Several additives are available for use in the manufacture of sand-cement mixes for plaster, but before specifying their inclusion, one needs to understand their role and consider several factors. Bryan Perrie, managing director of The Concrete Institute, gives some tips:
Hydrated builder’s lime
South African limes must comply with SANS 523 (type A2P). They are non‐hydraulic and cannot be used to replace cement in the mix. The lime tends to increase the water requirement of the plaster and therefore reduces the compressive strength slightly. However, the improved workability and water retention result in better bonding and lower permeability.
Depending on the fine content of the sand, up to one bag (25kg or 40 litres measured loose) may be used per bag of common cement, but builder’s lime should never be used with masonry cement.
Used to improve workability and water retention, these admixtures should comply with SANS 50934 and their dosage must be controlled to avoid adverse consequences. The most commonly used admixtures are so‐called “mortar plasticisers”, which are in fact air‐entraining agents, which should never be used with masonry cements.
Bonding aids (or bonding liquids) are often used in plaster mixes and spatterdash applications for bonding plasters to substrates. They are by no means a substitute for good surface preparation and workmanship, but they do impart good workability to the mix, although overdosing can lead to low strength.
Used to add colour, pigments should comply with BS EN 12878 or an equivalent quality standard. The pigments must be alkali‐tolerant and if exposed to sunlight, ultraviolet‐resistant and should not increase the water requirement of the mix unduly.
For any given combination of cement and sand, the dosage depends on the pigment colour and the desired final colour. Adding more than the saturation dosage not only wastes pigment, but can also weaken the mix by increasing the water requirement.
For uniform colour, the pigment should be thoroughly mixed with the cement prior to mixing with the sand. Variegated colour effects are also possible by partially mixing in one or more additional pigments. Note that both the sand and cement colour can have a distinct effect on the final colour, as can the amount of mixing water.
For white or pigmented plasters, the sand must be free of organic matter as these can cause localised retardation, staining and pop‐outs. Caution should also be exercised when using carbon black as a pigment, as it tends to fade with time.
It is impossible to patch pigmented plaster without the patch being visible, so all plumbing, electrical and other fixtures must be completely installed prior to plastering.
Water for plaster
Water used in the plaster mix should be fit for drinking.
Gypsum‐based plaster is a sulphate compound that attacks Portland cement paste, causing swelling, softening and disintegration, especially in damp conditions, and these two should therefore never be mixed.
The Concrete Institute
Tel: 011 315 0300
Caption main image:
The dosage of pigmentation added to plaster will determine the final colour of the surfaces.
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