Lime is known as a traditional material that has been used for many years. It is also a material that is being increasingly recommended by conservationists and architects because it does not have the potential to damage traditionally constructed buildings greatly in comparison to hard cement mortars and gypsum plasters which are well known to be inappropriate for a traditionally constructed building.
The reason why traditional lime plasters are more suitable for protecting traditional buildings than modern rendering materials lies in it being more porous and having the ability to absorb moisture from the air. When traditional lime render is used with a breathable paint, this can help reduce the possibility of condensation building up within the structure of the property, this condensation being the cause of damage to properties and traditional buildings.
To apply traditional lime render to a property, first its base component lime putty is to be made. Limestone (calcium carbonate, CaCO3) is burnt in a kiln to produce quicklime (calcium oxide, CaO). The quicklime (calcium oxide, CaO) is then mixed with water to produce a boiling liquid (calcium hydroxide, Ca(OH)2) which is passed through a sieve and then left to mature in a pit or tanks for a number of months. This process is called slaking and the resultant lime putty (calcium hydroxide, Ca(OH)2) ends up the consistency of cream cheese.
The mature lime putty (calcium hydroxide, Ca(OH)2) is then mixed with a sand to make a lime mortar. The type of sand used depends on the use of the lime putty i.e. coarse sands are typically used for buildings and fine sands are used for plastering. Animal hair is added into the mix to increase the strength of the lime putty. The mixed lime mortar is then left to mature for a period of one to two weeks before it is used to prevent it from cracking or shrinking.
Another use of lime was as a decorative finish to buildings through the application of limewash. Limewash is made by mixing slaked lime in water (calcium hydroxide, Ca(OH)2) which sets slowly by absorbing carbon dioxide from the air to produce crystals of calcite (calcium carbonate, CaCO3).
Limewash maintains the building allowing vapour to pass through/ permeate through the decorative coatings. If a building is unable to release water molecules, water becomes trapped in the external walls, encourages mould to grow and this results in the underlying material such as wood to rot. The trapped water vapour condenses and this can raise heating bills because damp walls absorb a lot of heat and prevent the warm air from circulating evenly around the property.
On the other hand, hard cement renders and many modern masonry paints prevent moisture from passing through the property to be evaporated. This may result in dampness, cold walls, condensation, flaking paint, rotten skirting boards, joists and other timber fittings, increased heating bills etc.