Special considerations for sandstone and marble ledges and copings.

Some types of stone can suffer from weathering from the sodium salt in pool water used with electrolytic chlorination, as the salt forms crystals as it dries.  Pool water seeps into small cracks and pores in the stone and erodes it over time as crystals are formed.  These pools are often set up to use ‘fresh water’ and liquid chlorine instead of salt chlorination to minimise the problem.   Unfortunately, so called ‘fresh water’ pools often get naturally salty as the mains water used to top up the pool also contains salt.

In addition, salt is naturally added with the liquid chlorine.  Liquid chlorine self-decomposes quite rapidly, producing sodium hydroxide (drain-cleaner) and sodium chloride (common salt).  Eventually, even ‘fresh’ pools may erode your stone.  In addition, the running cost and inconvenience of running liquid chlorine pools is substantially higher than that of salt chlorination, which is why some 90% of Australian pools use some form of salt chlorination.    In fresh pools, you really need to change the water  every few years to minimise built-up salts from evaporation, especially if you live in an area with ‘hard’ water.

One alternative you may wish to consider is to use magnesium chloride salt instead of common (sodium) salt.  Unlike sodium-chloride salt, magnesium salt will not generally have the same erosion effect on sandstone and the water is nicer to swim in, softer and much better with hair.  Magnesium baths are reputed to help tired muscles as well.

One proprietary blend of magnesium chloride salt is call ‘mineral salt,’ where the active ingredient is the same but blended with potassium salt, mainly for a difference in taste.

If you do switch to magnesium salt, there is no need to change your chlorinator in most pools as standard salt-water chlorinators work with magnesium or sodium salt.

There is a small cost to change over from sodium to magnesium and a higher cost to go to ‘mineral.’

Our experience has been that sandstone is not appreciably affected by magnesium pools over many years.


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