It is commonly supposed that purpose of lime mortar is to stick masonry units such as brick and stone together. Although this is the case with sand and cement mortars, lime mortars do not set super-hard. Instead, the purpose of lime mortar first and foremost is to provide a cushion to spread individual masonry loads evenly, particularly when dealing with softer stones. As lime does not ‘stick’ stonework together, this means that traditional stonework produced with a lime mortar will be held together through craftsmanship rather than cement.
When set, sand and cement mortars are non-porous and harder than many rocks. As water cannot be released through mortar joints, it is often forced out through the building material. As well as the possibility of forcing damp back inside buildings, common affects of this are salt stains and the ‘popping off’ of exterior brick and stonework. This leaves distinctive pock-marks and can also be identified by sand and cement pointing standing proud of stonework.
Many older stone buildings, or those incorporating cob or timber, shift and move subtly over time or with the seasons. As sand and cement mortars set hard and are rigid, this natural movement can cause cracks, especially when it has been used to repair traditional materials. Soft, durable and breathable lime mortars however, will move with other materials, and have self-healing properties.