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Lime plaster is a type of plaster composed of sand, water, and lime, usually non-hydraulic hydrated lime (also known as slaked lime, high calcium lime or air lime). Ancient lime plaster often contained horse hair for reinforcement and pozzolan additives to reduce the working time.
Traditional non-hydraulic hydrated lime only sets through carbonatation when the plaster is kept moist and access of CO2 from the air is possible. It will not set when submersed in water. When a very thick layer or several layers are applied the lime can remain soft for weeks.
The curing time of lime plaster can be shortened by using (natural) hydraulic lime or adding pozzolan additives, transforming it into artificially hydraulic lime. In ancient times, Roman lime plaster incorporated pozzolanic volcanic ash; in modern times, fly ash is preferred. Non-hydraulic lime plaster can also be made to set faster by adding gypsum.
Lime plaster sets up to a solid mass that is durable yet relatively flexible. Hydraulic lime plaster can be almost as hard as cement plaster. When cured lime plaster is unaffected by water and will not soften or dissolve like drywall and earthen or gypsum plaster. Unlike gypsum or clay plaster, lime plaster is sufficiently durable and resistant to the elements to be used for exterior plastering.
Compared to cement plaster, plaster made from hydrated lime is less brittle and less prone to cracking, requiring no expansion joints. It will not detach from the wall when subjected to shear stress due to expansion inflicted by solar radiation and moisture. Unlike cement plaster it will shield softer materials from shear stresses otherwise possibly causing the deterioration of the underlaying surface. It is usually not recommended to replace more than 20% of the lime content with cement when rendering the facade.
Lime plaster is permeable and allows for the diffusion and evaporation of moisture. The elevated pH of the lime in the plaster will act as a fungicide; mold will not grow in lime plaster.
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