Traditional buildings usually have solid walls with no cavity, and are built on insubstantial foundations, this leads to settlement and movement associated with seasonal changes in the ground conditions. Lime mortar is softer and weaker than the stone, and is therefore able to accommodate slight movements without significant cracking. Lime is also permeable and allows evaporation of rising damp from within the wall, it is this permeability or ‘breathing‛ that helps to keep the building dry inside without a damp proof course or chemical treatments.
Modern cement pointing is very different from lime mortar. It is hard, brittle and much less porous. It is harder than the stone and too rigid to allow settlement or movement in a wall. When movement does occur the edges of the stone are forced against the hard mortar, causing the stone to spall and cracking the mortar itself. Further damage is caused when rainwater seeps into the cracks in the pointing and around the edges of the stone, the water cannot evaporate through the mortar because it is not permeable, instead it is forced to evaporate through the face of the stone, leaving behind soluble salts in the surface layers of the stone, this leads to crumbling and decay, often so severe that the entire face of the stone is lost and the hard cement pointing is left standing proud.
Lime mortar is softer and more porous than masonry, allowing moisture to move and encouraging evaporation and salt deposition in the mortar joints. The lime mortar joint is sacrificial, and it is this that decays and not the stone.
It is much easier and far cheaper to repoint a wall than to repair or replace damaged stone.
More information about lime and its benefits can be found on the following websites:
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