Lime plaster is a type of

plaster Plaster is a building material used for the protective or decorative coating of walls and ceilings and for moulding and casting decorative elements. In English, "plaster" usually means a material used for the interiors of buildings, while "r ...

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 Fly ash, flue ash, coal ash, or pulverised fuel ash (in the UK) plurale tantum: coal combustion residuals (CCRs)is a coal combustion product that is composed of the particulates (fine particles of burned fuel) that are driven out of coal-fired ...

is preferred. Non-hydraulic lime plaster can also be made to set faster by adding

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and dr ...

. Lime production for use in plastering home-made cisterns (in making them impermeable) was especially important in countries where rain-fall was scarce in summer. This enabled them to collect the winter run-off of rain water and to have it stored for later use, whether for personal or agricultural needs.

Advantages

Lime plaster sets up to a solid mass that is durable yet relatively flexible. Hydraulic lime plaster is not as hard as cement plaster. Hydraulic limes and historic limes were graded as feeble, moderate and eminent. Modern hydraulic limes would be graded at 2, 3.5, or 5 newtons. Portland cement plaster on the other hand would typically be in the region of 25 to 35 newtons when cured; i.e. up to 10 times harder. Lime plaster is less affected by water and will not soften or dissolve like

drywall Drywall (also called plasterboard, dry lining, wallboard, sheet rock, gypsum board, buster board, custard board, and gypsum panel) is a panel made of calcium sulfate dihydrate (gypsum), with or without additives, typically extruded between thi ...

and earthen or gypsum plaster. Unlike gypsum or

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay part ...

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. This would otherwise possibly cause the deterioration of the underlying surface. It is usually not recommended to replace more than 20% of the lime content with

cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement m ...

when rendering the facade. Lime plaster is permeable and allows for the

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

and evaporation of

moisture Moisture is the presence of a liquid, especially water, often in trace amounts. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some commercial products. Moisture also refers to the amount of water vapo ...

. However, when properly worked with pozzolanic agents and animal fat, it becomes impermeable. The elevated pH of the lime in the plaster acts as a fungicide, preventing mold from growing in lime plaster.

Disadvantages

Non-hydraulic lime plaster sets slowly and is quite caustic while wet, with a pH of 12. Plasterers must take care to protect themselves or use mild acids as vinegar or lemon juice to neutralize chemical burn. When the plaster is dry, the pH falls to about 8.6. Non-hydraulic lime plaster requires moisture to set and has to be prevented from drying for several days. The number of qualified tradesmen capable of plastering with lime is in decline due to widespread adoption of drywall and gypsum veneer plaster.

Historical use in the arts

One of the earliest examples of lime plaster dates back to the end of the eighth millennium BC. Three statues were discovered in a buried pit at 'Ain Ghazal in Jordan that were sculpted with lime plaster over armatures of reeds and twine. They were made in the pre-pottery neolithic period, around 7200 BC. The fact that these sculptures have lasted so long is a testament to the durability of lime plaster.

Historical uses in building

* Lime plaster was a common multi-purpose material used throughout the PPNB Levant, Iran and Anatolia, including Jericho, 'Ain Ghazal, Çatalhöyük and Çayönü. It was used for internal walls, floors and internal platforms. At the archaeological site of 'Ain Ghazal in modern-day Jordan, occupied from 7200 BC to 5000 BC, lime plaster is believed to have been used as the main component of the large anthropomorphical figurines discovered there in the 1980s. * Qadad lime plaster is waterproof and used for interiors and exteriors * Some of the earliest known examples of lime used for building purposes are in ancient Egyptian buildings (primarily monuments). Some of these edifices are found in the chambers of the

pyramid A pyramid (from el, πυραμίς ') is a structure whose outer surfaces are triangular and converge to a single step at the top, making the shape roughly a pyramid in the geometric sense. The base of a pyramid can be trilateral, quadrila ...

s, and date to between the Ninth and Tenth Dynasties (~2000 BC). They are still hard and intact. * Archaeological digs carried out on the island of Malta have shown that in places like Tarxien and Hagar, lime stucco was also used as a binder to hold stone together as well as for decoration at sites dating back as far as 3000–2500 BC. * At el-Amarna, a large pavement on brick was discovered that dates back to 1400 BC. It was apparently the floor of part of the harem of King Amenhotep IV. * Ancient Chinese used Suk-wui (the Chinese word for slaked lime) in the construction of The Great Wall of China. * Ancient Romans used hydraulic lime (added volcanic ash, an activated aluminium silicate) to ensure hardening of plaster and concrete in cold or wet conditions. * The Aztec Empire and other Mesoamerican civilizations used lime plaster to pave streets in their cities. It was also used to coat the walls and floors of buildings. * This material was used in the San Luis Mission architecture.

References

External links

British Museum: Lime Plaster Statues

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{{DEFAULTSORT:Lime Plaster Building materials Plastering Pre-Pottery Neolithic B