How aggressive does water need to be, and how long does it take to visibly etch well-made plaster? And if plaster is not well-made, what can even balanced pool water do in terms of early deterioration and discoloration? Interestingly, the results of this study showed that a few improper plastering practices were harder on pool plaster than significantly aggressive water.
“Good” Plaster, “Bad” Water - The first part of our study was designed to determine the amount of calcium that can be dissolved or etched away from well-made pool plaster by aggressive water. Several plaster coupons were made with a proper (low) water-to-cement ratio, with no calcium chloride (an accelerator), and were allowed to cure and harden for 24 hours before being individually submerged in aggressive water for six months. The beginning calcium level of the water they were placed in was only 80 ppm, and the water was maintained continually with a Langelier Saturation Index (LSI or CSI) ranging from -0.5 to -1.1. After six months, the measurement of the calcium in the water tanks showed an average increase of 50 ppm. (Note: An increase of calcium in the water indicates a loss of calcium from the plaster surface). No discoloration of these coupons was visible, the coupons remained very white and smooth, and there was no visible shrinkage or craze cracking.
“Bad” Plaster, “Good” Water – The next part studied how much calcium can deteriorate or can be removed (dissolved), from poorly made plaster that was placed in balanced water. The beginning calcium hardness level was 180 ppm and in a LSI range of +0.2 to +0.5. The “poorly made” plaster coupons were made with a high water-to-cement ratio, a high amount of calcium chloride, and they were placed in water only one hour after final troweling, representing three possible and sometimes common improper plastering practices. After two days, the calcium level of the water in the tanks was tested, and the result was an average increase of 160 ppm! Since the ratio of plaster surface area of the coupons to gallons of submersion water was similar to that of an actual pool, we can calculate that this amount of calcium loss from a plaster surface is equal to about 26 pounds of calcium carbonate from a 20,000 gallon pool. Some gray discoloration and minor craze cracking of the coupons was also observed, which confirmed the known effects of using excessive amounts of calcium chloride and a high water-to-cement ratio.
As can be seen in this study comparison, three times more calcium was lost from the plaster surface due to the improper plastering methods than is lost from significantly aggressive water attacking well-made plaster for six months. A loss of calcium from the immediate surface increases porosity (and possible change in whiteness color) which directly affects the aesthetic durability of a pool plaster surface. Simply put, it reduces plaster’s ability to hold up well in a water environment and makes it more susceptible to staining and discoloration.
There is data from other sources that support the results of our study. The American Concrete Institute has documented the negative and detrimental effects from using high water-to-cement ratios, high calcium chloride contents, and submerging cement too early before sufficient and proper hardening.
Also, an objective review of the data from the studies of the NPIRC at Cal Poly reveals some similarities to our results. For example, in their Phase 1 protocol, the plaster sections that were submerged in water immediately after final troweling looked visibly worse in comparison to plaster sections that had six hours of drying time before being submerged in water. In Phase 2, several of the test pools were reported to have had either visible deterioration or discoloration in just six weeks’ time even though the water was balanced and non-aggressive. These results indicate something other than aggressive water chemistry as the cause of the deterioration and discoloration of the NPIRC test pools, although this information was not included in their final conclusions.
It should not always be assumed that an increase of calcium in the pool water (compared to the tap water readings) has to be the result of aggressive water dissolving calcium from the new pool plaster surface, but instead may be the result of improper workmanship practices. The discoloration of plaster, whether white streaks or spots on white or dark colored plaster, should not be blamed on unbalanced pool water. And grey mottling discoloration is most likely caused by calcium chloride added to the mix and/or late hard troweling. If the pool water has been maintained properly and reasonably well, and within APSP standards, a review with the plasterer regarding the quality and workmanship of their product is appropriate.
Another known improper plastering practice which correlates to the effects of a high water/cement ratio, is the act of adding water to a plaster surface while troweling. Doing this increases the water–to-cement ratio where the water is added and troweled into the surface. This procedure is known to cause increased laitance, increased porosity, weakness, softness, shrinkage (craze cracking), discoloration, and a non-durable finish. That is why both the ACI and the PCA advise against adding water while troweling a cement or concrete surface, which would include pool plaster.
Fortunately, today’s technology provides the ability to analyze pool plaster for its water-to-cement ratio, the content of calcium chloride, and whether etching has occurred due to aggressive water or whether the plaster has simply deteriorated, discolored, and leached (by balanced water) due to improper plastering practices and additives.
See also this post: ten-guidelines-for-quality-pool-plaster-t42957.html
And this post: white-spotting-of-new-plaster-pools-t56607.html