There are two types of lime which can be used for repairing older properties and building new ones: hydraulic and non-hydraulic. Both of these are different from the hydrated lime sold by most builders merchants and which is best used as a plasticiser in modern cement.
Non-hydraulic lime products are based on lime putty, and will only harden on contact with air. Hydraulic lime on the other hand will set in the absence of air, even underwater.
Traditional buildings, often with no damp course, were constructed with materials which ‘breathe’ i.e. which allow moisture to pass through them and evaporate off harmlessly. Modern cements and plasters are not breathable and if used on older buildings can lead to damp cold walls, condensation, flaking paint and rotten timbers.
In addition lime mortars are flexible and can accommodate the slight movement which occurs in older properties without cracking. The results of using modern cement on older properties can frequently be seen as cracked brickwork and ‘spalling’. This occurs when moisture which can no longer pass through the mortar course is forced to come out through the bricks, with the result that the face of the brick is blown off. (picture)
Whilst lime putty mortars are most often used for the repair of older buildings, hydraulic lime can fulfil structural requirements for new build where once Portland cements were the only answer. Hydraulic lime is a an environmentally friendly alternative to cement which uses less energy in manufacture, sets almost as quickly, allows a building to breathe and flex, and eliminates the need for expansion joints.