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Question on swimming pool chemicals

Are swimming pool chemicals aggresive to concrete?

Answer

Chemicals are only aggressive in their concentrated form, not when sufficiently diluted. Commonly used swimming pool chemicals are:

Aluminium sulfate is used as a floc to improve the filtration process. It is strongly acidic and measures must be taken to minimise concrete damage in the event of spillage.

Hypochlorite is used as a sanitizer. A family of chlorine compounds such as Calcium Hypochlorite and Lithium Hypochlorite, both granular, and the liquid Sodium Hypochlorite. When these compounds contact water, they release Hypochlorous Acid, the active sanitizing agent. Sodium hypochloite is strongly alkaline, in concentrated form it will attack concrete slowly.

Hydrochloric acid is used as a pH balancer. It is very aggressive to cement-based materials and measures must be taken to minimise concrete damage in the event of spillage

Sodium Hydroxide (caustic soda), another sanitizer, up to 10% is not harmful to concrete but at higher concentrations may cause damage by crystallisation and subsequent spalling.

Sodium Bisulfate: An granular form of acid, used to counteract a scaling condition by lowering pH and/or alkalinity. Sodium bisulphate is acidic but less aggressive than hydrochloric acid it will cause concrete to disintegrate - and measures must be taken to minimise concrete damage in the event of spillage.

1) Note that chlorine gas was once used as a sanitizer. When in contact with concrete chlorides can be released which may increase the risk of steel (fixings and reinforcement) corrosion. Similarly for hydrochloric acid.

2) Saltwater swimming pools may well have problems with chloride induced corrosion of the steel reinforcement.

 
Acknowledgement: The Concrete Society
 

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Swimming pools