The correct selection of materials is crucial to achieve successful plastering, says Bryan Perrie, MD of The Concrete Institute.

He has provided some guidelines on how to choose the right materials:

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 and masonry cements may be used in accordance with the requirements in SANS 2001-EM1 and/or the NHBRC Home Building Manual.

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.

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, i.e. 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.

Crusher sands are not generally suitable for use in plaster due to their angular particle shape. “Karoo” sands, which contain a high proportion of disc-shaped dark-coloured particles, should not be used for plastering because they exhibit excessive swelling and shrinkage on wetting and drying respectively. This causes excessive shrinkage
cracking in the hardened plaster.

Important physical properties of sands are:

• Clay content;
• Grading;
• Maximum particle size; and
• Particle shape.

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.

Increasing these fractions results in increased water requirement (with consequent lower strength and higher shrinkage), but improved workability and water retention. The optimum fines content is therefore a compromise between these properties.
For plasters, sand lacking in fines may be used in conjunction with hydrated builder’s lime, mortar plasticiser, or masonry cement, or it may be blended with fine filler sand.

Ideally, for good workability, the particle shape should be nicely rounded with a smooth particle surface texture.

The water used in the plaster mix should be fit for drinking.

In conclusion, Perrie says that provided sufficient attention is paid to the selection of materials, mix proportions, preparation of substrate surfaces and the application of the plaster, the results should be serviceable and aesthetically acceptable.

For more information on plastering, refer to the publications, Successful Plastering, and Common Defects in Plasters, both available from the website www.theconcreteinstitute.org.za or phone the Institute on 011 315 0300 or email info@theconcreteinstitute.org.za