There are proper steps to follow for the making of quality pool plaster, including color, quartz, and pebble finishes. There are also improper practices that can lead to early deterioration and discoloration. Following is a ten-point checklist that will help achieve a smooth, long-lasting, and discoloration-free swimming pool plaster.
1. Plastering in extreme weather conditions can lead to quality and durability problems. The ACI and PCA have specifically warned against using cement-based products in temperatures considered too hot or cold.
One solution is “tenting” the pool, which protects the plaster surface (and the plasterers!) from the elements. In extreme dry heat, tenting the pool, and perhaps directing air from an evaporative cooler beneath the tent, will help the plaster retain its moisture, and properly cure and harden.
2. The best cement/aggregate ratio is about 1 part cement to 1.5 parts aggregate (marble or limestone sand). If the plaster is too rich (cement-heavy), it tends to shrink and crack. If it’s too lean (more sand), it will be less durable and potentially unworkable.
3. When mixing plaster, a thick mix is best. Achieve a water/cement ratio of .48 or less. Both the American Concrete Institute (ACI) and the Portland Cement Association (PCA) maintain that lower water/cement ratios produce better-quality cement that can withstand occasional exposure to aggressive water.
Lower water/cement ratios boost density while reducing permeability, porosity, shrinkage (craze cracking) and water movement within the cement product. Higher water/cement ratios, by contrast, cause excess shrinkage and cracking, and fail to offer adequate protection or long-term durability against the effects of water and the environment.
4. A plaster mix should be mixed thoroughly, but also not too long. It is recommended that if the plaster has been mixed for more than 90 minutes, the plaster mix should be discarded.
5. Plaster should contain as little calcium chloride set-accelerant additive as possible – and never more than 2 percent to the amount of white cement. According to the PCA and other testing facilities, too much calcium chloride increases gray mottling discoloration and cement shrinkage. Colored plaster, of course, should not contain any calcium chloride due to severe mottling effects. Several alternatives to calcium chloride that do not exhibit these characteristics are now available.
6. Never add water to plaster surfaces while troweling. Both the ACI and PCA have found that this can increase porosity, shrinkage, and variable discoloration.
Never “work,” or force, additional water into the plaster surface when troweling. Doing so can weaken the surface and may accelerate deterioration and caused spotting or streaking discoloration. Dark color plaster is even more susceptible than white plaster to white spotting and blotchy discolorations due to water troweling.
7. Well-timed hard troweling can produce a smooth and dense plaster finish. But if the plaster becomes too hard before the surface is smooth, the result is often dark gray discoloration and spotting, especially when calcium chloride is also used. Cement “cream” (laitance) that accumulates on the trowel should be discarded and not re-applied to the surface.
8. Don’t fill the pool with water too soon. A new pool plaster surface is soft and vulnerable to being dissolved by water. Though conditions vary, filling should not be started for at least six to eight hours after the pool has been plastered and finished, even in hot weather. This should be enough time for the plaster to harden properly before being submerged in water.
Shrinkage cracks are more likely caused by high water mixtures and water troweling than being allowed to harden before submerging in water. Consider cement flatwork applications in comparison.
Even balanced fill water can dissolve soluble cement components (calcium hydroxide) if the surface has not adequately hardened. The end result is often greater porosity, early deterioration, and discoloration. And it only takes a few months to become visible.
9. Soft or aggressive fill water can harm new plaster surfaces. By contrast, baking soda startups will adjust the fill water to promote a superior plaster surface. Other new plaster problems such as drips, splotches, spotting, trowel marks, and hand- and footprints are the result of localized troweling and finishing errors.
Acid treatments of newly finished quartz finishes etches a plaster surface and can cause color differences and streaking that may not be visible until weeks later after the pool is filled with water. Those techniques simply age and shorten the life-span of pool plaster.
10. Once the pool is filled, balance the water (and keep it balanced). Balanced water helps help preserve the plaster. Aggressive water causes uniform etching, while over-saturated water scales the plaster. The Saturation Index is a good guide – to prevent scaling or etching, water should have a saturation index value in the range of -0.3 to +0.5.
With reasonably consistent maintenance, pool plaster has a life span of approximately 20 years. It’s an inherently strong surface, and should be able to withstand “real world” chemistry and/or maintenance challenges.
Although pozzolans and other materials such as quartz and pebble aggregates are generating good results, solid workmanship is still required. The above guidelines will benefit pool plasterers in the pursuit of a discoloration-free and durable pool finish.