Lime comes from limestone, it is quarried, crushed and heated in lime kilns to drive off carbon dioxide. This process converts the limestone in a highly alkaline (caustic) material called quick lime.
When quick lime is slaked with water a lime putty is formed. High calcium lime putty will keep indefinitely in a sealed container with a shallow covering of water and will mature and improve with age. This type of lime putty can be used as a binder in sand lime mortars which will only set by absorbing carbon dioxide and so reversing the process that occurred in the lime kilns.
Some limestones come from a quarry containing impurities which gives the lime the ability to set underwater. These limes are knows as naturally hydraulic limes and are useful where mortars need to set more quickly such as in exposed areas or in damp conditions where high calcium limes would just not set.
Impurities can also be added to high calcium lime putty mortars to give the mortar hydraulic characteristics. These added impurities are known as pozzolons.
Bagged hydrated high calcium limes are also available which are used to make lime putty from this powdered material 24 hours or more before use. These hydrated high calcium limes are usually used as a plasterciser in sand/cement mortars, hydrated high calcium lime is usually considered to be not strong enough to be used as the sole binder in mortar mixes.
Pre mixed lime backing and finishing plasters are also available for use where dampness has been a problem. These can be very useful in renovation work as the mixes have been factory controlled containing fibres to reduce cracking, salt retardants to reduce efflorescence and other salt damage and lightweight aggregates to help insulate and reduce condensation.
On site the correct choice of material or mix is essential together with skilled application if acceptable results are to be achieved using lime mortars.
Given the right conditions, correct material choice and application lime mortar and plaster mixes can be highly compatible with period and traditional buildings, allowing walls to breathe and accommodating slight movement in the building.
At White Chapel Art Plaster our surveyors will advise which mixes are suitable for exposed substrates and where lime mortars can be confidently used. It is essential that a skilled and experienced surveyor or craftsman is able to make these specifications if failures and disappointments are to be avoided.