Why you should not point with Cement based Mortar on buildings built with lime mortar and why damage is caused to stone and brick walls with cement based mortar?
Since the 1950s the use of lime in building construction had virtually ceased, cement based mortars were used to repair old buildings which created increasing problems with spawlling of the masonry walls, due to the freeze thaw process. Stone and brick walls built with lime mortar need to breathe allowing moisture to evaporate through the mortar joints. If the mortar joints are blocked by cement pointing to the walls, the walls will get wet and the only way out is for it to evaporate through the stone or brickwork.
The walls will be badly effected by salt accumulations and in the winter damp walls will freeze and spawlling to the walls will occur, due to the freeze thaw process, hence at the same time trapping water into the walls which can emerge inside causing inside bowling to the plaster internally.
The cement based pointing should be carefully removed to a depth which is equivalent to twice the width of the original mortar joint.
The walls need to them be re-pointed with Natural Hydraulic Lime mortar, this being softer than the existing stone or brickwork, thus giving the wall the best chance of breathing.
The choice of sands, aggregates and the strength of natural hydraulic lime combined with the ratio to be used can differ by the width of the mortar joints, also it depends on how hard or how soft the stone/bricks are, and taking into account how exposed the materials will be to weather damage, like chimney stacks and parapet walls, copings and paving. In most cases NHL3.5 hydraulic lime with a ratio of 3 sand to 1 hydraulic lime is commonly used and proven to be suitable.
As you will see below, we have some examples of cement based pointing and the effects that can occur with this, alongside we have examples where we have removed the cement based pointing and after raking out, we have repointed with natural hydraulic lime mortar
Please click on the pictures below for more information.