Kim – What Deteriorates Pool Plaster the Most?

Probably everyone in the pool industry has the heard the statement; “Water chemistry (aggressive water) has the most profound effect on the deterioration of pool plaster.” Is that completely true?

Most of the industry’s associations advocate an LSI range of -0.3 to +0.5. The pool plastering trade, however, suggests that any negative (-) LSI is unacceptable, and that many different plaster problems will quickly develop when the LSI is slightly negative for any amount of time. That claim is false.

We will review the effects of aggressive water on quality applied pool plaster and compare that to what happens when improper plastering workmanship is performed.

The onBalance research team has studied this issue and performed many experiments. Our studies have shown that maintaining water with a slightly aggressive LSI of -0.1 to -0.3 consistently for a full year will UNIFORMLY dissolve about 3 to 4 pounds of plaster material (calcium carbonate) from the surface of a 20,000-gallon pool. The result of that will not be visible. No discoloration or other issues.

It has also been determined that maintaining a moderately aggressive LSI of -0.6 to -1.1 for one year will dissolve about 12 pounds of calcium being uniformly removed from well-made plaster. The reason for the greater effect is because the LSI is logarithmic. And the reason for the uniformity is that quality plaster is relatively uniform across its face, so etching deterioration would also be even across the surface. The effect of maintaining water in that condition for one year will also not be visible to the naked eye. It takes several years of maintaining water in that condition for the etching to become visible. But that also does not cause gray mottling or white spotting and streaking discoloration.

What about poor plastering workmanship? Does that have a detrimental effect even in balanced water? Let’s compare the above with what happens when the plastering application isn’t done so well.

Poor quality plaster was made with a high water to cement ratio (which causes porosity and micro-cracking), a high amount of calcium chloride (hardening accelerator), and they were placed in balanced water (+0.3 LSI) one hour after final troweling. In three days, the calcium level of the water increased by 160 ppm, which means the equivalent of 26 pounds of calcium carbonate being uniformly dissolved and removed from the plaster surface of a 20,000-gallon plaster pool.

Craze cracking of the plaster coupons was also visually observed. Petrographic analysis and studies by the Portland Cement Association have shown that a high amount of mix water and adding calcium chloride can result in micro-cracking and excess porosity, which is not visible to the naked eye.

Incidentally, a similar negative effect will also occur to quartz and pebble pools. It is not the aggregate that is dissolved and affected; it is the paste. Both quartz and pebble projects contain a significant amount of Portland cement paste which is what is negatively affected by the above issues.

The increase in the porosity and micro-cracking of a plaster surface can also be caused by adding excessive water to the hardening surface and troweling that water into the plaster finish. That improper
and detrimental troweling practice (adding water while troweling) causes irregular patterns (streaks and
spots) of porosity damage to a plaster finish which slowly degrades and becomes distinctly visible and
discolored over time.

The Long-Term Consequences of Performing Poor Workmanship

The above poor-quality plaster conditions allow even balanced pool water to penetrate and enter through the porous and micro-cracked areas and dissolve the soluble plaster compound known as calcium hydroxide and remove it thereby creating greater porosity, weakness, and low durability over time. That increasing porosity is also what causes colored pigment plaster to lose its’ color and turn “whitish” over time.

Of course, porosity and cracking of the plaster surface will also allow water that is aggressive to enter those areas and dissolve calcium hydroxide and even some non-soluble plaster compounds. But that would not happen if the plaster surface hadn’t been already compromised by poor workmanship.

Improper Plastering Practices Far Worse than Poor Water Balance

As can be seen, poor plastering applications can result in a greater loss of calcium from a plaster surface (and an increase in the calcium level of the pool water), than does slightly aggressive LSI water for whole year! And if excessive water troweling occurs, that also contributes to increasing porosity and micro-cracking which leads to more rapid plaster degradation in those affected areas.

Unfortunately, a lot can go wrong with the plastering application, including with quartz and pebble pools that result in defects, discolorations, and shorten the life-span of plaster finishes. Therefore, why hasn’t the pool plastering trade established some standards and limits for the water/cement ratio, calcium chloride content, water troweling, and other problematic issues?

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