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This is written for builders, remodelers, and pool plastering contractors who are interested in providing a long-lasting and pleasingly aesthetic pool plaster finish, especially colored plaster.

Recent studies by onBalance have determined that plastering in hot and dry temperatures can have a detrimental effect to the quality and durability of the plaster surface. At issue is the fact that when a plaster surface dries out, hydration and hardening stops. This results in a plaster surface that is soft and weak and that will not be ready to withstand being submerged in water.

To demonstrate this issue, a plaster coupon (A) was formed that was then exposed to the sun all day, with temperatures reaching 90 degrees at about 30% humidity. The next morning, this coupon was placed in balanced water (actually +0.3 SI). After five days, plaster dust was visible on the bottom of the tank. The coupon was removed and the plaster dust (calcium carbonate) that developed in the water was dissolved by adding acid. The resulting calcium hardness level in the tank was determined to have increased 65 ppm.

Next, the experiment was repeated (Coupon B) when the ambient air temperature was hotter at about 100 degrees, and humidity was lower at about 20%. An additional coupon (C) was formed also, but this coupon was immediately placed in a controlled environment for 24 hours to harden, where the temperature was 75 degrees at about 35% humidity.

The next morning both coupons (B and C) were placed in separate water tanks. After five days, the water in the tank with plaster coupon B, which was exposed to the hot sun, had a calcium hardness increase of more than 120 ppm, after the plaster dust was dissolved. That is the equivalent of about 20 pounds of solid calcium carbonate being dissolved from the plaster surface of a 20,000 gallon pool.

Plaster coupon C, that hardened in the temperature- and humidity-controlled environment did not develop any visible plaster dust. The calcium hardness of the water increased a nominal 10 ppm.

In these tests, it was also determined that if calcium chloride (a hardening accelerator) is added to the plaster mix, an even higher amount of plaster dust formed in comparison to plaster samples that did not have any calcium chloride added, regardless of temperature.

When calcium is lost from a plaster surface, it causes a loss in density and an increase in the porosity of the surface. This in turn will result in a loss of durability of the plaster surface and will likely trap dirt and minerals easier, causing discoloration sooner than would otherwise occur.

Contractors should be aware that with dark colored pool plaster, an increase in the porosity will cause whitening or a lighter color developing over time as the surface loses calcium or other plaster material. This has been a sore issue for the pool industry for a long time. Of course, other improper plastering practices also contribute to this problem besides plastering in hot dry weather. Adding calcium chloride to an integrally colored plaster mix, or applying excessive water while troweling can also result in increased porosity and lightening. Improper water chemistry is often incorrectly blamed for the whitening of colored plaster, including pebble and quartz pool finishes. (Of course, whitening due to scale depositing on the plaster surface is a water chemistry issue).

Some plastering people claim that the solution to plastering in hot and dry temperatures is to immediately fill the pool with water after final troweling; that it is better if plaster is cured under water, which helps prevent the plaster from cracking in the heat. That is only partially true. Yes, the pool plaster matrix does gain added strength while curing under water, but the plaster “surface” needs to be sufficiently hardened before being submerged in water. Otherwise, the water filling the pool overwhelms the correct ratio of water to cement at the surface, which can cause a number of defects, including a general dissolution of plaster material from the surface. This has been documented. (See my prior blog post “Poor Plastering Practices vs. Aggressive Water”) Only after the surface has been properly hardened is curing under water beneficial.

In regards to cracking, good plasterers know that using a low water: cement ratio, little or no calcium chloride, and no excessive water troweling are very effective in avoiding cracking, even in hot weather.

Finally, the best solution to plastering in hot and dry weather while avoiding the detrimental effects it causes is to “tent” a pool and use an evaporative cooler to supply cool and humid air underneath the tent. This will also protect pool finishers. Tenting should be used during the entire plastering process, the six hour waiting period before filling, and until the pool is full of water.

The Portland Cement Association (PCA) and American Concrete Institute (ACI) have conducted many studies showing that cement work in hot and dry temperatures has a detrimental effect to the quality and durability of such products. They also suggest various methods to protect concrete surfaces from hot and dry temperatures. Pool plastering is a more, not a less challenging process than other cementitious applications due to the fact that pool plaster is generally troweled to a smoother finish than is concrete flatwork, as well as the thinner application, the rich mix and the need to be maintainable in the pool water and chemistry environment. Thus it is even more critical for pool finishers to follow optimal practices derived from cement science.