Mineral-based plasters can be divided up into plasters or renders for external use, and plasters for internal use.
Plasters and renders for external use obviously have to be weatherproof – to cope with moisture and temperature changes without cracking, and provide protection against driving rain. Cement and lime-based plasters meet all requirements well, less so synthetic resin and silicate dispersion plasters.
Plasters for indoor use ideally should be breathable, durable, and suitable for accepting a painted, papered, or tiled finish. Interior plasters are mostly based on gypsum, gypsum-lime, or lime-cement, although more recently clay-based plasters have been making a reasonable impact due to their excellent environmental credentials.
Plasters and renders consist of a number of ingredients:
Binding agents, which when mixed with water become adhesive and hold the mix together – they are responsible for strength and permeability. Binding agents include cement, lime, gypsum, and clay.
The structural filler makes up the bulk of the volume and is usually sand, although other aggregates can be used. When water is added the binding agents cause the filler particles to adhere to one another, creating a pliable material that spreads smoothly and bonds onto walls. As the water evaporates, plasters set, or cure, and the binding agents create a hard, protective finish.
Fibres add bulk to the plaster, as well as tensile reinforcement and improved elasticity – allowing it to be applied in a thicker coat. Traditionally horse hair was used, these days any number of materials might be used such as hemp, straw, plastic fibres or even steel fibres.
Additives improve plasticity, workability, durability, curing time, and/or provide colour. Traditionally additives such as pig’s blood, cornflower, or mustard were used for their properties as well as their colour – although resultant colour was more about which additive was available at the time.
Apart from its weatherproofing quality, a good plaster or render also has other advantages. Because it is applied wet, there is likely to be continuous coverage and this gives good airtightness. Plasters and renders also have an amount of thermal mass, depending on thickness and density, and are thus able to store or release heat helping to maintain a more steady internal temperature.
Moisture permeability and hygroscopicity are more or less the same thing, although generally both are used positively – cement is relatively impermeable and will even act as a vapour barrier at about 40mm thick, and so unsurprisingly is not hygroscopic. Clay is hygroscopic, enabling moisture to be absorbed and then let out again, but is also therefore quite permeable. Lime is unusual, its permeability is closer to cement than clay, whilst still being hygroscopic.
To read more about different types of plaster and render, click on one of the links in the table below:
Lime Render & Plaster
Clay Plaster & Render
Thin Coat Renders
Cement renders are used both internally and externally, particularly in applications where an extra-hard or water-resistant surface is required. Cement render is usually the cheaper alternative, which has meant its use has become wastefully ubiquitous.
Cement renders are much tougher than lime and more impermeable. However, they are also more inflexible, meaning that they cannot move or flex with the building and are much more liable to cracking.
As discussed elsewhere on this site, cement production relies on destructive quarrying and is energy intensive. It has the highest level of embodied energy of any plaster or render, and is the main industrial source for NOx and CO2 emissions. Cement cannot be reused, but only crushed and recycled as aggregate.
Lime renders and plasters are made from lime putty combined with sand and fibre. They are much less rigid and brittle than cement and so less liable to crack, even on buildings liable to flex. If hairline cracks do appear it is able to ‘self-heal’ and larger cracks can easily be filled with a limewash.
Lime plasters and renders are hygroscopic, so they will absorb moisture when humid and let it out again when dry. This is very useful for ‘breathing walls’, which maintain steady internal atmospheres and temperatures, and also improve internal air conditions by absorbing carbon dioxide.
Lime plaster/render is relatively sensitive compared with cement, and must be carefully stored and applied. It is slightly more difficult to work with than cement plaster, which is more forgiving. Lime plasters/renders are generally ‘sealed’ with limewash, which must be repeated every 5 years on exposed facades (25 years on protected facades).
Adding a little gypsum to the mix shortens the curing time of pure lime plaster – this is known as gauged plaster (for internal use only).
Lime plasters/renders can be reused and recycled.
Gypsum is very vulnerable to moisture, and so can only be used internally, and not in very moist/humid situations. Gypsum plaster expands as it cures, reducing cracking.
Gypsum plaster is made by removing 75% of the water from gypsum, resulting in Plaster of Paris, which is considered a Class A ‘unretarded plaster’. Plaster of Paris sets too quickly to be used as a wall plaster, and so has to have a retarder added, such as TriSodium Citrate (a common additive in foods) resulting in a Class B ‘retarded hemi-hydrate plaster’.
Class B plasters are further divided into Type A - Undercoat plaster (plaster with sand), and Type B: Final coat plaster (using finer powder).
The environmental issues are those as with plasterboard - the extraction of raw material and quarrying can destroy natural habitats and landscapes; production involves a lot of energy; and sulphate wastes such as plasterboard are not permitted in general landfill.
Recovered gypsum can be combined with fresh gypsum or FGD to produce new plasterboard.
The oldest form of plaster/render, clay is less vulnerable to moisture than gypsum, and can be used as an external render in protected conditions, but is ideally used as an internal plaster.
Traditionally the clay would have been sourced locally, even on site, but nowadays it is most common to buy a standardised product. Sourcing locally or on site is of course still possible, but experimentation and testing will be required as clay varies in particle size, colour, shrinkage, and strength.
Clay plaster is ‘hydrophilic’, i.e. it attracts water, and expands as it gets wet, thereby becoming more resistant to water. This makes it ideal for use with timber or straw construction. This also means that erosion is a characteristic of clay render, which must be protected from rain and splashback. Erosion can be arrested by the inclusion of fibres such as chopped straw, or several coats of limewash. Sometimes a top coat of lime render is applied, although this is not ideal due to the potential bonding issues between the two materials.
Clay, unlike cement and lime, does not chemically change during drying and so can be reactivated by water. Clay plasters and renders can therefore be wetted and re-worked indefinitely, are completely reusable, or can be simply dug back into the ground.
Clay is hygroscopic, meaning it is able to regulate internal atmospheres. Plasters are not as tough as some, but damage can be repaired easily.
Clay plaster has very low embodied energy, is non-toxic, and emission-free.
These are a more modern alternative to traditional cement external render. Silicone fibres are added to the cement, giving a high degree of water repellence whilst still allowing moisture to pass through the render and therefore letting the wall breathe. Proprietary renders often require a nylon reinforcing mesh.
As well as the environmental issues of cement, the silicone additive in proprietary renders is a low toxic substance, synthesised mainly from chlorosilanes and tetraethoxysilane - a bi-product of this process is hydrogen chloride, which is hazardous.
'Thin coat' render offers superior performance and durability compared to standard cementitious renders. It is a form of polymer render, typically containing silicone, quartz, and calcite reinforcing fillers. Thin coat render requires sealing or coating once cured – the system is applied in two coats, with the base coat incorporating a nylon or glass fibre mesh, and a finishing coat typically based on an acrylic, silicate or silicone formulation.
Thin coat render is highly flexible, and is particularly useful where crack resistant performance is critical – for example walls and facades incorporating materials of very different expansion/contraction rates.
If a glass fibre mesh is used, the complete system is relatively resource efficient, whereas nylon is an oil-based mesh and so embodied energy is increased where this is used. As above, thin coat renders contain a silicone additive, which has environmental impacts.
Lightweight plasters weigh up to 60% less than sand-based plasters because the sand filler is substituted for a lightweight aggregate, such as exfoliated vermiculite, and expanded perlite. These aggregates are mixed with Class B gypsum plasters for undercoat and finish coat applications. Lightweight plasters have improved fire resistance and better insulation performance.