME
o,l’%
(c)
ElSTRAIN
o.1’%
cmtraction
LLH_Hl”/
3 45678 9 10 11 12 TIME (CiEty$)
I I
DEFORMATIONVSTIME
A= . 1ST &2~JE-SAT
FIRST DRY* ● C
, 2ND DRY 3RD DRY. .b. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..z.
STRAINSAT END OF EACH DUGGAN CYCLEDUGGAN ZERO STRAIN, EARLY E)(PTS
Figure 7.6Temperature Testing R6gimes, Data and Examples Strains in Duggan and Attiogbe Tests
chapter 7 Page 78
2,5
2.0
1.5
EXPANSION‘Ye
1.0
0.5
0.0
/’
~— CEMENT A
~ CEMENT B
~ CEMENT C
~ CEMENT D
●
o 5 10 15 20 25 30 35
TIME (days)
Figure 7.7Expansion of Paste Cores in Attiogbe Test (data from ref. 12)
w/c=O.5, cores taken from 150mm cubes
Example expansion results for cement pastes are shown in Figure 7,7, Note the extremely largeexpansions that were observed with the pastes made with cement B and cement C, - -
The pamllel tests performed by Attiogbe are perhaps more interesting than the main series, Theresearchers looked at expansion behaviour of pastes and concretes stored in saturated NaCl solu-tion rather than in water, Companion specimens stored in water expanded, but specimens storedin NaCl did not expand and even contracted in some cases. In the presence of chlorides the volu-bility of ettringite increases; therefore, no ettringite is observed in NaCl soaked specimens and noexpansion occurs, This is strong evidence that the expansions in water are due to the formation ofettringite,
Fundamental examinations confirm that pastes and concretes that exhibit large expansions con-tain “massive formations of ettringite” and those with small expansions have “little or no visibleamounts of ettringite”, The results suggest that “late formation of ettringite” is the cause of expan-sion, Attiogbe’s research is in agreement with Gillott’s in this respect; the Duggan test, or testssimilar to Duggan’s test, indicates expansion due to ettringite formation and not alkali-aggregatemction.
7.1.4 Correlations Between Glllott’s and Attiogbe’s Results
Both research programmed have come to similar conclusions concerning the Duggan test, Thetest is a measure of secondary ettringite formation and not alkali aggregate reaction, at least inthe fust 20 days of soaking. Ftuthermore, the findings in both studies do not contradict the
chapter 7 Page 79
0.25
0.2
0.15
0,1
0.05
0
t
CEMENT B ❑
1JCEMENT D
■ CEMENTB~ CEMENT C
tn CEMENTA
I
1, CEMENTC
CEMENT AI @ CONCRETE
PETE_o 0.5 1 1,5 2 2.5
STRAINSMEASUREDBY ATTIOGBEAT 20 DAYS,%
Figure 7.8Scattergram of Strains Measured by Gillott [110] and those Measured by Attiogbe [12]
Data are Iabelled according to origin of Cement
mechanism of secondary ettringite formation first proposed by European researchers (see Chap-ter 5).
Attiogbe and Gillott used some of the same types of cement in their studies. Thus, it is possible toexamine whether any comelation exists in the expansions obtained by the two studies. Figwe 7.8shows a scattergram that was obtained from the available results.
The very large stmins shown in the Attiogbe study and confiied by Wells et al [317] are some-what surprising. Apparently the Attiogbe heat-treatment r6gime is much more severe than that ofthe Duggan r~gime. This is also somewhat surprising, since Gillott subjected his specimens to asimulated precast r6gime prior to the Duggan test. One aspect that could explain the very highstrains of the Attiogbe paste specimens is the higher w/c ratio used (0.5 vs 0.4 for Gillott).
In any event, there is little correlation between these two studies, despite the fact that the sametypes of cement were used. Although it has been shown that the Duggan test is reproducibleamong laboratories, it is clearly important that a single test procedure is used for all studies.
7.2 Interpretation of Duggan-Test Resuits
Duggan & Scott initially proposed a limit of acceptable behaviour for concrete as 0. l% expan-sion after 20 days of soaking. The proposal of this limit for the Duggan test was criticized.Lawrence et al [177], for example, had several concerns about the test which they say prevent sat-isftiory interpretation of results:
chapter 7 Page 80
@ 22mm diameter cores are not representative of the concrete structure. The testing ofsmall concrete cores could magni~ the intensity of the effect of secondary ettringite for-mation, Note that Lawrence tested mortar bars which am also not representative of theconcrete
= Drilling cores could itself produce &mage and thus create sites for additional expansionof ettringite
@ Site concretes may not have been heat-treated and therefore may not be susceptible tosecondary ettringite formation. They become susceptible when the core is heat-treated inthe Duggan test
= Expansions of the cores during testing maybe the mult of a number of different effects
EIJ the heating cycle proposed by Duggan does not represent practical heating regimes. Theheating programme is very severe and may nxult in the rejection of cements that mayperform adequately in pmctice,
= Duggan’s test results have not been correlated with field concrete,
7.2.1 The Zero Strain Datum
A primary criticism centres about the definition of zero strain on a dry datum, Figure 7.6b showsa schematic of the heating/cooling regime followed in the Duggan test, Also shown (Figure 7,6c)are example values of observed strains measured at the end of each component of the Duggantest and during soaking (typical values extracted from Duggan & Scott [92]). Figure 7.6a showsthe temperature regime followed by Attiogbe et al [12],
In this example, first drying results in a shrinkage strain of about 0,07?40(bottom left of Figure7.6c). Most, but not all, of this deformation is recoverable on first resaturation, Second dryingproduces less relative shrinkage than in the 1st cycle, but the total shrinkage is slightly greater,Second resaturation is not much different than 1st resaturation and the magnitude of 3rd drying isapproximately the same as second drying,
The length of the specimen at the third drying point is the definition of zero strain fust used byDuggan. Thus, when bars are placed back in water for swelling vs. time measurements, a substan-tial component of that swelling deformation is due to the uptake of water and not due to swellingas a result of secondary ettringite formation, Under these conditions, then, the definition of O,1°/0expansion at 20 days as “unacceptable behaviouf’ is shaky, at best, because the swelling compo-nent due to water ingress will vary from concrete to concrete independent of whether secondaryettringite forms,
Afier criticism by ASTM committee C09.02.02, Duggan & Scott [92] redefined the zero datumas the length of the saturated core hefizcethe start of the test regime (see Figure 7,6), This newzero was also used by Gillott et al [11O, 111] in their experiments. This is a better approach be-cause it attempts to remove the component of swelling vs time that is due to water uptake only.The rationale behind this definition is that the swelling due to water uptake atler 3rd drying isequal to the total shrinkage that occurs between point A and point B (Figure 7,6c). Thk new defi-nition led Duggan & Scott to define a new Pass/Fail limit of 0.05°/0at 20 days, rather than O.10/o.
This is, however, still not correct because during the Duggan r6gime the cores undergo a signifi-cant amount of permanent deformation. In the example shown in Figure 7.6 this amounts to 0.01to 0,02% (compare point A with the 1st and 2nd resatwation points). It is well known that mostpermanent (irrecoverable) deformation in concrete that is subjected to shrinkage/swelling r4gi-mes occurs during the first cycle; shrinkage and swelling deformation during subsequent cycles
chapter 7 Page 81
Table 7.1Analysis of Strains During Duggan Test (data from ref. 92)
Type of corvxete tested
A Non-reactive agg., low alk. cemen
B: Non-reactive agg., silicafum~C Non-reactive agg., silica fume, repaa
D Non-reactive agg., low alk. cemen
E: Expensive oonc., failed in servkx
F: Non-reactive agg., high elk. cemen
Q: Reactive agg. #l, low-alkali cemen
H: Reactive agg$2, low-alkali cemen
STRAINAT
20 DAYS%
0.35
0.3
0.25
0.2
0.15
0.1 “
o.05-
0-
-o.05-
S3-0.067 0.046 -0.021
-0.065 0.055 -0.010
-0.051 0.045 -0.006
-0.021 0.015 -0.006
-0.064 0.060 -0.004
-0.050 0.050 0.000
-0.020 0.016 -0.004
-0.056 0.062 0.006 %
-0.030 0.041 4.010
-0.054 0.074 0.010
-0.052 0.062 0.024
-0.050 0.044 -0.012
4.079 0.067 0.004
-0.072 0.062 -0.010
0.010 0.000 0.006
-0.066 0.050 -0.030
-0.045
-0.062
-0S)62
-0.044
-0.076
-0.060
-0.027
-0.050
OldZeroZ1
0.062
0.066
0.056
0.096
0.322
0.304
0.119
0.096
Expans.NewZeroZ2
0.027
0.016
0.02
0.04
0.246
0.234
0S)98
0.016
R&isedZeroZ3
0.041-ma-0.024
0.052
0.235
0.242
0.119
0.046
❑ FIRST DUGGAN ZERO - Z1 ----
- r —■ SECOND DUGGAN ZERO - Z2
❑ REVISED ZERO - Z3----
----
---- ---- ---- ---- ---- --
---- ---- ---- ---
I I I I I I I r’A B c D E F G H
CONCRETEDESIGNATION
Figure 7.9Comparison of 20 day Expansions Using Three Zero Definitions
chapter 7 Page 82
are then approximately constant. Therefore, a more correct estimate of water-swelling after the3rd drying cycle is the swelling that occurs between the 2nd chying point (point C) and the 2nd re-saturation point (point D).
Results reported by Duggan & Scott [92] were used to compare the effect of the three definitionsof the zero point on the deformation at 20 days. The extracted data and analysis am shown in Ta-ble 7.1 and Figure 7.9. The zero data are defined as:
= Z1: Initial Duggan Zero, point B, Figure 7.6
~ Z2: New Duggan Zero = Z1 - estimate of water swelling (pt A - pt B)
= Z3: Revised Zero = Z1 - estimate of water swelling @t D-pt C)
It should be noted that Attiogbe et al [12] also attempt to compensate for the water-swelling com-ponent of deformation by defining the zero point two days after the last heat cycle (see Figure7.6). The disadvantage of this definition is that strains over the fwst two days that are, in tit, dueto internal chemical reactions and expansion will not be measured,
The use of the revised zero, or any other zero, and definition of pass/fail criteria based upon abso-lute deformation values is not the best approach for two reasons:
= Regardless of the zero datum that is used, there must still be some uncertainty in themeaning of a given deformation at, say, 20 days. This uncertainty occurs because we can-not precisely differentiate between deformation caused by water-swelling and deformat-ion caused by secondary expansions.
~ A single point criterion is proposed to describe a time-dependent rate process — be it aaror secondary ettringite formation. Any number of reaction mechanisms could be envis-aged which pass through the proposed window of, say; between Oand 0.05% expansionat 28 days. A single point criterion is not satisfactory,
A much more mtional, and still simple, approach is one which can be developed from the workof Gillott et al [110]. Gillott calculates rates of expansion over two time periods, among others —i.e. from 3-22 days and from 22-90 days. He also defines a ratio of deformation rates= rate (3-22) /rate (22-90).
It is suggested that if the Duggan testis to be developed into a standard test method, then defini-tion of pass fail criteria along the lines of “rate of deformation” must be used, There are three dis-tinct advantages:
~ The expansion processes involved are rate processes and therefore the use of rate criteriaare fimdarnentally valid
= The use of rates of deformation to define pass/fail eliminates the uncertainty associatedwith having to differentiate between water-swelling and other types of swelling, Dugganand Scott [92] indicate that the time-dependence of wetting-swelling is effectively com-plete 8 hours after the start of soaking. Thus, mtes of deformation determined afler thiseight hour period should be related to the extent and consequences of secondary et-tringite formation.
~ Comparison of tates of deformation over different time periods (e.g. 3-22 days and 22-90days) allows a rapid determination of whether the time-dependent process is acceleratingor decelerating, A single deformation-time criterion as proposed by Duggan & Scott can-not do this.
Chapkr 8 Page 83
Chapter 8General Discussion
A review of case studies indicates that deterioration of concrete is usually a result of a combina-tion of effects. Alkali-aggregate niwtion, corrosion and sulphate attack appear to be principalmechanisms. Some case studies indicate that secondary ettringite formation may also be a princi-pal mechanism for some types of concrete. The number of cases that can be directly related to sec-ondary ettringite formation is small; however, a detailed review of research in the area combinedwith a few simple deductions indicate that the rate of incidents that can be wholly or partially at-tributed to secondary ettringite formation will increase.
The main question posed at the beginning of this report was: is there, or is there likely to be, a po-tential secondary ettringite formation problem in North America? Examination of the available lit-erature does not reveal any incident of deterioration in North America dhectly attributed tosecondary ettringite formation. lherefore, one conclusion based on this review is that there is notpresently a secondary ettringite problem in North America.
However, good quality laboratory research confirms that secondary ettringite formation can pro-duce expansion and cracking of heat-treated concrete made with North American commercial ce-ments. In North America it is not unusual to find Type 30 cements with S03 contents in the range3.5 to 4.0% — and could be as high as 4.5Y0.It is not unusual to find alumina contents of 570 or
more and C3A contents of 9% or more. Calculation of the WA or ~/A ratios for such cementsplaces them in the maximum expansion ranges determined by Heinz and Ludwig and by Gillott-
Cement with high sulphate and alumina contents results in the potential for production of a largequantity of ettringite and thus high potential expansion. The high BMnes currently used for Type30 cements results in a heterogeneousmicrostructure, easier ion transport to nucleation sites and m-requireseven higher sulphate contents to prevent rapid set.
The presence of cement, aggregate and steel together in concrete result in a weak interracial re-gion which has high calcium hydroxide content, with orientated crystals, high in porosity and highin ettringite content. This is also the region that cracks when excessive heat treatment is applied.
On the concrete production side, the author is aware of concrete treated with maximum tempera-tures in the range 60-70”C and delay periods in the range 1-3 hours. Furthermore, some produc-tion processes using high heating and cooling rates (as high as 25-30°C/hr) have been observed. A70”C maximum temperature is within the range where ettringite starts to decompose and sul-phates integrate into the structure of calcium silicate hydrate; research indicates that sulphate inthis form can slowly re-enter the pore solution at a later age, and react with aluminates to producesecondary ettringite. The potential inadequate delay periods and excessive heating and coolingrates could &mage the concrete to provide sufficient nucleation sites for secondary ettringite for-mation and destruction.
In Canada we often have precast concrete that is subjected to frequent wetting and drying andfreezing and thawing cycles; cycles which will exacerbate any potential secondary ettringite prob-lem. Fortunately, much of the concrete is also air-entrained, which appears to result in substantialdampening of the secondary-ettringite deterioration process.
The conclusion is that given current cement production and construction practices there does ap-pear to be a potential for a secondary ettringite formation problem in North America.
Possible remedies in the cement plant are to strive towards the reduction of C3A, S03 and the tW-nesses of cements used by the precast industry. Better test methods need to be developed to evalu-ate the potential long-term stability of cements in concrete.
Chapter 8 Page 84
The precast concrete industry should strive towards more stringent specifications. It is suggestedthat the new German sgx!cificationsmaybe appropriate for consideration by North Americanauthorities. There is substantial evidence that air-entrainment, pozzolams and lime-stone aggregatecan all contribute towards Educing the potential for secondary ettringite formation.
Much more research is also necessary, Much of past research has been performed on cmnmercialczments. Clearly, to determine the conditions under which secondary ettringite forms, and thus topropose more precise specifications, we need to perform careful experiments on specially pr-eparedcements — cements where cement composition, gypsum content, fineness, and other fac-tors are strictly controlled. At the same time, experiments on commercial cements should alsocontinue in order to provide relevance to the results from the controlled experiments.
The development of adequate test methods to evaluate long-term durability is also a crucial themefor fiture research, It is unfortunate that the Duggan test started out on a poor footing — lack ofproper attention to detail and lack of control during early experiments severely limited the signM-cance of the early findings. More careful research by Gillo~ however, and maUzation that someimprovements to the test method can still be made suggest that a future variation of the Duggantest may prove to be a useful practical test method, and one that could be developed into an accu-rate standard test method.
Acknowledgements
The author is grateful to the Portland Cement Association (Project 92-05) for funding this project.Thanks also go out to members of the Secondary Ettringite Task Group (H, Chen, S. Cumming,P, Grattan Bellew, P. Breeze, C, Rogers and J,F, Scott) for forwarding copies of papers and otherinformation relevant to the subject, Special thanks are due to Paddy Grattan-Bellew and the Na-tional Research Council for doing the computer-database search on topics related to ettrlngite.Paddy also provided tome english translations of some important German papers. I would alsolike to acknowledge the painstaking work of the Interlibrary Loans Oftlce at the University of Cal-gary and Mr. CMS Huizer for doing the legwork to obtain access to the relevant literature.
The contents of this report reflect the research and views of the author, who is responsible for theaccuracy of the report. “fhe opinions expressed and conclusions drawn in this report am not neces-sarily those of the Portland Cement Association.
References Page 8S
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Subject Index Paze 114
Subject Index
Accelerated Tests . . . . . . . . . . . . . 6,69-82
Aggregate
limestone . . . . . . . . . . . . . . . . . . . . . . . . . . 6,39
andtransition zone, . . . . . . . . . . , . ...6.7,39
Airentrainment . . . . . . . . . . . . . . ...52
Alkali-aggregate reaction . . . . . . . . . 4,6,71
Alkalis
effect onettringite formation . . . . . . . . . . . . 25
Attack mechanism . . . . . . . . . . . . . . ...4
Blast furnace slag . . . . . . . . . . . . . . ..39
Calcium silicate hydrate
andsulphates . . . . . . . . . . . . . . . . . . . ...23.33
Calcium aluminates
nXardationof. . . . . . . . . . . . . . . . . . . . . . ...18
Calcium hydroxide . . . . . . . . . . . . . . .,4
orientation in transition zone . . . . . . . . . . 3942
Carboaluminates . . . . . . . . . . . . 21,28,39
carbonates
effect onettringite formatb . . . . . . . . . . . . 25
Carbonation . . . . . . . . . . . . . . . . . ...4
Cement
llneneas . . . . . . . . ..o . . . . . . . . . ..o .$.24.45
hydradonchemistry . . . . . . . . . . . . . . . . . . . . 16
Cement chemistry
andettringite formation . . . . . . . . . . . . . . 60-68
chlorides
diffusion of . . . . . . . . . . . . . . . . . . . . . . . . ,4,6
penetration of . .,,..,.0.. . . . . . . . . . . . ,,, . 4
Chloroaluminate . . . . . . . . . . . . . . .. 6,28
Corrosion . . . . . . . . . . . . . . . . . .. 4,21
Creep . . . . . . . . . . . . . . . . . . . . ..10
Crystal growth
andpressure exerted . . . . . . . . . . . . . . . . ...29
Duggantest, . . . . . . . . . . . . . . . .69-82
Ettringite
wdmwtitimofdudnti, . . . . . . . . . . . 18
andthe transition zone . . . . . . . . . . . . . . . 39-42
chemistry of . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Crystal structure of . . ..o . . . .. o.... . . . ...17
decomposition of. . . . . . . . . . . . . . . . . . . . 6,22
deposition of . . . . . . . . . . . . . . . . .4.6.7.9.39
expansion mechanism . . . . . . . . . 29-32,44-68
formation atTectedby alkalis ..,....., ,.. .25
formation a.fkcted by temp. . . . . . . ..8.22.24
formation of . . . . . . . . . . . . . . . . . . . .16.20.36
morphology of. , . ., . . . . . . . . . . . . . . . . ...28
precipitation of . . . . . . . . . . . . . . . . . . . ...4.36
stability of . . . . . . . . . . . . . . . . ..2O. 24,25,35
Expansion . . . . . . . . . . . . . .4.6.24.44.74
andtype of cement, . . . . . . . . . . . . . . ...57.73
due to ettringite formation. . . . . . 30,47-51,71
typeof . . . . . . . . . . . . . . . . . . . . . . . . . . . ...56
Flyash . . . . . . . . . . . . . . . . . .. 39,45
Frost durability, . . . . . . . . . . . . . .. 8,36
Gypsum, , . . . . . . . . . . . ..l9.24.33.46
effect of percent on expansion. ,....46,58, 62
Heat curing
guidelines for. ,., . . . . . . . . . . . . . ..lO.64-67
Humidity
of storage, effect on expansion . . . . . . . . ...51
Hydration
intransition zone, . . . . . . . . . . . . . . . . .. 41-42
Ion substitution
andeffectoftemperature. .,...... . . . . ...24
inettringite crystal structure . . . . . . . . . . ...18
Jouravskite .,, . . . . ...,.,... . ..18
Microcracks . . . . . . . . . . . . . . . . .. 6,33
at aggregate/paste interface . . . . . . . . . . . ..7, 8
infilling of . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Monosulphoalumi nate
formation of . . . . . . . . . . . . . . . . . . . . . . . ...16
stability of . . . . . . . . . . . . . . . . . . . . . . . . 20,35
Permeability .,.......,...,,. . ..12
pH
andstability of aluminates, ...,... . . . . ...21
effect ofalkalis on. , .,.,..... . . . . . . . ...25
Pore solution
ionsin . . . . . . . . . . . . . ..o. o. . . . . . . . . . ...33
Subject Index Page 115
Porestructure . . . . . . . . . . . . . ... .4,36
Porosity
andthe transition zone . . . . . . . . . . . . . . . ...42
Precast c.mcrete .,...... . . . . . 7,8,9-14
Precipitation inpores ..,......,...4,36
Precuringperiod. . . . . . . . . . . .8,10,33,46
Relative humidity
effect on expansion. . . . . . . . . . . . ,., ...,, 46Secondary ettringite
mechanism of formation. 32-35,47,52-54,58
Shrinkage . . . . . . . . . . . . . . . . . ,. .10
Silica fume . . . . . . . . . . . . . . . . ,, .42
Sleepers . . . . . . . . . . . . . . . . . . see Ties
Soaking period . , . . . , , see Precuringperiod
Volubility product . . . . . . . . . . . . . . ..21
Strength
andgypsum content . . . . . . . . . . . . . . . . . 60-62
effect of elevated temperature on . . . 10, 13, 61
of ties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...8
Sulphate
effect oftype onexpansion . . . . . . . . . . . . . . 46
release ofions withtime ..,.,, , . . . . . . ...47
SulphateJaluminate ratio . . . . 50,55,62-64,76
Swelling pressure........,,.. ., ..30
Temperature
effect ofrate of rise . . . . . . . . . . . . . . . . . . ...9
effect instability . . . . . . . . . . . . . . .21.33.54
elevated, effect of . . . . . . . . . . . 6,9-12,44-68
Testing
andthe Duggantest. .,,,..,,.,., . ...69-82
Thaumasite . . . . . . . . . . . . . . 6,7,18,26
l%ermalexpansion ..,,..... ... ...14
Ties . . . . . . . . . . . . . . . . . . . . ..7.8
Transition zone
andcalcium hydroxide . . . . . . . . . . . . . . ...39
andettringite content . . . . . . . . . . . ., . . . ...39
at aggregate/paste interface. . . . . . . . . . . 36-42
atsteel/paste interface . . . . . . . . . . . . . ..20.42
Trass . . . . . . . . . . . . . . . . . . . . ...45
METRIC CONVERSIONS
Following are metric conversions of the measurements used in this text.They are based In meet cases on the International System of Units (S1),
1 In,1 sq In,1 ftlsqff1 sq ft per gallon1 gal1 kip = 1000 Ibf1 lb1 lb fwr cubic yard1 psf1 palNo, 4 sieveNo. 200 sieve1 bag of cement (U,SJ1 bag of cement (Canadian)1 bag per cubic yard (US,)deg. c
= 25,40 mm= 545.16 mmz= 0,3048 m= 0.0929 ,mz= 0,0245 mzlL= 3.785 L= 4.448 kN= 0.4536 kg= 0.5933 kglm’= 4,682 kg/m~= 0.006895 MPa= 4.75 mm= 75 ~m= 94 lb = 42,6 kg= 881b = 40kgw 55,8 kg/m3= (deg. F - 32)/1.8
PALABRAS CLAVE: pruebas aceleradas, reacci6n alcali-agregado, alcalis, aluminates de calcio, hidr6xido de calcio,carbo-aluminates, carbonataci6n, cemento, quimica del cemento, cloruros, cloru-aluminates, agrietamiento, formaci6n deetringita retrasada, prueba de Duggan, durabilidad, etringita, expansi6n, cal, tratamiento de calor, hidrataci6n, iones ensoluci6n, microagrietamientos, monosulfa-aluminato, pH, soluci6n para poros, concreto precolado, periodo de precurado,orientaci6n preferida, formaci6n secundaria de etringita, resistencia, sulfates, relaci6n sulfato/aluminato, presi6n deexpansion, temperature, ensaye, taumasita, durmientes de concreto, zona de transici6n.
SINOPSIS: Este reporte comprende una revisi6n y an~lisis de la literature existente relacionada con las causas, 10Sefectosy la prevenci6n de la etringita secundaria (retrasada) en el concreto. Se examinaron m&sde 300 publicaciones. Se tratanprimeramente Ios estudios en 10Scuales el daiio en el concreto posiblemente fue causado por la formaci6n de etringitasecundaria. Posteriormente, se trata la investigaci6n fundamental relacionada con la formaci6n de la etringita secundaria,su quimica y 10Smecanismos de formaci6n. Se analizan as investigaciones mi%importances sobre el tema. Asimismo, sedetermina la importancia potential (a)del m&odo de curado por calentamiento (b)de la qufmica del cemento. En el tiltimocapitulo, se evalua una prueba rapida sobre la evaluaci6n de la suceptibilidad del potential de la etringita secundaria(ensaye de Duggan). El an~lisis indica que parece haber potential en Norte America para la formaci6n de etringitasecundaria; es muy probable que la formaci6n de etringita secundaria pueda llevar a un deterioro significativo del concretotratado con calor. Sin embargo, es poco probable que la formaci6n de etringita secundaria pueda ser el iinico mecanismoresponsible del deterioro premature. Los factores criticos que determinant la extensi6n del dafio debido a la formaci6n deetringita secundaria, son: (a) la duraci6n del periodo de retraso antes del calentado del concreto, (b) la severidad del regimende calentado o enfriamiento, y (c) la relaci6n SOJA1,OS del cemento. No existe evidencia de que 10Sconcretos no tratadoscon calentamiento son suceptibles a este fentimeno. Mayor investigaci6n y mejoras en el ensaye de Duggan puede resultaren el desarrollo de un mdtodo de prueba estandard para analizar la estabilidad y la durabilidad del concreto a largo plazo.
REFERENCIA. Day, R. L., The Eflect of Secondary Ettringite Formationon the Durability of Concrete: A LiteratureAnalysis,Research and Development Bulletin RD108T, Portland Cement Association [El efecto de la formaci6n de etringitasecundaria en la durabilidad del concreto: Un an~lisis de la literature, Boletin de Investigation y Desarrollo RD108T,Asociacion de Cemento Portland], Skokie, Illinois, U.S.A., 1992.
STICHWORTER: Schnellverfahren, Alkali-Zuschlagreaktion, Alkali, Kalziumaluminate, Kalziumhydroxid,Kohlenstoffaluminate, Carbonation, Zement, Zementchemie, Chloride, Chloraluminate, Rit3bildung, verzogerteEttringitbildung, Duggan-Test, Haltbarkeit, Ettringit, Expansion, Gips, Warmebehandlung, Hydration, geloste Ionen,Bildung von Mikrorissen, Monosulphoaluminat, pH, Porenloslichkeit, Porositat, Fertigbeton, Vorharteperiode,wiinschenswerte Ausrichtung, sekundare Ettringitbildung, Festigkeit, Sulfate, Sulfat-/Aluminatverhaltnis, Treibdruck,Temperature,Testen, Thaumasit, Bindung, ~ergangszone.
AUSZUG: Der Bericht umfat3teine Besprechung und Analyseder verfiigbarenLiteratur uber die Ursachen, Auswirkungenund Vermeidung von sekundarer (verzogerter) Ettringitbildung in Beton. Es wurden uber 300 Publikationen untersucht.Fallstudien i.iberSchaden in Beton, die wahrscheinlich durch sekundare Ettringitbildung hervorgerufen wurden, werdenzu Beginn untersucht. Die Grundlagenforschung uber die sekundare Ettringitbildung, deren Chemie und dieAblagerungsmechanismen werden danach behandelt. Wichtige Punkte uber das Thema werden detailliert analysiert.Danach wird die potentielle Bedeutung (a) einer Methode der Warmebehandlung und (b) die Chemie des Zementsbesprochen. Im letzten Kapitel wird ein Schnellverfahren zur Bewertung moglicher Empfindlichkeit auf sekundareEttringitbildung (“Duggan’’-Test) beschrieben. Die Analyse weist darauf bin, daf3in Nordamerika ein Potential fi-irdieBildung sekundaren Ettringits besteht. Es ist sehr wahrscheinlich, daf?die Bildung von sekundarem Ettringit zu starkerDegenerierung von warmebehandeltem Beton fuhrt. Es ist jedoch unwahrscheinlich, dafl sekundare Ettringitbildung dereinzige Mechanisms ist, der zu vorzeitiger Degenerierung fuhren kann. Die kritischen Faktoren, die das Ausmafl derSchaden aufgrund von sekundarer Ettringitbildung bestimmen, sind (a) die Dauer der Verzogerungsperiode vor Beginnder Warmebehandlung des Betons, (b) das Ausmai3 des Warme-/Kuhlbereichs und (c) das SO~/Al,O~-Verhaltnis imZement. Es gibt keine Hinweise, dafi nicht warmebehandelter Beton fur dieses Phanomen anfallig ist. Weitere Forschungund Verbesserungen des Duggan-Tests konnten zur Entwicklung eines verwendbaren Standardtests fuhren, der dielangfristige dimensional Stabilitat und Haltbarkeit von Beton bewertet.
REFERENZ: Day, R.L., The Eflecf of Secondary EttringiteFormation on the Durability of Concrete: A Literature Analysis, Researchand Development Bulletin RD108T, Portland Cement Association [Die Auswirkungen sekundarer Ettringitbildung auf dieHaItbarkeitvonBeton: Eine Analyse der Literatur,Forschungs-und EntwicklungsbulletinRDlO8T, Portlandzementverband],Skokie, Illinois, U.S.A., 1992.
PCA R&D Serial No. 1929
This publication is intended SOLELY for use by PROFESSIONALPERSONNELwho are competent to evaluate the significance andIimifations of the information provided harein, and who will accepttotal responsibility for the application of this information. ThePortland Cement Association DISCLAIMS any and allRESPONSIBILITY and LIABILIW for the accuracy of and theapplication of the information contained in this publication to thefull extent parmitted by law.
Portland Cement Association 5420 Old Orchard Road,SMie, Wzoia 60077-1083 (708) 966-6200,FAX (708) 966-9781
mII An organization of cement manufacturers to improveand extend the uses of portland cement and concretethrough market developmen~ engineerin~ research,education, and public affairswork.
Printed in U.S.A. RD108,O1T
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