User:Rubblerat/Lime plaster

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Lime plaster is a type of plaster that uses lime as a binder, most often slaked (or hydrated) lime. Lime plaster has been used in construction and art for thousands of years.

Composition

The most common binder used for lime plaster is slaked lime putty, which is pure calcium hydroxide mixed with enough water to produce a stiff paste. If a hydraulic set is required natural or artificial hydraulic lime (almost always initialised as NHL and AHL respectively) can be used. NHL is made from limestone that naturally contains impurities, which allow it to set hydraulically. AHL has impurities added during its production to achieve the same effect. Alternatively, pozzolans such as fly ash can be added to slaked lime to give it a hydraulic set.

Slaked lime putty shrinks significantly as it sets which makes it unsuitable for plastering on its own, so it is mixed with stone dust, sand or other aggregates to increase its durability and coverage. The finish of the plaster is determined by the size and type of the aggregate used; a coarser aggregate, such as <4 mm (0.16 in) sand, will produce a rougher finish than finer <2 mm (0.079 in) sand. In the present day lime plaster is normally applied in three layers, the scratch coat (also known as the base or render coat), the floating coat, and the final finishing or setting coat. The scratch and floating coats are normally mixed at a ratio of about 2:1 coarse aggregate to lime, while the finishing coat is 1:1 with a finer aggregate.

When more strength is required, animal hair, natural fibres or fibreglass strands can be added to the plaster when it is being mixed to create hair plaster. The fibres reinforce the structure of the plaster, making it less likely to crack.

Properties

Once it has dried, lime plaster is more flexible than gypsum or cement based plasters. This means it is less likely to crack because of thermal expansion or movement in the surface it is applied to. It also allows water vapour to pass through, which stops water building up in surrounding wood, masonry or brickwork. Lime plaster is strongly alkaline when wet, and can cause chemical burns if it is left in contact with the skin. Its alkalinity goes down as it dries, with a final pH of about 8.5. CHECK THIS.

Lime plaster can take months to fully harden, but it normally sets firm in about two weeks. The setting time depends on what kind of lime has been used, the temperature and humidity of the air around it, and whether it is exposed to direct sunlight or wind. The plaster must be kept damp as it sets, to stop cracks forming and to help the process of carbonation. Lime plaster cannot Fresh or setting lime plaster can be damaged by frost, so it should not be used if the temperature is expected to fall below 5 °C (41 °F) in the two weeks after it is applied. Low temperatures also slow down the carbonation process, so it will take longer to set.

Use

Lime plaster and render is often used in the repair and maintenance of older buildings that were built with lime based materials, because they can be damaged if modern cement plasters and renders are used on them. The lime plaster allows water absorbed by the structure to evaporate out whereas modern impervious coatings trap water, leading to damp and decay.