Posted by Kim Skinner on August 27, 2009 at 4:19pm
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? The following are results of our study.“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 80 ppm, and the water was maintained continually with a Langelier Saturation Index (LSI) ranging from -0.6 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 in the amount 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 from poorly made plaster that has been maintained in balanced water. The beginning calcium hardness level was 180 ppm and was maintained in a continual 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 common improper plastering practices. Only two days later, 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 already 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 quickly 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 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 or discoloration.There are data from other sources that support the results of our study. The ACI and (PCA) have documented the negative and detrimental effects from using high water-to-cement ratios, and high calcium chloride contents.Also, an objective review of the data from 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, many of the test pools were reported to have had either visible deterioration or discoloration in just six weeks time even though they were in balanced (non-aggressive) water. 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.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 less-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. This improper practice simply adds to the negative effect mentioned above.It should not always be assumed that an increase in calcium (from the tap water readings) had 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. Additionally, any discoloration of plaster, whether white plaster with dark blotches or gray streaks, or dark colored plaster with white streaks, should not automatically be blamed on aggressive water. 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. 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 and discolored due to improper plastering practices.onBalance is a consulting group comprised of Que Hales, Doug Latta and Kim Skinner
Charlie, borate would not cause this problem. Another possibility is a high pH over 8.4, a high calcium level over 800 ppm, and a high alkalinity combined. That could cause a scaling condition. But again, you stated that the chemical readings are okay, so perhaps water chemistry is not the problem. Question: After rubbing off the white residue, can you scratch off plaster with your fingernail or plastic object? Does it "dust" everywhere, or just in most areas of the pool?
i'm knew at this - i see you commented - i had just wanted to add that i had used borate extreme and wondered if that would have had anything to do with it? would you recommend acid washing even though i'll have to get rid of the borate? i guess i'll bring back the marble duster to show him what it does. thanks for your reply. i've never seen anything like this before. i think the water does have a lot of iron in it i have experienced before but it's not effecting the chemisty this year.
Hi Kim, i wrote back to you the other day about being able to wipe my hand along the marlbe dusting of one my pools - 2 yr old marble dust, it feels very smooth, almost too smooth, and when you wipe your hand over it a layer of marble dust comes off & clouds up. i wanted to write back & mention that the only thing different that i have done with this pool to any other pool this year is to put borate in it which i've never experienced before. other than that i've hardly had to do anything to it bc the borate seems to have been maintaining the chlorine in it - but maybe that has been the deceiving factor? or is the calcium too low at 300ppm? - it shows ideal when tested? looks like i should do light acid wash in the the spring but now i don't want to lose all the borate i just put in but i can't have all this marble dust just wiping off can i and turning into cloudy grey? if i don't wipe it off it feels slippery and like very very soft cement - but looks good. thanks for any advice, Charlie
Hard to say, but there are several possibilities. It would require very aggressive water to cause that condition, but you state that the water is balanced. Another possibility is very poorly made and porous plaster that is deteriorating. Review my two recent blogs on "Making Durable Pool Plaster" for possible poor workmanship issues. Of course, I assume that the pool hasn't been painted?
do you happen to know why, on a second year marble dusted pool, i can rub my hand on the marble dust, it feels soft,kind of slippery & a top layer of dust appears to come off into the pool and clouds like a wet dust & goes cloudy grey when it wipes into the pool. all chems are ok, well calcium is at 300ppm. Thanks, Charlie
There are a couple of ways, but this is best done by a professional cement lab. We use R.J. Lee Group. They use backscattered electron imaging (BEI) and scanning electron microscope (SEM) and compare with known samples and determine or estimate w/c ratio.
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you said said there is technology for analyzing cement to water ratios. How is this done?