In a fresh state, plaster must be workable and cohesive, in other words, it must be plastic and have good water retention.
In a hardened state, plaster must be strong enough to withstand local impact and abrasion, and it must be able to hold paint. It must also be free of cracking, well-bonded to the substrate, and have an acceptable surface texture and surface accuracy.
In South Africa, CEM I and CEM II A cements are used in plaster. Masonry cements may be used in accordance with the requirements in SANS 2001-EM1 and the NHBRC Home Building Manual. The NHBRC Home Building Manual does not permit the use of MC 22,5X or MC 12,5X in plaster work for residential construction unless a competent person has designed the mix.
Sand contributes the biggest volume to the plaster mix. In South Africa, natural sands (pit, river and dune sands) are habitually used for plaster mixes.
It is important that sand be free of organic matter, such as roots, twigs and humus. If the sand has lumps, it must be passed through a sieve.
Properties of sands for plastering:
• Grading: Grading influences the water requirement, workability and the water retentivity of the mix. Increasing the grading fraction requires more water, but it improves workability and water retentivity. The optimum grading fraction is therefore a compromise between these elements. Sand with good grading may be used with cement types CEM II and hydrated builder’s lime, or a plasticiser may be added to the mix.
• Maximum particle size: for conventional smooth plaster, all the sand should pass through a sieve with 2,36 mm holes. For coarsely textured decorative work, the corresponding sieve size is 4,75 mm.
• Clay content: plaster sand should only have a small proportion of plaster sand.
Warning: Gypsum-based plaster should never be mixed with a plaster made from portland cement. Gypsum is a sulphate compound which attacks portland cement paste, especially in damp conditions, causing swelling, softening and disintegration of the plaster.
Table 16.5: Mix proportions prescribed for normal plaster in SANS 2001-EM1:2007
|Type of Plaster||Common cement, kg||Limea, kg||Masonry cement
MC 22,5X or MC 12,5, kg
MC 12,5X, kg
|Maximum volume, litres||Number of standard wheelbarrows|
|External plaster||50||0 to 25||-||-||150||2,5|
|Internal plaster||50||0 to 25||-||-||200||3|
|a The addition of lime to common cements is optional. A maximum of 30 kg is permitted. Lime shall not be used as a replacement for a portion of the cement in the mix. Lime is not used with masonry cements.
Preparation of specific types of substrate
Monolithic concrete: Concrete is normally placed in situ but may be precast. Provide a rough surface for plastering and remove curing compound, if any, by mechanical means. Clean the surface using a water jet or a vacuum. Smooth off-shutter high-strength concrete surfaces will require a spatterdash coat.
Concrete masonry: The texture of the surface should be sufficiently rough. Clean the surface by brushing, water jetting or vacuuming it.
Burnt clay stock brickwork: The texture of the bricks should be sufficiently rough. Clean the surface by brushing, water jetting or vacuuming it. Assess whether the stock brick walls have suction by wetting the wall. If the suction is high, wet the wall at least one hour before plastering and leave it to dry in this time.
Burnt clay face-brickwork: Brick texture may be smooth, almost glazed or rough. Joints must have a recess of 10 mm deep. If the brickwork has been treated with a sealer or waterproofing agent, it must be mechanically removed. Apply a spatterdash coat if the brick surface is smooth.
Sundried, unburnt or poorly burnt soft clay brickwork: Protect the wall from rain or running water. Rake out the joints to about 10 mm deep. Brush down the wall to remove any loose material. Lightly dampen the wall and apply a spatterdash coat that incorporates a polymer emulsion to improve adhesion.
Batching: Batching sand by loose volume should be satisfactory. The batch should be small enough to be used up within two hours.
Mixing: Machine mixing is preferable, but hand mixing can be done on a smooth, clean concrete floor or steel sheet. Sand should be spread about 100 mm thick over the surface, and the cement spread uniformly over it. Mix sand and cement well before gradually adding water and continually mixing. Mix until the correct colour and consistency is reached.
Plaster thickness: The recommended thickness for the first undercoat is 10 - 15 mm. 5 – 10 mm is recommended for the second undercoat and the finish coat.
Conventional architectural applications of plaster for home building
Applying the plaster: Plastering should be protected from the sun and wind. The plaster should be used up within two hours of being mixed and should never be retempered by mixing in additional water. Plaster should be cut through to the substrate where different substrate materials meet, and the edges should be neatly finished off.
premix plasterThe general procedure for applying plaster is as follows:
• Apply screed strips to act as guides for the striker board before the wall is plastered.
• Using a rectangular plasterer’s trowel, push plaster onto the wall or ceiling, compacting the plaster to ensure full contact with the substrate.
• Once the plaster starts to stiffen, it should be struck off to a plane or curved surface using a light striker board.
• If plaster is to be applied in more than one coat, the undercoat(s) should be scored with parallel lines about 20 mm apart and 5 mm deep.
• For the final coat, use a wood float to remove ridges made by the striker board. Fill in any depressions and float flush with the surrounding plaster.
• If a very smooth texture is required, a steel trowel may be used on the surface.
Grinning: Grinning occurs when the positions of the mortar joints are visible through the plaster. It may be avoided by applying a spatterdash coat or an undercoat.
Crazing: Crazing is a network of closely spaced fine cracks caused by over-trowelling in a rich mix.
Cracking: Cracking of plaster is caused either by structural movement or by shrinkage of the plaster. Shrinkage occurring during the time between application and hardening is caused by excessive loss of water from the plaster due to using sand that is badly graded, not using lime or masonry cement when the sand lacks fine material, not eliminating excessive suction of the substrate, or exposure to direct sun or wind. Hardened plaster will shrink as it loses moisture. A well proportioned plaster will relieve stresses induced by shrinkage, by developing fine cracks which will not be apparent.
Lack of hardness: Soft and friable plaster results from using a mix with poor water retention properties, using a slow-hardening cement, plastering in the sun or wind, not dampening absorbent substrates, using re-tempered plaster, using a very lean mix, using sand containing an excessive proportion of fines, and using partially hydrated cement.
Debonding: Debonding of plaster may result from poor surface preparation, relative movement of the background, using over-rich mixes, and excessive plaster thickness.
Expansion: This includes swelling, softening, layer cracking and spalling of the plaster. It can be caused by including a gypsum plaster in the mix.