Calcium Sulfate in Pools

When calcium ions in pool water precipitate onto tile and interior pool surfaces as a solid white crust or crystal, the most common form of precipitate is calcium carbonate (CaCO3), commonly referred to in the trade as “scale”.Calcium Carbonate scale has some specific, known characteristics, such as being soluble in acid, having a value of 3 on the Mohs hardness scale (with 1 being the softest – such as talc, and 10 being the hardest, such as diamonds).Calcium Carbonate scale is generally relatively uniform in its deposit pattern across the interior surfaces of a swimming pool or spa, although a certain amount of it will concentrate at the waterline as an evaporite.

Calcium scale that deposited over a long period of time (in what we would refer to as a “natural” process) tends to be somewhat smooth to the touch and low in profile, influenced in patterning by flow and/or porosity of the surface it precipitates onto, and colored by incorporated dirt or other suspended water impurities (see tile samples above). Calcium scale that deposits in a short period of time (usually through an environmental trigger such as high temperature, high pH, high alkalinity and calcium, etc.) tends to be totally uniform in deposition, pure white, with sharp defined crystals and rough to the touch:

An interesting (although fortunately not too common) variation to calcium carbonate precipitation is the precipitation of calcium sulfate. Calcium sulfate is chemically designated CaSO4, and is commonly called gypsum.If an appreciable amount of sulfate is found in pool water, it can combine with calcium (if also high in content) in the water and form a crystalline deposit. Sulfate can be introduced as a component of treatment chemicals. These treatment chemicals can include one type of liquid pool acid (sulfamic acid), dry granule acid (sodium bisulfate), dechlorinators (sodium sulfite and sodium thiosulfate), and even non-chlorine shocks (such as persulfates).Occasionally in some isolated parts of the country, high amounts of sulfate may also be present in tap water, generally from well or ground water.Calcium sulfate crystals which form as evaporites in pools are distinctive and different from calcium carbonate precipitates in several key ways. They are softer (only 2 on the Mohs scale), soluble in acid only if that acid is at boiling temperatures, and are in what is called a swallowtail crystal formation.Here is a pool with tile covered in calcium sulfate crystals:

You can see that the tile is almost totally obscured by the deposited crystals, but the grey plaster under water is not.Here is a close-up of that tile:

Here is a close-up of the crystals from the pool, showing the distinctive swallowtail shape:

Fingernails have a Mohs hardness of 2.5, while calcium sulfate is 2 and calcium carbonate is 3. Here is a calcium sulfate crystal that has been scratched on the side with a fingernail to show that the crystal is softer than the nail:

We also previously mentioned that “room temperature” acid will dissolve calcium carbonate, but not calcium sulfate… but that heated (boiling) acid will dissolve the sulfate crystals. Here is a picture sequence showing us sanding some of the sulfate deposition layer off the top step:

We then poured (non-heated) acid on the sulfate deposit near the sanded area, getting no reaction:

But when the acid hit the bare plaster, which contains calcium carbonate, it reacted, in the form of white effervescence:

We don’t have pictures of using the boiling acid, because we were too busy trying not to breathe the fumes… but it worked!This pool was located near a mountain mine, where the well-pumped source water had sulfate in it. Since that water was also high in both calcium and alkalinity, the regular use of dry acid also contributed to the problem.

The homeowner chose to resurface his pool with an exposed aggregate finish, so we didn’t do a boiling acid wash on the entire pool. This condition is also relatively rare, so you may never see one with calcium sulfate deposits. But now, if you do, you’ll know what you are looking at!Provided by onBalance – Que Hales, Doug Latta, and Kim Skinner
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  • Que, I wondered what happened to that pool. I can' t even remember if I did the new interior or not. I can tell you that I never used soduim bisulfate to lower high alkalinity or control pH, however the pool was in that condition when I took it over. I built the pool 12 or so years ago for another company and I know the tech who was maintaining it originally. I doubt he used dry acid either. Larry Rylance
  • Que,
    Great article as usual and enlightening. Wouldn't alum (aluminum sulfate) also be a potential for this in high calcium. I didn't see it mentioned in the article. I am not sure I would feel good encouraging pool techs to use "boiling acid". Fortunately, as you stated this occurence is rare! Handling regular muriatic acid is scary enough, but boiling. You are a brave soul! I hope to see you at the Western Show in Feb.

    All the best!
  • Richard - You'll note that I referred to the swallowtails as an evaporite - 90% of the calcium sulfate in the pool was either an evaporite on the tile, or broken-off crystals which fell to the pool floor. The evaporite crystals were about 1/2 inches long. There was a thin film of of calcium sulfate over all of the pool (see sanding/acid pictures). So there was enough of both products to form a thin underwater precipitate, but more than enough to make those cool crystals.
    This pool was resurfaced a couple of years ago - I can look in my notes to see if I saved the water analysis. He was filling the pool from a well on the property, and the area is next to old mines in the Tucson Mountains. The pool tech was using sodium bisulfate to combat the high alkalinity of the fill water... I'm sure that didn't help. The homeowner has a dozen houses across the country, and spends a week or two at a time at each - he got to Tucson with the family after dark on a Friday evening and hopped into the pool - ouch ouch ouch as he hopped out! He ended up telling the pool tech to have it redone in pebble because he always wanted to try an exposed aggregate surface...
  • Thanks for the photo link. The bulk of the precipitation is in the area above the water line, so wetting and evaporation concentrate the substances and since the crystals doesn't dissolve readily in water, they just grow and grow with each splash and evaporation. So the bulk pool water can have much lower concentrations than saturation with calcium sulfate.

    I presume that when the homeowner resurfaces his pool, that the water was exchanged so we'll never know the sulfate level. Nevertheless, it might be interesting to look at a water report to see the CH and sulfate level in the fill water. Of course, we don't know how much more sulfate was added via dry acid or non-chlorine shock (MPS).
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  • Richard - I wish I would have had an opportunity to test the water before the precipitation event... unfortunately testing the water afterwards (when the service tech called me in) only shows what was left afterwards. Here are a few more pictures:
  • Muriatic acid boils at around 215ºF - In my lab I put crystals in acid in a flask and boiled it... just previous to a full boil the crystaIs disappeared (went into solution). I did it with a propane camping stove at poolside, and boiled two gallons in a metal pot to try it out. I did not attempt to breathe the fumes ;o)
  • Que, you must have had extraordinarily high sulfate and calcium levels to precipitate gypsum (calcium sulfate dihydrate). Even at 500 ppm Calcium Hardness (CH as ppm calcium carbonate), it should require at least 2000 ppm sulfate to precipitate calcium sulfate dihydrate. At 1000 ppm CH, it would take at least 900 ppm sulfate to precipitate. Do you have any idea what the sulfate level was?
  • Very good information, Most of my business is dealing with the roughness of plaster. Scale (calcium carbonate) is hard to remove as oppossed the calcium sulfate. I have never tried a boiling acid wash. At what temperature does acid boil and how dangerous is it in that state.
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