High-magnesia Portland Cements

U DC 666.942.7.017

High-magnesia Portland cements: studies on cements prepared with reagent-grade chemicals* S. S. Rehsi, MSc(Tech), PhD and A. J. Majumdar, PhD

MINISTRY OF PUBLIC BUILDING AND WORKS: BUILDING RESEARCH STATION

SYNOPSIS Several Portland cement clinkers containing up to

15 % MgO have been prepared with reagent-grade chem-icals, and the properties of cements made from them have been investigated. In particular, the effect of varia-tions in composition, burning temperature and cooling conditions of the clinkers upon the autoclave expansion and strength of the corresponding cements has been examined in detail. It is not possible by controlling these factors alone to produce 'sound' (i.e. volume-stable) Portland cements having MgO percentages much above the present limits recommended by the Standards in different countries. The addition of a suitable reactive silicate material in finely divided form, e.g. pulverized-fuel ash, effectively stabilizes high-magnesia cements and these mixtures pass the ASTM test for soundness of Portland cements. Data on the autoclave expansion and mechanical strength of these mixtures of high-magnesia cement and pfa are presented.

Introduction Portland cements containing up to 5 % MgO remain

'sound', i.e. volume-stable, in use. With more than 5 % MgO, a cement exhibits instability, commonly known as 'unsoundness'. The reason for such behaviour is that, except for a small amount which may enter solu-tion in the silicate liquid and so become part of the glass, in a normal, slowly cooled, cement clinker most of the MgO remains as free, hard-burned, crystalline MgO (the mineral periclase). This crystalline MgO hydrates slowly in water at normal temperatures. (1) In cement paste, the hydration reaction may be retarded even further because the MgO grains are usually em-bedded in the matrix. The hydration of MgO to

*Crown copyright.

Mg(OH)2 is accompanied by a volume increase of 118 %(2), and the delayed expansion on hydration of free MgO may disrupt the mortar or concrete in a structure long after the cement has hardened. The damaging effect of delayed expansion due to MgO was first observed in 1884 on a number of bridges and viaducts in France and on the Town Hall of Cassel in Germany(3). It was the investigations which followed that led to the introduction of limits for the total MgO content in cement specifications all over the world. The British Standard (4) restricts it to a maximum of 4 %, whereas the ASTM recommends (5) a level of not more than 5 % by weight of MgO in Portland cement.

The restriction of a maximum level of MgO in cement to 5 % means that only limestones of very high quality can be used in cement manufacture. For coun-tries with unlimited reserves of such limestones, this is not a serious handicap. On the other hand, there are a large number of countries (India, for instance) where the reserve of low-magnesia limestone is limited. Efforts are, therefore, being made in some of these countries(6-10) to devise suitable means by which sound and durable cements with MgO levels significantly higher than 5 % may be manufactured. In this respect, reports of research carried out by Rosa(11-13) in Czechoslovakia have been very encouraging. His re-sults demonstrate that cements containing as much as 15 % MgO can be made sound (i.e. volume-stable) by mixing with them sufficient quantities of a finely divided, reactive silicate material such as pulverized-fuel ash (pfa). These mixtures were shown to pass the autoclave expansion test, both after a short hydration period and after having been stored under water for ten years. Bridges and office buildings are reported to have been built in Bratislava with this type of cement.

An investigation was undertaken at the Building Research Station two years ago which aimed at exam-ining the possibility of producing Portland cement

67

Magazine o.fConcrete Research: Vol. 21, No. 67 : June 1969

from Indian limestones containing large percentages of MgC03. In the exploratory phase of this work, it was necessary to study clinkers of widely differing compositions and to permit variations in the condi-tions under which they were produced. Such clinkers were produced by burning reagent-grade chemicals in large enoug"t quantities for subsequent measurements to be made of the properties of the cements. Experi-mental results obtained with these clinkers and ce-ments made in the laboratory are presented in this paper.

Experimental PREPARATION AND TESTING OF SYNTHETIC CLINKERS

Compositions of synthetic clinkers Compositions of ordinary Portland cements are such

that, if one ignores the minor constituents of the clinkers, they can be plotted on the phase diagram for the CaO-Alz03-SiOz system (14). This particular dia-gram becomes less important as the percentage of MgO increases in the clinker because the delineation of the phase boundaries changes quite sharply. Sandra-Deudon and Cavalier(lS) have recently published phase diagrams of the CaO-Alz03-SiOz-MgO system at 5, 10 and 15 % MgO levels which confirm this belief. How-ever, it is well known that ordinary Portland cement compositions lie close to the ternary CaO-3CaO.SiO[ 2CaO.SiOz invariant point, and in the persent investi-gation the base compositions of the clinkers at the 75, 10 and 15 % MgO levels were selected with this fact in mind. Other considerations which governed the selection of clinker compositions were: (I) the range and proportions of 3CaO.SiOz(C3S),

2CaO.SiOz(CzS), 3CaO.AlzOiC3A) and 4CaO. Alz0 3.FePiC4AF) normally present in ordinary Portland cement;

(2) specified limits on lime saturation factor and AIP3/FeP3 ratio;

(3) the compressive strength of the cement so ob-tained.

The compositions of some of the clinkers prepared in this study are given in Table 1. It will be noted that, at each MgO level, several compositions were syn-thesized. They differed considerably in their Alz03/ FeZ0 3 ratios. Other variations included burning tem-perature and the method of quenching. It has been claimed from time to time by various investigators that, if clinkers containing large amounts of MgO are burned at temperatures lower than clinkering tempera-tures used industrially, cements made from them do not show excessive volume expansion. Budnikov(16) recently supported this view. To confirm these obser-vations, a few cements were prepared from clinkers of

68

a particular composition burnt at different tempera-tures in the range 1300 to 1500C. The composition of the clinker was deliberately chosen to have a high C4AF content in order to reduce the clinkering tem-perature. Various properties of these cements were measured.

The synthetic clinkers were prepared from reagent-grade chemicals CaC03 (999 %); crushed quartz, SiOz (9995 %); AIP3 (9958 %); Fep3 (Mathey 'Specpure') and MgO obtained by decarbonating MgC03 (999 %) at 1300C. The powdered materials passing a 200 mesh BS sieve were accurately weighed in required propor-tions, mixed dry in a shaker and then decarbonated in platinum boats at 1000C. The decarbonated mix was passed through 100 mesh sieve, mixed in the shaker and then pelletized to form cylindrical pellets of about 25 x 25 mm under pressure.

The pellets were burnt in a platinum boat at the desired temperature for 2 h in a muffle furnace wound with 08 mm diameter Pt. 80 % Rh. 20 % wire, fitted with a motorized variac controller and a Kent multi-point temperature recorder. The control of tempera-ture was within 1 Co. One pellet was burnt at a time. After burning, the pellet was cooled, either by leaving it out in the laboratory atmosphere (air-cooled) or by submerging it under water (water quenched).

A batch of 20 g clinker was made in one run.

Analysis of cement clinkers The free CaO in the clinker was determined by the

Lerch and Bogue method(17) as specified in the ASTM Designation C114-63(18). Uncombined MgO in clin-kers was estimated by quantitative X-ray diffrac-tometry, following a procedure described by us recently(19) .

Phases present in the cement clinkers were identified by X-ray powder diffraction. A focusing camera of the Guinier-Nonius type, having a diameter of 114 mm, and Ni-filtered CuKoc radiation were used for record-ing the photographs. Typical exposure time varied from Ii to 2 h. The powder photographs were com-pared with similar photographs obtained with 'stan-dard' specimens of substances expected in these samples. When such a standard specimen was not available in the pure form, X-ray data published by Taylor(ZO) and those appearing in the ASTM powder data file(Zl) were consulted.

Quantitative determination of the major phases in the cement clinkers was carried out by microscopic point-counting on clinker specimens following the procedure described by Midgley and Dharmadhikari (22). About 1000 to 1200 counts were made on each section. The counting method gives the percentages by volume; these were converted to percentage weight by multi-plying by the densities of the phases identified. The densities used were 313, 328 and 340 g/cm3 for C3S, CzS and the interstitial phases respectively.

0"-\D

TAB LEI: Compositions of high-magnesia cement clinkers.

Clinker Ref. No.

I

CaO

Oxide composition (% by weight)

Si02 I AlP3 I Fe,03 MgO

Lime saturation

factor

Al,03 Fe20 3

Burning tempera-

ture

(CC)

Conditions I Calculated compound comp:Jsition of (% by weight)

cooling clinker*

C3S C,S C3A I C.AF Free CaO

Mineralogical analysis (% by weight)

Free MgO C3S C,S

Inter-stitial

320 5212 2505 19'57 I I I ' I I 1450 a-c 44-46 2898 006 1 61268 21807 4824 4601 750 I 0'8.78 1048 1500 w-q 44.21 29.17 500 1400 0.12 l(a) I . I ,

I, ---.. ~ 1 a-c 5500 2000 000 7-10 1 57'55 I 24'37 1098 260 53-93 2091 22'44

2 I 1450 w-q 5444 2044 012 665 5984 2123 12.16 2(a) 61920 21-450 4660 1970 I 1000 0'9. 16 ~'370 a-c 55.00 20'00 9'00 600 0'00 5-45 5980 23'74 11.01 ;~~~ , 1500 w-q 53-95 20'81 024 515 61-41 18-43 1477

1-- : I~~ - I a-c 5500 2000 000 623 5521 2603 112'53 4 I I I 1450 I w-q 5500 2000 I 000 615 5640 2320 1425 4(a) 61681 21-450 4-404 2-465 I 1000 0923 I 1786, I a-c 5500 20'00. 7.5087.50 000 535 5612 2571 1282 4(b) I +- 1500 w-q 5424 2058 I 018 515 5804 21'85 1478 4(c) 1450 I +-------- - --- - - a-c 5500 2000 I 000 530 5517 2192 1760 15, 'I I 1450 w-q 5473 20'21 006 495 5662 1923 19'14 15(a) 61279 II 21'45013'985 3286 1000 0916 I 1'213. II I a-c 54.73 20.21 500 I 1000 006 4'70 I 5576 : 2173 1775 15(b) I 1500 , w-q 54-49 2039 , ! 012 465 I 5683 II 1777 2063 15(c) I ---+ Itt- I i I . I .----+------t-- ,- I -- -- ,a-c 5300 1700. 000 I 970 '50-43 2395 1592 3 'I I I I 1450 I w-q 52-48 17-40 I I I 0'12 955 5161 21-45 1727 3(a) 58011 19879 4150 2960 15'00 0927 1-402 I I a-c 52'73 17.21 1 6'00 900 006 9'20 5123 2334 16.17 3(b) ; I 1500 w-q 52.48 INO I I 012 905 5310 1979 1794 3 (c) r- -=T' -- 1:::1- a-c 52-47 17-41- r-0-'1-2 1020 5334 24-45 1189

16 , I 1450 I w-q 5223 I 1759 I ' I 018 955 5458 2215 13-54 16(a) 1 58 '252 19879 4-404 2-465 1500 0933 1786 I a-c 52-47 I INI 750 I 750 i 012 970 5475 23-89 1154 16(b) I I 1500 I w-q II 52-47 1 17-41 I I 012 9-40 5523 2125 1400 16(c) I I I

! -I a~~ 115l2~43. IN5 ! I 012 1025 5501 23'77 1085 l7 i I 1450 w-q 52.68 17.26 ; 006 985 5601 22-45 11.63 17(a) I' 58-491 19879 4660 1970 1500 0935 2365 a-c 52.68 17.26 900 6'00~. 006 970 5554 23-44 11.26 l7(b) 1500 _ 52-43 IN5 012 960 5651 1999 \378 17(c) ------.l_ wq l

*a-c air-cooled. w-q water-quenched

VJ ~ ~ ~ :::-r;q' :::-

~ ~ :::: ~ is'

~ ~ is"' :::: t:).,

~ ~ r;;-

Magazine of Concrete Research: Vol. 21, No. 67 : June 1969

PREPARATION AND TESTING OF CEMENTS

Preparation of cements The clinker samples were first crushed in a per-

cussion mortar to pass a 100 mesh sieve. Four per cent by weight of gypsum (Analar grade CaS04.2Hp and passing a 100 mesh sieve) was then added and the material ground in a small steel ball mill of 50 g capacity, to a fineness in terms of specific surface of about 035 m2/g. Ordinary Portland cements have been generally found to be of this fineness.

Determination of fineness The fineness in terms of specific surface of cement

samples was determined by the air permeability method, the Lea and Nurse apparatus as specified in BS 12: 1958 being used(4).

Determination of compressive strength Compressive strengths of cement samples were

determined by using 13 mm cubes, according to the method developed by Parker(23) and currently used at the Building Research Station for testing strengths of experimental cements. The relationship of these strength results with those obtained with the 70 mm cube recommended in the BSI2:1958(4) has been established by Welch and Gutt(24) to be

Strength of 70 mm cube = 22 x strength of 13 mm cube + 8274 N/mm2

Determination of soundness The extent of the potential danger of delayed expan-

sion due to MgO was assessed by the performance of cements in the autoclave expansion test recommended by the ASTM(25). In this test, hydration of cement is substantially accelerated in the presence of super-heated steam (at 203 007 MN/m2) and it is con-sidered that all free MgO is hydrated. The expansion after the autoclave treatment should not exceed 08 % for sound cements. In the present study, this standard test was slightly modified and 13 mm cubes were used in place of bars measuring 25 x 25 x 285 mm with a 250 mm effective gauge length. The need for such a modification arose from the fact that experimental cements were produced in small quantities, whereas about 500 g of cement are needed to cast only one specimen of the size recommended by the ASTM standard procedure(25).

Details of the 13 mm cube method and a comparison of results obtained with this method and with standard bar specimens have been reported by us earlier(26).

The use of additives The experimental basis of the present ASTM restric-

tion on maximum permissible MgO contents in ce-

70

ments is to be found in the classic study by Gonnerman, Lerch and Whiteside(27) who demonstrated, with syn-thetic clinkers made in the laboratory, that cements containing more than 5 % MgO show large volume expansion when autoclaved. Results obtained in the present study confirm this view. (26) The addition of a suitable pozzolanic material, however, stabilizes these high-magnesia cements, as first pointed out by Rosa (11). In the present investigation, confirmatory evidence of this technologically important observation was sought by incorporating suitable quantities of a British pul-verized-fuel ash (pfa) in the cement mixes. The pfa fraction passing a 300 mesh BS sieve was used as the additive and the mixtures were tested for autoclave expansion and compressive strength.

The chemical analysis, mineralogical composition and the specific surface of the pfa fraction used in this study are given in Table 2.

EXAMINATION OF SET CEMENTS Autoclaved cements and cement-pfa mixtures were

examined by X-ray diffraction to identify the phases present.

Results In Table 1, the oxide compositions of the synthetic

clinkers and their potential phase compositions calcu-lated according to ASTM Designation C 150-65(5) are given as well as the experimental results of analysis carried out on them.

In Table 3 are listed the specific surfaces, autoclave expansions and compressive strengths of high-magnesia cements and of mixtures of these cements

TAB L E 2: Analysis of pfa. CHEMICAL ANALYSIS

Oxide Si02 AI20, Fe20,

Percentage by weight 498

CaO MgO NazO KzO TiOz

Acid-soluble SO, Loss on ignition

SPECIFIC SURFACE = 3642 cm2Jg MINERALOGICAL COMPOSITION

249 92

37 21 14 33 11 0'8 41

(I) Glassy phase, observed under microscope to be present as small spheres

(2) Crystalline phases, based on X-ray powder diffraction data: Quartz (Si02) Mullite (2Si02.3AI20,) Haematite (FeZ03) Magnetite (Fe304)

with variou proportion of pfa. It will be noted that the compre ive trength f the cement-pfa mixture are Ii ted only for tho mi ture which howed auto-clave expan ion below the ASTM pecified limit f O %. To facilitate a ready compari on, the compre -ive trength of an ordinary P rtland cement deter-

mined on 13 mm a well a 70 mm cube are given at the bottom of Table 3.

igure 1 and 2 are photograph which h w the relative ize of the aut laved pecimen of high-magne ia cement ha ing varying MgO content with and without addition of pfa. ome of the re ult on autoclave expan ion Ii ted in Table 3 are h wn in Figure 3 to 6.

The compo ition of the mi u ed to prepare clinker at lower temperature i gi en in Table 4. Thi Table a l 0 contain data on free CaO ana ly e and computed

Figurt J: Condilion 01 auloc/avtd J 3 mm Clllx> sptcimtns ol .. arious umtnl wilhoul pia. Tht umtnl ... trt prodllctd Irom high-magn ia umtnl clinktrs containing (from righl 10 11'11) O. I , 2. 3, 4. 5.5'5.6, 7'5. 10 and 15% MgO rtspl'cli\tly.

Figurt 2: COlldilions 01 aUlac/avl'd 13 mm Cllbt sptcimtn 01 aml'nts wilh and wilhoul pIa. Tht umtnt wt,t prodllud Irom high-maglll'Sia umtnl dinkt'r as. J4 a1ld 16 conlaining 10 and 15 % MgO

rtsPt'cli~tly. UpPt'r row: Ctmtnl prodllud Irom high-l1lQgnt ia alii nl clinktr o. 16 (f'O/1/ Itft (0 righ/) (i) ""ilhoul pia. (ii) wilh 25% pia, (iii) wilh 30 % pia, (iv) wilh 35 % pia and (v) wilh 40 % pia. Middlt: Ordinary Porllolld amenl (comlll rcial, MgO ('onltnl 104"/.) wilhoUI pIa. Lcwtr row: Ctmtnl produced Irom high-magnesia ctmtnl dinktr Q. /4 (from lell IQ righl) (I) wilhour pIa, (ii) 25 % pIa, (iii) wilh 30% pia, (il') wilh 35 % pia and (v) with 40% pia.

Studies of high-magnesia PorI/and cements

p tential pha e compo ition for the e clinker. The la t two column of thi Table gi e the figure for pecific urface of cement made from the e c1inke

and the autoclave e pan ion alue of them.

Di cu ion of result Data pre ented in able 3 c nfirm Rosa' b er a-

tion (ll) that P rtland cement ha ing more than 5 to 6 % MgO are not' ound' on their own (ee igure I) but their volume e pan ion can be brought under control by addition of uitable quantitie of finely divided pfa . Although present re utt apply to aut -cia e condition only, Ro a ha hown that the e re ull are valid at ambient temperature al 0 over a period of to 10 years. Furthermore. the cement-pfa mi tllre have adequate trength to pa the tandard pecificati n .

71


TAB L E 3: Autoclave expansions and compressive strengths of high-magnesia cements without pfa and containing different amounts of pfa by weight.

Clinker Content Specific surface Autoclave Compressive strength (N/mm') at of pfa of cement expansion Ref. No. (% by weight) (cm 2/g) (%) 3 days 7 days 28 days

0 3555 12640 - - -1 10 3564 s.c.* - - -

15 3568 3550 _. - -20 3572 0445 916 13-48 1906 0 3529 11-400 - - -

l(a) 10 3540 s.c. - - -15 3546 1180 - - -20 3552 0265 951 14'18 19'77 0 3533 17090 - - -

20 3555 s.c. - - -2 25 3560 s.c. - - -

30 3566 0'750 1029 1542 2137 35 3571 0457 - - -

I

0 3498 15145 - - -20 3527 s.c. - - -

2(a) 25 3534 0518 - - -30 3541 0-410 1051 1544 2168 35 3548 0264 - - -0 3482 16'960 - - -

2(b) 25 3522 s.c. - - .-30 3530 0548 10'45 1404 2165 35 3537 0'426 - - -

0 3513 l3-780 j--------

- - -

2(c) 25 3545 0568 - - .-30 3552 0-421 10'37 1503 2264 35 3558 0286 - -

-

0 3513 16-465 - --

14 25 3545 s.c. - - -30 3552 0597 1034 1580 2033 35 3558 0-479 - - -

15300 .~--j--._----

0 3572 - - -14(a) 25 3589 0628 - - -

30 3593 0-406 1007 1393 22'17 35 3596 0313 -- - -0 3577 14050 - - -

14(b) 25 3593 1117 - - -30 3596 0-493 1076 1453 2099 35 3599 0392 - - -

--c--0 3491 l3-21O - - -

14(c) 25 3529 0558 - - -30 3536 0-403 1073 13-90 2176 35 3544 0'278 - .-

-

---~-.

0 3543 16032 - - -15 25 3568 4015 - - -

30 3573 0573 990 13-65 1867 35 3578 0'397 - - -

----- --c----------r------------0 3503 15112 - - -

15(a) 25 3538 0'538 - --

30 3545 0379 1015 1429 2162 35 3552 0284 - -

-

0 3501 13990 - --

15(b) 25 3536 1102 - --

30 3543 0-476 1009 13-82 1980 35 3550 0374 - - -

. - ---------

0 3537 l3-240 - --

15(e) 25 3563 0583 - - -30 3568 0332 10'15 13-71 1958 35 3574 0277 - - -

* s.c. = specimen cracked

72

Studies of high-magnesia Portland cements

I ~-

Clinker Content Specific surface Autoclave Compressive strength (N/mm') at Ref. No. ofpfa of cement expansion 3 days 7 days 28 days (~~ by weight) (cm'/g) ( '~~)

0 3470 21395 - - -25 3513 s.c. - - -

3 30 3522 0787 - - -35 3530 0-497 932 1186 1829 40 3539 0388 - - ---~

0 3515 20080 - - -25 3547 s.c. - - -

3(a) 30 3553 0633 - - -35 3559 0-411 929 1255 1848 40 3566 0314 - - -

--1--------0 3553 20165 - - -

25 3575 s.c. - - -3(b) 30 3580 0645 - -

35 3584 0-417 907 1316 1856 40 3589 0329 - - -

- -~ ~---

0 3587 19925 -- - -25 3601 s.c. - - -

3(c) 30 3603 0631 - - -35 3606 0-408 921 13-13 1806 40 3609 0323 - - -

----

-~- ----~--

0 3551 22375 - - -25 3574 s.c. - - -

16 30 3578 1044 - - -35 3583 0695 924 1175 1806 40 3587 0-491 - - -

---~ -----

0 3594 20590 - - -25 3606 s.c. - - -

16(a) 30 3608 0980 - - -35 3610 0540 9-43 1172 1671 40 3613 0-425 - - -

--- "--------

0 3542 20750 - - -25 3567 s.c. - - -

16(b) 30 3572 0995 - - -35 3577 0612 935 1175 1732 40 3582 0-423 - - -

----[----- -~---0 3508 20175 - - -

25 3541 s.c. - - -16(c) 30 3548 0761 - - -

35 3555 0508 957 1183 1724 40 3562 0-413 - - -

--

0 3483 22990 - - -25 3523 s.c. - - -

17 30 3531 1520 - - -35 3539 0786 830 1183 1591 40 3547 0610 - - -

3527 ------- ------- ~------ ------- - -----0 21820 - - -25 3556 s.c. - - -

17(a) 30 3561 1 265 - - -35 3567 0690 830 11-42 1624 40 3573 0525 - - -

--~.--- -- ------ - ~

0 3514 21525 - - -25 3546 s.c. - - -

17(b) 30 3552 1315 - - -35 3559 0740 8-41 1128 16-44 40 3565 0578 - - -

---- ------

----

- -

0 3539 21 170 - - -25 3565 s.c. -~ - -

17(c) 30 3570 0985 - - -35 3575 0628 833 1167 1619 40 3580 0-478 - - -

-----

ORDINARY PORTLAND CEMENT

13 mm cube I 00 3440 70 mm cube 3440 ~ _____ _L _____ ~ __

0-401 1210 13-76 ---]----20-:s2---0_-4_0_1 _~ __ 2_6_61 __ -L-. _4_0_-4_7__ __~~ ____ _

73

MaKazine of Concrete Research: Vol. 21, No. 67 : June 1969

~ air-cooled clinker

~ water-quenched clinker

Key to Figures 3 to 6

10 15145 1709

08 -

~ I

z Q

~ 06 z ~ X 05 w w > 04 ~ U 0 f-:>

02

@ 25 30

(a) C3A,C4AF = 15 (b) C3A!C,AF = 10 (c) C3A!C4AF = 0'5

1530 16465

35 25 30 35 PFA CONTENT-% by weight

15112 16032

25 30 35

Figure 3: Autoclave expansion of cement with and without different amounts of pfa. The cements were produced from high-magnesia cement clinker Nos. 2, 2(a), 14, 14(a), 15 and 15(a).

10 1378 1696 1321 1405

08 - - - --

~ I

z Q ~ 06 z Q. X 05 w w > :5 04 u 0 f-:>

02

@ 25 30

(a) C3A;C.AF = 15 (b) C3AC,AF = 10 (c) C3AiC.AF = 0'5

35 25 30 35 PFA CONTENT-% by weIght

1324 1399

25 30

Figure 4: Autoclave expansion of cements with and wiThout different amounts of pfa. The cements were produced from high-magnesia cement clinker Nos. 2(b), 2(c), 14(b), 14(c), 15(b) and 15(c).

@ 35

TAB L E 4: Composition of clinker and the autoclave expansions of the cements produced by burning it at temperatures from 1300 to 1500C.

Burning Conditions Free Calculated compound composition Specific Autoclave Composition temperature of CaO (% by weight) surface expansion

of the mix cooling of cement eC) clinker (%) C,S C,S C,A CAF (cm','g) (%)

1300 air-cooled 1020 12-45 4885 3618 s.d.* CaO = 61844 % water-quenched 924 1635 4591 3592 s.d.* SiO, = 20316% -------

air-cooled 480 34-43 32'27 3521 1300 AI,O, = 5-411% 1350 water-quenched 5-40 3198 3412 3606 1259

Fe,O .. = 4929% ... ---.-.~- ----------MgO = 7500% 1400 air-cooled 036 5250 1864 600 . 1500 3651 12-41

water-quenched 018 5323 1809 3585 1239 .. _ .. _--

----

Lime saturation 1450 air-cooled 012 53-47 1791 3579 1231

factor = 0929 water-quenched 012 53-47 1791 3511 1238 ---.--~---- --- --.~---1500 air-cooled 012 53-47 1791 3637 1232 AI,O,;,Fe,O, = 1098 water-quenched 006 5372 1772 3598 1227

* s. d. specimen disintegrated

74

16

1-4

12

~ I

z 10

2 ~ z "-

x 08 w w > --'

u 0 f- 06 :J

05 ------

04

02

30 35 40

(a) C3A/C.,AF = /5 (b) C3A/C,AF = /'0 (c) C3A/C,AF = 067

2059 22375

30 35 40 PFA CONTENT-% by weight

2008 21-395

@ 30 35 40

Figure 5: Autoclave expansion of cements with and without different amounts of pfa. The cements were produced from high-magnesia cement clinker Nos. 3, 3(a), 16(a) /7 and /7(a).

The amount ofpfa necessary to stabilize a particular cement depends, as is to be expected, upon the free MgO content of the cement. However, for each cement, there is a critical percentage of pfa below which the mixture does not have any rigidity (see Figure 2). Menzel(28) had pointed out a long time ago that there is a minimum in the relationship between strength and CaO/SiOz ratio for cement and silica mixtures. Present work suggests that a similar relationship may exist in the case of mixtures of high-magnesia cement and pfa also.

Results presented in Tables I, 3 and 4 show that it is not possible to produce sound Portland cements having more than 5 to 6 % MgO by altering the condi-tions of burning and cooling of the clinker or by chang-ing its composition. When clinkers were prepared at a temperature as low as l300C, it was noted that these 'dusted' extensively. For clinkers burnt at 1350C, the dusting was less vigorous and it was not observed to


16 21-17 24525

14

12

~ I 10 z

2 ~ z

~ x 08 w w > 5 u 0 f- 06 :J

05

04

02

30 35 40

(a) C3A/CAF = 1'5 (b) C3A/CAF = 1'0 (c) C3A/C,AF = 067

20175 20 75

30 35 40 PFA CONTENT-% by weight

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Figure 6: Autoclave expansion of cements with and without different amounts of pfa. The cements were produced ji-om high-magnesia cement clinker Nos. 3(b), 3(c), J6(b), /6(c). 17(b) and 17(e).

occur when clinkers were prepared at 1400C and above. As is well known, dusting of a clinker occurs due to inversion of fJ-C 2S to y-C2S. The latter modi-fication of C 2S does not possess any hydraulic proper-ties and is deliberately avoided in cements. Budnikov l161 surprisingly makes no mention of dusting of clinkers prepared by him at 1350C or below. Autoclave expal,1-sion results in Table 4, however, show all clinkers made from the same mix but at different temperatures in the range 1300 to 1500C to be unsound.

Examination of the data presented in Table I shows that variations in the AI20 J/Fe20 1 ratios in the clinker compositions as reflected in the variations in ClA/C 4A F ratios influence the amounts of uncombined MgO in these clinkers. With increase in Fe20 3 content in clin-ker composition. the amounts of uncombined MgO decrease. A rise in the burning temperature of clinkers from 1450C to 1500C also results in some decrease in the quantity of free MgO present in the clinker.

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Rapid cooling of the c1in ker has a similar effect. These observations confirm the belief(Z9) that the glassy phase in the clinker does incorporate a substantial portion of the MgO present in the mix composition. Conditions which favour the formation of this glass produce less uncombined MgO in the clinker. These clinkers were found relatively easier to grind.

Results presented in Tables 1 and 3 show quite clearly that there is a direct correspondence between the amount of uncombined MgO present in the clinker and the autoclave expansion of the cement made from it, variables such as burning temperature, cooling con-ditions and Alz0 3jFez0 3 affecting this.

All clinkers (listed in Table I) gave identical X-ray powder patterns in respect of the phases present. Inten-sity variations were noticed, however, in the case of a large number of powder lines. This is to be expected because the clinkers differed widely in their potential phase analyses. The principal phases identified were alite, (3-CzS, C3A and a ferrite phase belonging to the CzF-'CzA' solid solution series. These are the principal phases of an ordinary Portland cement c1inker(30). In addition, there is conclusive evidence of the presence of a large amount of uncombined MgO. This is also expected, because the total MgO in the clinkers studied was equal to or greater than 75 % and only a small portion of this enters the glassy phase (produced in the burning process) and other crystalline phases. There was no evidence to suggest that y-CzS was present. The amounts of MgO, FeZ0 3 and Alz0 3 as well as the glassy phase present in the clinker were presumably adequate for the stabilization of (3-CzS.

Photomicrographs taken in the course of this study indicate that water quenching of the clinker produces smaller periclase crystals than air cooling. Results of quantitative microscopic examination on high-mag-nesia cement clinkers, presented in Table I, indicate that the proportions of C 3S and the interstitial phase increase and that of the CzS phase decreases when a clinker is water-quenched instead of air-cooled. An increase in the Fe Z0 3 content in the clinker composi-tions reduces the percentages of C3S and CzS phases and increases that of the interstitial.

In photomicrographs of autoclaved samples of ce-ments which were stabilized by the addition of suitable quantities of pfa, periclase crystals were not positively identified. It must be pointed out, however, that micro-scopic examination of hydrated cement is rendered particularly difficult by the presence of extremely fine-grained matrix material of hydrate compounds.

All autoclaved high-magnesia cements gave an iden-tical X-ray pattern. The phases which could be identi-fied positively in the photographs were Ca(OH)z, Mg(OH)z, C3S-hydrate and anhydrous cement min-erals, mainly ferrite. The presence of small quantities of xonotlite (6CaO.6SiOz.HP), hillebrandite (2CaO. SiOz.HzO), MgO and the hydrogarnet phases cannot be ruled out. No evidence was seen, however, of the

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presence of II A tobt;rmorite (5CaO.6SiOz.5HzO) or 738 A serpentine (3MgO.2SiOz.2HP) in these photo-graphs.

When adequate quantities of pfa are added to these cements and the mixtures autoclaved, nearly all Ca(OH)z is used up and C3S.H gradually disappears and crystalline 11 A tobermorite makes its appearance. This is due to the fact that the CaOjSiOz ratio of the mix changes markedly with additions of pfa and appropriate hydrate phases are formed. Present know-ledge about the dependence of the stability of these phases upon the CaOjSiOz ratio of the mix under autoclave condition has been summarized by Taylor(3!). Present experimental results are in substantial agree-ment with Taylor's assessment. The amounts of Mg(OH)z also progressively decrease, partly owing to the dilution effect of adding pfa but also probably owing to the formation of hydrates containing mag-nesium ion in their crystal structures. Periclase is not completely hydrated to brucite in the autoclave treat-ment. Among other hydrate phases which are probably present in small quantities, xonotlite, hillebrandite, IX-CzS.H and the hydrogarnet phase may be men-tioned. A certain proportion of the cement and the pfa remain un hydrated even after the autoclave treat-ment. This explains the presence of the powder lines corresponding to the ferrite phase in the X-ray patterns of the autoclaved cement, and ferrite, quartz, iron oxide and mullite in the X-ray patterns of the auto-c1aved cement containing pfa.

Although no conclusive evidence of the formation of any magnesium silicate hydrate was seen in these photographs, a very diffuse band was seen at 152 A and sometimes at 245 A in a large number of photo-graphs. The most abundant calcium silicate hydrate formed was C3S.H in autoclaved high-magnesia ce-ment and II A tobermorite in autoclaved cement-pfa mixes. Products of long-term room temperature (I r 1C) hydration under distilled water in the case of one cement when analysed by X-rays show that even after three months, no Mg(OH)z or tobermorite was formed; only traces of CSH gel were seen. Hydration at room temperature, of course, leads to the formation of ettringite (3CaO.Alz0 3.3CaS04.32HP), which is not stable under autoclave conditions.

Conclusions It is fair to conclude from this work that high-

magnesia Portland cements containing as much as 15 % MgO can be stabilized by the addition of suitable quantities ofpfa. The cement-pfa mixtures which pass the ASTM autoclave expansion test also possess ade-quate strength to pass the standard recommendations for ordinary Portland cement. Although it has not

been possible in this work to collect data on volume stability of these mixtures at room temperature, Rosa's work shows the autoclave results to be valid at ambient temperatures also. Mixtures of high-magnesia cement and pfa can therefore be used as substitutes for ordinary Portland cement. Present work indicates that these mixtures could be used in precast products which are to be steam-cured subsequently.

ACKNOWLEDGEMENT The work forms a part of the training in high-tempera-

ture silicate chemistry and cement technology of one of us (SSR) on attachment to the Building Research Station under the Colombo Plan Scheme.

REFERENCES 1. BOGUE, R. H. The chemistry of portland cement. Second

edition. New York, Reinhold Publishing Corporation, 1955. p. 141.

2. BROWN, L. s. and SAWAYZE, M. A. Autoclave tests and the microscope. Rock Products. Vol. 41, No.6. June 1938. pp.65-69.

3. LEA, F. M. The chemistry of cement and concrete. Second edition. London, Edward Arnold (Publishers) Ltd, 1956. p.315.

4. BRITISH STANDARDS INSTITUTION. BSI2:1958. Portland ce-ment (ordinary and rapid-hardening). London. p. 40.

5. AMERICAN SOCIETY FOR TESTING AND MATERIALS. ASTM Designation: CI50-65. Standard specification for Portland cement. 1965 Book of ASTM Standards: Part 10. October 1965. pp. 98-102.

6. DOLEZSAl. K. High-magnesia Portland cement. Proceedings of the Seventh Conference on the Silicate Industry, Budapest, 1963. Budapest, Akademiai Kiad6, 1965. pp. 633-649.

7. BUDNIKOV, P. P. Physico-chemical properties, microstruc-ture and phase composition of high-magnesia cements. Proceedings of the Seventh Conference on the Silicate Industry, Budapest, 1963. Budapest, Akademiai Kiad6, 1965. pp. 651-659.

8. GILLE, F. Untersuchungen tiber das Magnesia-Treiben von Portland cement. (Investigations on expansion tendencies in Portland cement due to MgO.) Zement-Kalk-Gips. Vol. 5, No.5. May 1952. pp. 142-150.

9. GUYE, F. Ciments a haute teneur en magnesie stables a l'essai a l'autoclave. (High-magnesia cements stable under autoclave tests.) Revue des Materiaux de Construction et de Travaux Publics. No. 567. 1962. pp. 333-348. Translated from the French by V. McEachran. Garston, Building Research Station, May 1965. pp. 16. Library Communica-tion No. 1291.

10. BUDNIKOV, P. P. and VOROBE'V, KH. s. Properties of Portland cement with high MgO contents. Tsement. Vol. 26, No.1. 1960. pp. 14-21. (In Russian.) Chemical Abstracts. Vol. 54. 1960. p. 11430.

11. ROSA, J. Raumbestandige Zemente mit hohem MgO-Gehalt. (Sound cements with high MgO content.) Zement-Kalk-Gips. Vol. 18, No.9. September 1965. pp. 460-470.


12. ROSA, J. High-magnesia cements with steady volume changes and their hydration. Tsement. Vol. 31, No.6. November-December 1965. pp. 6--8. (In Russian.)

13. ROSA, J. The mechanism of the stabilisation of high-magnesia Portland cement. Proceedings of the Eighth Con-ference on the Silicate Industry, Budapest, 1966. Budapest, Akademiai Kiad6, 1966. pp. 263-273.

14. LEA, F. M. The chemistry of cement and concrete. Second edition. London, Edward Arnold (Publishers) Ltd, 1956. p.57.

15. CAVALIER, G. and SANDRER-DEUDON, M. Laitiers quater-naries CaO-MgO-Alz0 3-SiOz. (Quaternary system clinkers CaO-MgO-AlzOrSiOz.) Revue de Mhallurgie. Vol. 57, No. 12. December 1960. pp. 1143-1157.

16. BUDNIKOV, P. P. Physico-chemical properties, microstruc-ture and phase composition of high-magnesia cements. Proceedings of the Seventh Conference on the Silicate Industry, Budapest, 1963. Budapest, Akademiai Kiad6, 1965. pp. 651-659.

17. BOGUE, R. H. The chemistry of portland cement. Second edition. New York, Reinhold Publishing Corporation, 1955. p. 98.

18. AMERICAN SOCIETY FOR TESTING AND MATERIALS. ASTM Designation: CI14-63. Standard methods of chemical analysis of Portland cement. 1964 Book of ASTM Stan-dards: Part 9. January 1964. pp. 88-135.

19. REHSI, s. and MAJUMDAR, A. J. Quantitative determination of uncombined MgO in Portland cement clinker by X-ray diffractometry. Journal of Applied Chemistry. Vol. 18, No. 10. October 1968, pp. 297-300.

20. TAYLOR, H. F. w. The chemistry of cements. London, Academic Press, 1964. Vol. 2. pp. xi, 460.

21. AMERICAN SOCIETY FOR TESTING AND MATERIALS. Index to the X-ray powder data file. 1958. Special Technical Publica-tion No. 48-G.

22. MIDGLEY, H. G. and DHARMADHIKARI, P. v. The point count-ing microscopic method for the quantitutive determination of the silicate phases in Portland cement clinkers. Garston, Building Research Station. Current Papers, Research Series No. 13, pp. 1-7.

23. PARKER, T. w. Influence of the heat treatment of Portland cement clinker on the properties of cement. Journal of the Society of Chemical Industry. Transactions and Communi-cations. Vol. 58. June 1939. p. 203.

24. WELCH, J. H. and GUTT, w. The effect of minor components on the hydraulicity of the calcium silicates. Proceedings of the Fourth international Symposium on Chemistry of Cement, Washington, D.C., 1960. Washington D.C., National Bureau of Standards, 1962. NBS Monograph No. 43. Vol. 1. pp. 59-67.

25. AMERICAN SOCIETY FOR TESTING AND MATERIALS. ASTM Designation: CI51-63. Standard method of test for auto-clave expansion of Portland cement. 1964 Book of ASTM Standards: Part 9. January 1964. pp. 163-169.

26. REHSI, s. s. and MAJUMDAR, A. J. The use of small specimens for measuring autoclave expansion of cements. Magazine of Concrete: Research. Vol. 19. No. 61. December 1967. pp. 243-246.

27. GONNERMAN, H. F., LERCH, W. and WHITESIDE, T. M. Inl'esti-gations of the hydration expansion characteristics of Portland cements. Chicago, Portland Cement Association, June 1953. pp. 168. Research Department Bulletin 45.

28. MENZEL, c. A. Strength and volume changes of steam-curved portland cement mortar and concrete. Journal of the American Concrete Institute. Proceedings Vol. 31. Novem-ber-December 1934. pp. 125-148.

29. BOGUE, R. H. The chemistry of Portland cement. Second edition. New York, Reinhold Publishing Corporation, 1955. p. 361.

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30. MIDGLEY, H. G. The formation and phase composition of Portland cement clinker. The chemistry of cements edited by H. F. W. TAYLOR. London, Academic Press, 1964. Vol. 1. p.90.

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31. TAYLOR, H. F. w. The steam curing of Portland cement products. The chemistry of cements. London, Academic Press, 1964. Vol. 1. pp. 417-432.

Contributions discussing the above paper should be in the hands of the Editor not later than 31 December 1969.

High-magnesia Portland Cements

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