FEATURE
Material selection important for successful plastering
By John Roxburgh
Sand-cement plaster is used extensively in building work as a decorative or protective coating to concrete and masonry walls and concrete ceilings . But there are plenty of pitfalls on the road to successful plastering - a building aspect that continues to pose challenges , even to experienced contractors .
John Roxburgh , senior lecturer at the School of Concrete Technology of Cement and Concrete SA ( CCSA ), here provides some guidelines on the selection of materials for successful plastering :
Cements for plaster should comply with the requirements of SANS 50197-1 or SANS 50413-1 . CEM l and CEM ll A cements are used in plaster with good results . CEM ll B-V or W cements are recommended for plaster exposed to damp conditions during service ( for example , plastered plinths below damp proof course level and freestanding walls ) to reduce the risk of efflorescence . Cement plaster should be applied in accordance with the requirements of SANS 2001-EMI .
Cements with slow early strength development should be used only if protection of the plaster on the substrate ( against sun and wind resulting in evaporation of moisture from the surface ) will be adequate to minimise early cracking before the plaster has developed sufficient tensile strength .
Bags should be clearly marked with the strength grade ; cement type and a Letter of Authority ( LOA ) number issued by the National Regulator for Compulsory Standards and should provide guidance on mix proportions for plaster .
The choice of cement should be based on the properties of the sand to be used in the plaster .
Sand is by far the major constituent of plaster and has a significant influence on its performance and material cost . In South Africa , natural sands , such as pit , river , and dune sands , are almost invariably used . An important requirement is that sand should be free of organic matter such as roots , seeds , twigs , and humus .
If sand includes lumps that are not easily broken between the fingers , it is not ideal for use in plaster ; if such a sand is to be used , then the lumps should be removed by sieving .
‘ Karoo ’ sands , which contain a high proportion of disc-shaped dark-coloured particles , should not be used for plastering because they exhibit excessive swelling
John Roxburgh , senior lecturer at the School of Concrete Technology of Cement and Concrete SA ( CCSA ).
and shrinkage on wetting and drying , respectively . This causes excessive shrinkage and cracking in the hardened plaster .
Important physical properties of sands are :
• • Clay content ;
• • Grading ;
• • Maximum particle size ; and
• • Particle shape .
ALL IMAGES : CCSA
An important requirement is that sand should be free of organic matter such as roots , seeds , twigs , and humus .
Only a small proportion of clay can be tolerated in sand used in plaster . Clay normally causes a high-water requirement and high drying shrinkage .
Ideally , the sand should have a continuous grading , from dust up to the largest particles . The fractions passing the 0.150- mm and 0.075-mm sieves (‘ fines ’) are important because they significantly influence the water requirement , workability and the level of water retention of the mix .
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