Calcium Hydroxide is a buzzword in the pool plaster business because it is one of the most important compounds in cement.
Calcium Hydroxide, Ca(OH)2, is the most soluble form of calcium in cement. It's important in pool plaster because it takes longer to cure than the rest of the compounds in cement. As the hydration process continues, moisture leaves the cement, which causes microcracks and shrinkage as the material loses volume. Calcium hydroxide is incredibly valuable when this happens because it fills in those cracks before eventually being carbonated, and converting into calcium carbonate (CaCO3).
How is calcium hydroxide formed?
When water (H2O) is mixed with portland cement, a compound in the cement called calcium oxide (CaO) combines with the water to create calcium hydroxide, Ca(OH)2. This process is called hydration.
Why is calcium hydroxide so important?
Calcium hydroxide has a very high pH (12.6). When water is aggressive–meaning below -0.30 on the LSI–it will begin dissolving calcium carbonate. Underneath the outermost carbonate layer, calcium hydroxide is still present in the cement for years. It takes a long time for carbonation to penetrate into the cement matrix past the surface.
So when aggressive water seeks its own LSI balance, it is most likely to dissolve calcium hydroxide and pull it out from within the cement. This draws calcium hydroxide to the surface, where its high pH causes a pH spike in the water, and can force carbonation to occur, turning the surface white. This is often mistaken for scale. Most plaster discolorations are related to calcium hydroxide.
For more on calcium hydroxide, take Orenda Startup Academy™