Recently I have been called out to several jobs where stone as well as tile has been damaged around salt treated swimming pools. In both cases a sealer was applied and the initial client reaction was to blame the sealer. As salt treatment is becoming more and more popular I think it is prudent to investigate this technology and see how it relates to tile, stone and sealer technology. Traditionally pools have been sanitized using chlorine; however in recent times salt has become the new buzz word. In many cases people use the term salt as if it replaces chlorine in the pool environment. This is not true. Chlorine is a gas at room temperature. Regular table salt (sodium chloride, NaCL) is half chlorine and with simple electrolysis chlorine gas can be created from table salt and water. The same reaction produces sodium hydroxide (a good degreaser) and by mixing chlorine gas with sodium hydroxide you create sodium hypochlorite (NaOCL), common bleach. So you can see chlorine and salt are very closely related.
When pools use a salt chlorinator they are in fact using salt to create chlorine. The process itself is rather complex. However in simple terms it works as follows. Electrolysis is achieved by passing a mild saline (salt) solution through an electrolytic cell. The cell houses two electrodes – one anode and one cathode usually coated (sometimes made out of) with metals such as platinum, titanium or iridium. The result of the electrolysis with salt water is the production of chlorine gas.
How much salt is used in the pool to create the right amount of chlorine? This depends on the size of the pool and can vary from season to season as high water temperature and strong sunlight can reduce available chlorine. However as a general rule the concentration is fairly weak with solutions of between 0.3% and 0.7% (3000ppm to 7000ppm) being effective in creating sufficient chlorine. To put this into some type of perspective these levels are between one fifth and one tenth of the level of salt in sea water. The maintenance of these pools is the same as traditional chlorine pools in that the ph should be maintained at approximately 7.6. This is done by adding chemicals’ including salt as the chlorine is lost through splash out, high temperature, and high UV exposure as well as rain water contamination. So this is how it works. But how does it react with tile, stone and sealers? The main issue with these products is the ability of the salt to crystallize during evaporation and their ability to withstand this process. When the salt treated water evaporates it leaves behind a crystal which is taken back into solution once re-wetted. This process happens not only on the surface of a tile or stone but also inside, if the stone or tile is suitably porous. It is this constant expansion and contraction caused by the crystal that creates shear and expansion in the stone. If the pressure is too great for the tile or stone they will react, with parts of them dislodging to accommodate the pressure. This most commonly manifests itself by way of a spalling of the surface.
Sealers have been seen as the saviour of this problem. It is reasoned that it is porosity alone that creates this problem. However this is not true. The problem is created by porosity (the water needs to travel sub-surface) as well as the internal structure of the stone and the ability of it’s components to withstand this pressure which is further related to the water and temperature expansion of these components themselves. The bottom line is that to have a successful tile or stone installation around a salt treated pool the tile and stone must be able to work in this environment without a sealer. The sealer is applied only to improve the stain resistance and maintenance of the pool surround and not to make a non-salt safe surface safe.
The types of stone that have difficulty working in these salt environments are the more porous and reactive types. Limestone for example covers a large group of stone; however most of these will not work well in this situation. Likewise many of the sandstone varieties struggle in salt environments. Examples of stones that will work well are granites and quartzite’s due to their low porosity and high content of low expansion components such as feldspar and quartz. As for suitable tiles, porcelain (both unglazed and glazed) is the most durable. However some glazed single fired products as well as some of the higher fired quarry and terra-cottas are recommended by manufacturers. In all cases it is prudent to seek the advice of the tile or stone supplier to determine if a tile or stone is suitable for installation around a salt treated pool. As a general guide though selecting a product that is frost proof (not frost resistant) will insure you have a material that is also suitable for a salt environment. As for the sealer it is important to note that not all sealers are suitable for this situation and therefore you must also ask for a sealer that is suitable for a salt chlorinated pool.
In conclusion, the advent of the salt chlorinator has certainly brought with it a need to use the correct products when using tile or stone in this environment. The problem jobs I have inspected over recent times have all been the result of incorrect stone or tile selection. In all cases a sealer was applied to reduce porosity and in doing so render the products salt safe. This as I have shown is not the case. To have a successful tile or stone installation around a salt pool you must have either a product that has no porosity or a suitable stone or tile impregnated with a salt safe sealer.