In this regular column, Consultant Barry Hunt answers reader enquiries. If you have a question, email it to: nss@qmj.co.uk marked for Barry’s attention.

Q: Can I use limestone for lining a swimming pool?

The simple answer is yes, but there must be extreme care in doing so as there is a wide range of problems that may be difficult to overcome.

Because limestone is porous, it is not ideal for the permanently wet environment of use in a swimming pool. Other potential issues include its relative softness and propensity to suffer under acidic conditions.

Under permanently wet conditions it is essential that the substrate materials are impermeable. It is presumed that the swimming pool is a monolithic concrete construction, which should provide this function. The limestone must be installed using adhesive and grout that are resistant to prolonged exposure to moisture. For more detailed advice, consult BS 5385: Part 4.

It is essential that the adhesive bed has no voids in order to prevent moisture being trapped. Voids also create habitats for algae and other organisms to breed that can then prove difficult to control. Black spot, which is a particularly tenacious growth of algae, may need strong algaecides to remove it, although there is a chance that they will lead to the greening of the limestone as a bi-product of the copper compounds typically used in such treatments.

The construction of the pool must have dried out thoroughly before the limestone is sealed. Sealing is not to prevent moisture passing through the limestone, but to reduce the potential for the movement and deposition of salts and other materials at or close to the surface. The most critical location to get right is the zone just above the water line where capillary action may draw the water up into the stone.

Sealing should ideally take the form of a resin type application to fill all voids and pores. The chosen treatment should exhibit minimal vapour transmission, something that many treatments are unable to achieve under constant contact with water. The sealer must be applied in coats until no more can soak into the stone surface. The sealer should also be allowed to cure thoroughly before the pool is filled as organic compounds within the sealer may provide nutrients that feed bacteria and fungi, leading to colonisation that results in staining of the stone.

Limestone is susceptible to acidic attack and the only way to prevent this is either to apply the sealer to a point where it actually forms a protective coating or ensure that the water treatment processes prevent the water from becoming acidic. Where pools rely on the addition of chemicals to balance the pH, it is unlikely that occasional acidification of the water can be avoided. Reverse osmosis, UV light and ozone systems are more capable of maintaining balanced water pH and are preferred in this instance.

Finally, it is generally best to choose a good quality limestone, particularly those that take a polish as these are likely to be less susceptible to the various potential deterioration mechanisms. Where possible, softer limestones with high porosities ought to be avoided as they might be difficult to use even with the greatest of care.

Limestones containing clay minerals that may suffer damaging swelling under continuously damp conditions should not be used.

Reference

BS 5385: Part 4: 1992. Wall and floor tiling. Code of practice for tiling and mosaics in specific conditions. Published by the British Standards Institution, London.

Barry Hunt is a Chartered Geologist and Chartered Surveyor who has spent 20 years investigating issues relating to natural stone and other construction materials. He now runs IBIS, an independent geomaterials consultancy undertaking commissions worldwide to provide consultancy, inspection and testing advice. Tel: 020 8518 8646

The advice offered in answer to readers’ questions is intended to provide helpful insights but should not be regarded as complete or definitive. Professional advice should always be sought with all stone-related issues as each problem will inevitably have its own unique set of circumstances.