Calcium Scale
Calcium scale forms on pool surfaces due to very hard and out-of-balance pool water. Calcium scale is usually uniform and covers both plaster and pebbles in a pool. Calcium scale usually feels rough, like sandpaper, on the pool surfaces.
Calcium scale can occur on plaster, vinyl liner, or fiberglass pool surfaces.
Calcium scale is normally from calcium carbonate but can be from calcium sulfate (CaSO4). Aqua Magazine:The Mystery of the Pointed Crystals describes a calcium sulfate scale situation. Orenda Understanding Calcium Sulfate Scale Crystals further describes calcium sulfate scaling and solutions for it.
Calcium phosphate can also form scale and we rarely see that in pools.
It is important to identify the type of calcium scaling to know how to treat it. Calcium carbonate will fizz up in acid, while the other two either don't react at all or just slowly dissolve. By testing the calcium level and calculating the waters CSI as well as testing for sulfates and phosphates you can identify which chemicals are unusually high.
Calcium scale is different then waterline scale on tiles, evaporative scale on surfaces outside of the water, efflorescence, or calcium nodules.
What Causes Calcium Scale?
If there are too many calcium and carbonate ions in the water, then they will combine and form calcium carbonate scale when pH is high. We measure the calcium in the water and the carbonate as TA in the water. We then use PoolMath to calculate the waters Scaling Index (CSI) using pH, CH and TA to tell us how likely scale will form. A positive CSI above +0.6 indicates there is a risk of scale formation.
Scale does not form overnight with a high CSI. It takes many weeks of the high CSI water to have the calcium and carbonate ions to combine and form scale deposited on the pool surface.
Calcium scale is often discovered when a pool or spa is opened after being closed for the winter. While the pool is closed the pH will tend to rise to reach equilibrium with the TA. With high TA pH can rise into the high 8's or 9. Water left still for a few months with high pH, TA and CH can for scale on the surfaces which will feel rough.
Why Are Calcium Crystals Forming in Pools? In recent years, calcium crystals have been forming during the winter at a higher rate in some newly plastered swimming pools. From the reports we have heard, this problem seems to occur more often in the northeast of the country. Apparently, pool builders, plasterers, and service techs involved have not been able to determine the cause of these sharp and abrasive crystals and how to prevent them from forming on new plaster surfaces, including quartz and pebble.
High pH alone does not cause scale with balanced water. Experiments have been done with some pools keeping the pH in the range of 9.0 to 10.0 and keeping the CSI within the balance range without any scale formation. But that was in vinyl pools, not plaster pools. Scale did develop in plaster pools BECAUSE the plaster is not impervious to the water. Another way to describe this is to say that plaster is slightly permeable and can bleed out calcium hydroxide (with certain conditions) and that can significantly affect the pool water at the immediate plaster surface.[1]
How to Treat Scale
You have three ways of handling scale:
- Run your water chemistry at a low CSI of around -0.6 or even a bit below and it may melt the scale off over a few months. This is the least damaging to your plaster finish but will take the longest.
- Drain the pool and sand the plaster using diamond sanding pads with power sanders.
- Do an acid wash of the pool that will remove the scale but also etch the palster, remove a layer of it, and weaken the plaster.
