Structural Use of Portland Clay-Pozzolana Cement

7/31/2019 Structural Use of Portland Clay-Pozzolana Cement

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K.A.Solomon-Ayeh

Structures, Building design and Planning Division

CSIR-BRRI

Kumasi

IntroductionAbout 90% of modern, masonry buildings in urban areas

of Ghana require the use of Ordinary Portland Cement(OPC)/Portland Lime Cement(PLC) for:

concrete ;

sandcrete + pavement blocks and bricks;

jointing;

plastering and screeding; tiling.

Increasing housing stock and other infrastructuredemands increase use of cement and otherbinding(cementitous) materials, however:

Use of OPC/PLC imply;

increased importation of clinker

increased mining of limestone (from Buipe,

Otekpelu, Nauli)

high cost of input raw materials

Solution

Use of Clay Pozzolana as additive toOPC/PLC

Advantages

Clay fairly abundant inalmost every district ofGhana.

Requires relatively cheapstart up capital.

In presence of lime has

cementitious properties,among others.

Engendering small

production units in

almost every district

Employment

Increased housing

Increased rate Increased purchasing

of construction

Properties desired of hardened

cement mortar and concrete High compressive strength

Durability

Economic

Portland Clay- pozzolana cement when

used in concrete should achieve these to make it worthwhile.

Chemical and Mechanical properties

of Portland clay-pozzolana cementProduction of OPC/PLC

Main components of Portland cement:

C3S Silicates, main cementitious compounds

C4AFhydration

SILICATES + WATER C-S-H gel + LIME

Beneficial and unbeneficial products

of hydration Beneficial product – C – S – H gel

Unbeneficial by-products:

(i) Free and combined lime

(ii) Magnesia

(iii) Calcium sulphate

(i), (ii), (iii) when react with water may expand andcause unsoundness of concrete.

(iii) by formation of ettringite

Clay pozzolana as binderClay pozzolana sequestrates lime to form acementitious material which, in addition to

OPC/PLC improves binding property of Portlandclay- pozzolana cement, among other usefulattributes.

CLAY POZZOLANA + FREE LIME

C – S – H gel

Level of cementation depends on:

- Mass of Free lime available to sequester;

- Type of pozzolana;

- Amount of pozzolana present.

Beneficial structural properties of concretesand rendering mortars made from Portland clay-

pozzolana cement.1. Strength

- Designed mixes to C12, C25, C40

- Part replacement of Portland lime cement byclay pozzolana at 0, 10, 20, 30, 40, 50% by wt.

- Complete immersion in water.

- Test of compressive strength at 7, 28, 120 days

Designed mix ratios, slumps and compressivestrengths are shown in Table 1.0 and Figs. 2,3 and4.

0 20 40 60 80 100 120 140

m p r e s s i v e s t r e n g t h ( M P a )

Age(days)

Fig. 2 - Compressive strengths for C12 concrete

0POZ:100PLC

10POZ:90PLC

20POZ:80PLC

30POZ:70PLC

40POZ:60PLC

50POZ:50PLC

0 20 40 60 80 100 120 140

m p r e s s i v e s t r e n g t h ( M P a )

Age(days)

Fig.3 - Compressive strengths for C25 concrete

0POZ:100PLC

10POZ:90PLC

20POZ:80PLC

30POZ:70PLC

40POZ:60PLC

50POZ:50PLC

0 20 40 60 80 100 120 140

C o m p r e s s i v e s t r e n g t h ( M P a )

Age(days)

Fig. 4 - Compressive strengths for C40 concrete

0POZ:100PLC

10POZ:90PLC

20POZ:80PLC

30POZ:70PLC

40POZ:60PLC

50POZ:50PLC

1.Compressive strengths for all % replacements are

lower than those without replacement at early

ages(less than 28 days); and, with the exception of

C12 mixes, tend to be higher than mixes without

replacement at higher ages.

2. The property in (1) tends to increase with increase

in concrete grade.3.Maximum compressive strengths are obtainable, for

normal and high strength concrete mixes, for

pozzolana replacements of between 10 to 20%.

Recommendations(i) Concretes

Concretes mixes designed to normal and high strength grades can made with Portland clay- pozzolana cement with an OPC/PLC replacement of between 10 – 20%.

Such concretes need to be cured with the best available means to obtain maximum results.

Clay- pozzolana cements are excellent when large concrete elements are to be cast, since gain of strength is low at early ages (because the cement- water reaction produces low heat ) and stresses due to rapid differential hardening will be minimal; reducing the likelihood of internal cracks.

(ii) Sandcrete blocks Sandcrete blocks must have a minimum wetcompressive strength of 1.4 MPa at 28-days. In fact,most sandcrete blocks do not achieve this, yet theyseem to perform adequately because they are usedfor cladding ( the load being carried bybeams/columns or when laid flat, as load-bearingmasonry units).

A maximum replacement of 30% of OPC/PLC in the sand/cement mortar mix should satisfy this requirement .

(iii) Jointing

In masonry construction, the desired result is to havea weak bedding and transverse joints and stronger

blocks/bricks.

cracks will occur within joints rather than

through blockwork. joints can be caulked out and repaired, or when

‘pointed’ can conveniently hide cracks.

mortar of OPC/PLC and sand is considered too

harsh and traditionally a more ‘friable’ or ‘plastic’mortar has been preferred, and ‘masonry cement’or OPC and lime mix has been used.

A 30% pozzolana replaced cement behaves like masonry cement

(iv) Tiling, Screeding and PlasteringMortar for tile laying, wall plastering and floor screedingmust be as ‘plastic’ as possible (similar to jointing mortar).

A maximum of 30% OPC/PLC replacement by clay pozzolana is recommended for both tile laying and wall plastering.

Tiles must be soaked in water for at least 2hrs prior to laying, to prevent the absorption of the mortar

moisture by the dry tile, which will lead to the spalling- off of tiles. For screeding, the percentage replacement should be

a maximum of 20%, to offer higher resistance to abrasion by pedestrians.

2. Durability Durability relates to how a structural element

appears and performs whilst in service. Poor

durability, whilst no imminently dangerous islikely to make users of a facility feeluncomfortable and unsafe and may prove costlyto maintain. It may eventually lead to a collapse

of the structure.

Durability may present itself in 4 ways.

(i) Cracking

Cracking in cement and concrete may be from:

Shrinkage cracks (when mortar is prevented by formworkor other surfaces from moving when it is setting and by

the heat evolved during hydration). Excessive use of water in batching.

Rapid loss of water necessary for hydration.

Use of fine aggregate with too much silt and clay content

(max. should be less than 4%).The use of Portland clay-pozzolana cement will reduce

the heat of hydration and resist the sulphates and

chlorides in silts and clays.

(ii) Sulphate and Chloride attack

Typical sources of sulphate and chloride solutions are:

- ground water of some clays;

- some sands;

-construction in saline environments (near sea or river);

- sub-soil construction;

-sewers, manholes, septic tanks and other harsh

environment; - construction in areas of high water table.

These waters contain sodium, calcium or magnesium sulphates. The

sulphates react with both Ca(OH)2 (lime) and the hydrated C3A toform gypsum and ettringite, respectively.

Sulphate attack is shown by a whitish appearance, starting at theedges and corners, followed by cracking and spalling of concrete.This appearance is because sulphate attack results in the formation

of gypsum and ettringite , both products occupying a greater volume than the compounds which they replace so that expansion of hardened concrete takes place.

Extent of sulphate attack depends on its concentrationand the permeability of the concrete (ease with whichsulphate can travel through the pore system).

If the concrete is very permeable:

water can percolate through its thickness andlime will be leached out.

evaporation at far face of concrete leaves behinddeposits of CaCO3 (formed by reaction of limewith CO2). This is known as efflorescence.

Attack by sea water

- Though sea water contains sulphates andchlorides, the presence of the latter ensures that

sea-water attack is not generally accompanied bythe expansion of concrete. The is gypsum andettringite are more soluble in a chloride solution thanin water, which means that they can be more easily

leached out by the sea water.- Expansion may take place as aresult of the

pressure exerted by the crystallization of salts in thepores of the concrete.

Crystalallization occurs above the high-water level,and it is by capillary action in the concrete. At thepoint of evaporation of water.

Prevention:

Use of Portland clay-pozzolana cement;

More importantly, by the use of concrete of low

permeability.

3. Economy

Structural Use of Portland Clay-Pozzolana Cement

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