If your property is built from a soft stone such as slate or sandstone, and has been pointed with cement, you may notice that the stonework is crumbling and eroding faster than the surrounding pointing.
This is a function of the process of salt deposition, and is heavily exacerbated when cement is used for pointing instead of lime.
When it comes to old cob and stone buildings, salts will be present within the wall substrate as a matter of course. They may also enter when groundwater wicks up into the walls, when salt spray from the sea comes into contact with the building, or when a chimney is leaking.
Why is Lime Much Better at Dealing with Crumbling Stonework than Cement?
Old stone buildings are designed to shed trapped moisture through their pointing joints, but when cement (which has a low breathability) is used for pointing instead of lime, this process cannot take place. Instead, any moisture in the wall substrate will evaporate through the stonework.
Surprisingly, most stone is porous to a degree – more so with sedimentary and metamorphic stone, though granite may also have a degree of porosity due to microscopic faults and fractures. When evaporation takes place on or near the face of the stonework, salts are deposited in its’ pore structure.
The deposition of salts exerts a massive pressure on the stonework and causes it to fracture and flake away. This process may be seen at its worst in coastal villages such as Marazion, where the buildings are of soft stone and catch a high level of salt spray.
Lime is much softer and more permeable than cement. When it is used for pointing it provides any moisture within the wall substrate with an evaporation front. As it is more breathable than the surrounding stonework, evaporation occurs through the pointing rather than the stone.
Any salts tend to be deposited on the surface of the pointing, and where they do cause damage, it is to the pointing, which acts as a sacrificial element.
How Soon Should I Replace the Cement Pointing with Lime?
If you do notice this phenomenon occurring on your building, it is best to treat it as soon as possible. The longer it is left the more damage will be caused to the stonework – the more erosion that takes place, the faster rain will be able to saturate the wall, resulting in a vicious circle of saturation and erosion.
If possible employ the services of an experienced lime mason to help assess the building and carry out any pointing works.
There is a great deal of variation in the quality of the lime work on offer, so make sure to check credentials and view past projects where possible.