PH minus stained brand new plaster finish...can it be fixed

[TreeFiter]

The company I work for has a pool in which someone added ph minus directly to the pool a few days after the plaster was done. There is a big white stain in the middle of this pool. We have been brushing the stain in addition to the normal brush down, but I'm wondering if there is anything else that can be done to fix this?

Would acid washing fix this type of stain? Would lowering the pH for a period of time have a similar effect?

I'm also interested in knowing what happens chemically between the sodium bisulfate and the plaster. Is the plaster dissolving, and then recrystallizing back onto the finish in those areas, or is the white stain a new compound?

 

[JoyfulNoise]

What is the actual color of the plaster? Did the customer just dump the sodium bisulfate straight into the water without dilution?

Essentially it's an acid-salt created by reacting sulfuric acid with a sodium base. The solid and solution formed by its dissolution will be more dense than water and will therefore create a very low pH "hot spot". So, in effect, it's like dumping a huge load of acid straight onto the plaster. My guess is that the extremely low pH etched away the freshly cured plaster surface (mostly calcium carbonate) and exposed the underlying plaster (calcium oxide and calcium silicate). If there was any colorant to the plaster, it probably got etched away as well.

I don't think any further etching of the plaster with pool water will help much and my guess is that the spot will only diminish slightly over time. One option to fix it might be to drain the pool and acid wash it to try to homogenize the surface relief but only the plastering company can tell you if that would work.

There is a slight chance that some calcium sulfate crystals formed but that is usually only seen in pools that have excessive sulfate levels from years of using dry acid and typically only forms under scaling conditions (high pH, high TA and high CH). That's the only other adverse reactions I can think of from using dry acids.


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