For the past 150 years modern artificial cements have slowly been replacing traditional lime based mortars and plasters, to such a degree that now virtually all construction is carried out using only modern materials. Whilst many of these materials are perfectly suitable for modern buildings they have been found to be incompatible with the construction of old buildings.
The need to understand the different technology involved in historic and modern structures is essential if successful repair and maintenance programmes are to be carried out.
Lime has been the primary binder used in mortars and plasters for thousands of years and the vast majority of all buildings constructed before 1900 made use of lime. Despite this, in many cases today lime is ignored, so why Lime?
Modern cements are harder and less permeable than lime mortars, the general aim when selecting a traditional mortar or plaster is that it should breathe more freely than the material which it is applied to and that it should have less composite strength than the substrate with which it is used. This is essential if you are to prolong the life of the historic fabric.
Modern buildings generally rely on an outer layer to prevent moisture penetrating the walls, whereas buildings constructed before 1900 generally rely on allowing the moisture which has been absorbed by the fabric to evaporate from the surface. In essence old buildings exposed to the elements are continually absorbing moisture and the ability for the moisture to evaporate again is crucial to the well being of the structure. Using cement based mortars and plasters in traditional buildings runs the risk of locking-in moisture which could result in dampness internally and general building fabric decay. Problems generally arise when the building has been “repaired” with inappropriate materials through lack of knowledge.
It is interesting to note that many structures built using lime technology 500+ years ago and maintained correctly are still in excellent condition today. It remains to be seen how modern structures will fare in 500 years!
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