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The Effect of Salt on Stone
Stone is becoming ever more popular in outdoor areas – in stone walls, around the pool, in entertaining areas, and so forth. But salt, which is prevalent in coastal areas and also in homes that have salt water pool systems, can have an effect on stone. What does it do and can it be prevented?
Salt and Stone in Outdoor Areas
The use of stone in outdoor areas, especially in entertaining areas or other areas close to pools is popular, but the suitability of stone depends on the quality of the stone that is used. Stone that easily absorbs water or moisture and has a small pore size is particularly susceptible to the effects of salt. This is because salt moves through the pores through a process known as capillary action and then crystallises. When it crystallises it expands, causing stress on the surrounding stone which can cause cracking and fragmentation. Ultimately, the stone may end up with surface pitting with a fine, powdery residue, as well as discolouration.
However, this does not mean that you cannot use stone around the pool. It is just a matter of being careful and choosing stone that is suitable for its intended purpose. Once the stone is selected and laid, it needs to be professionally fixed and sealed so that salt cannot enter the stone. For example, quartzite and granite are much more resistant to salt attack than sandstone, which needs to be very well sealed.
How to Prevent Salt Affecting Stone
Salt has the worst effect on porous stone. Salt crystallisation can be seen as a visible growth or stain on the surface of the stone. It is usually a light colour. This film dries and can take small parts of the stone with it. In coastal areas, this can be a dramatic problem as crystallisation can cease to be a surface problem and actually lead to the stonework itself decaying. To prevent this from happening, remove the signs of crystallisation with a soft brush and water regularly.
About Salt Damp
Salt damp is the term for the damage in buildings that is caused by the action of moisture and salt. The main cause of salt damp in walls is rising damp. This occurs when the damp-proofing course in the wall is not adequate and allows moisture from the ground to soak upwards into porous masonry walls. The dissolved salts in the ground travel into the wall along with the water.
Salt damp can cause problems such as fretting mortar, bubbling paint and plaster, and serious decomposition of the wall materials. The damage occurs in two main ways. Firstly, the dampness causes some materials to deteriorate. Secondly, the salts crystallise as the moisture evaporates, creating enormous pressure within the pores of the stone wall itself, physically breaking down the masonry. Eventually the masonry breaks down and flakes off in layers. Some salts also cause a chemical reaction that destroys the masonry structure.
If you do have damp problems this needs to be corrected – either with undersetting, or with a silicone damp course.
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Damp Proofing
Stonemasonry