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Some in the pool industry believe that calcium “naturally” releases from a new swimming plaster (cement-based) surface while it cures (hydrates) and hardens under water.  And then that causes the calcium hardness (CH) of the pool water to increase 50 to 200 ppm during the first month or two.  That is a false belief.

The fact is that if the plastering workmanship is performed properly and is of high quality, and the water balance is maintained properly, virtually no calcium (less than 10 ppm) is lost from a new plaster surface.

However, if the tap water used to fill the freshly plastered pool is soft and aggressive, then yes, calcium will be etched from the plaster surface, thereby increasing the CH level of the pool water 40 – 100 ppm depending on how aggressive the tap water is.  And that loss occurs within the first few days.

Also, several improper plastering practices can create conditions that allow calcium to be leached (dissolved) from a new plaster surface by balanced water.  High water/cement ratios, high calcium chloride content, wet troweling, and turning on the fill water too early, allows greater calcium dissolution and loss from the plaster even when chemistry factors are optimal.  As much as 160 ppm of calcium can be lost from the plaster surface within the first 48 hours upon submersion due to these factors, even when the tap water and pool water is balanced. 

If the plaster quality and the chemical factors are both good, it is not true that plaster “naturally” loses calcium from pool plaster.  Calcium levels do not automatically rise, and therefore, low levels of CH of start-up water need immediate correction, and preferably before filling to prevent calcium loss.

A good general recommendation for new plaster pools is to pre-test the fill water, and pre-condition it as needed before it is allowed to come in contact with the new fresh plaster.  Raise the calcium level to a minimum of 200 ppm, the TA to a minimum of 120, and ensure that the LSI is a slightly positive number for the first month.  The main point being; new and soft pool plaster requires a positive LSI for one month to protect it and to prevent a loss of calcium from the surface. 

After 30 days, a typical plaster finish generally has become “carbonated,” which means it is more durable and less soluble.  At that point, the LSI can then be lowered to zero.  If that program is followed, very little calcium will be dissolved from the surface, ensuring a smooth, dense, and more stain resistant plaster surface. 

The “Bicarb” start-up actually facilitates the best and easiest program to obtain a positive LSI of the tap water which ensures that no calcium is removed from a new plaster surface, and no “plaster dust” develops. 

However, this is contingent on the plaster quality being good.  If there is any continued increase in the calcium level (accounting for evaporation or Cal Hypo additions, etc.), it is possible that the workmanship and other practices of the plastering work were sub-standard.