Pool Plaster Issues

One of the complaints companies that build pools get after a period of time is, “The pool plaster YOU GUYS put on is failing”. People like to blame the builder, when in reality it is beyond our control and is usually the fault of the pool maintainer. Pool surfaces or plasters are made of a cementitious mix and part of that mix is Calcium Hydroxide. Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca(OH)2. It is a colorless crystal or white powder ..

Calcium hydroxide is one of the main reaction products resulting from the hydration of Portland cement with water. Calcium hydroxide, is susceptible to acid leaching. This is where things get complicated, Pool water is often treated with chlorine to correct the PH level, but more importantly pool tablets which contain Cyanuric Acid. Cyanuric (CYA) is a pool balancing product used to help chlorine last longer. Chlorine, in its natural form, is unstabilized—which means it degrades when exposed to sunlight. Adding Cyanuric Acid reduces the sun’s impact on chlorine loss. A lot of pool shock products also contain Sodium Dichlor: Dichlor for short, is stabilized pool shock, meaning it contains Cyanuric Acid to protect your chlorine against degradation from the sun. However, Acid is heavier than water and will sink to the bottom. This is part of the reason why dilution is so important; pre-dilution reduces the average density of the acid.

A typical situation like this occurs, during the regular season the pool is kept balanced with the addition of an acid. But at some times of the year an algae bloom can occur. The typical response is to add a few more Chlorine pucks to the float to try and stop the algae and most likely get some pool shock to rapidly raise the chlorine level and kill the algae that way. But in doing so you are raising the cyuranic acid levels as well. The other critical issue is the shock pellets have to be completely disolved in a bucket of water first before adding them to the pool, failure to do this will have severe consequences, as I will explain.

It is extremely common for discoloration and leaching of Calcium Hydroxide to occur on the top step of a plastered pool. You may wonder why but the answer is simple and it lies with your pool float. If you leave the float to just float free a lot of times it gets stuck in a corner, being in a corner is not bad thing, unless that corner happens to be the area of the top step of your pool. If the pool float sits in a corner just above the step as the pool pucks degrade they release cyanuric acid, since acid is heavier than water this acid will leach the Calcium Hydroxide out of the plaster leaving a rough surface with pitting. Pitting while small, is a big problem it allows water to carry the cyuranic acid into the plaster it self, leaching out more Calcium Hydroxide and can also introduce chlorine which is a powerful oxidizer to metals under the plasters surface which will cause rapid corrosion of most metals.

If we introduce shock into a pool to stop algae growth and there are some grains of shock that are not dissolved, they naturally settle on the bottom, and dissolve over time, but meanwhile they release cyuranic acid in close contact with the plasters surface. Once again the cement reacts with the acid and the Calcium Hydroxide is leached out of the pool plaster, leaving a tiny pinhole, and a very rough patch, which allows further corrosion over time. Very typically you will see a small quarter sized area of discoloration but eventually that area will show a small pinhole in the center. The more undissolved shock granules the more spots appear blending together and if the shock process is repeated every so often the leaching of the Calcium Hydroxide in those pinholes gets worse. Eventually allowing the pool water under the plaster layer.

When pool water gets below the plaster layer several things may occur. It may cause spalling and lifting of small areas of plaster that will get larger with time. The pool may show signs of rust on areas of the finish, due to the oxidizing effect of the chlorine leaching into the pools subsurface over a length of time.This process can easily occur within just a few years.

The first place you should look for discoloration and issues is that top step as it is an indicator of issues to come. But the most important thing to remember is pool tablets contain acid and your pool is also balanced with acid, too much acid is not a good thing nor is over chlorination of a pool. Pool shock granules have to be completely liquefied prior to putting them in the pool, one whole grain can ruin a small portion of your pool’s finish resulting in a cascade effect with time.

This started out as a pinprick and grew

Upper right is small pinhole, center is about 1 years progression
Area of discoloration with pinhole from Shock granule allowed to rest till disolved
Pinholes from shock after 6 months.
Top step damaged from pool float and cyanuric acid release.

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