For 50 years, the pool industry has considered pool water within an LSI of -0.3 and +0.5 to be acceptable and balanced. But recently, and without providing any supporting science or research, the NPC is trying to convince the industry that any negative LSI (-0.1 to -0.3) is unacceptable and immediately detrimental to pool plaster. Their theory also suggests that an alkalinity below 80 ppm, or a low calcium level (below 200 ppm) independently as being automatically aggressive even if the LSI is positive.
It is also being claimed that those water conditions cause gray mottling (of white plaster), craze cracking, variable white blotchiness and streaking of colored plaster, random calcium nodules, and plaster flaking (spalling) to occur in just a few weeks or months. That is all nonsense.
Contradictions to Those Claims
Let’s consider that plaster consultants recommend adding 8 to 10 gallons of acid to a 20,000-gallon pool for one week to improve the aesthetic appearance of new plaster finishes, including quartz and pebble. That acid treatment (or acid start-ups, now disingenuously referred to as “hot start-ups” by plasterers) lowers the pH to about 3.5, the alkalinity to a negative number (below zero), and the LSI to -4.0. That water is about 5,000 times more aggressive than an LSI of -0.3. If that type of water doesn’t cause the above plaster issues, how then could water that is 5,000 times less aggressive do that?
Plaster experiments show that those acid treatments dissolve and etch up to 30 pounds of calcium material from the plaster surface and, assuming the surface is relatively of consistent quality, do so uniformly without causing discoloration. (btw, the total weight of plaster material in a 20,000-gallon pool is about 4000 pounds).
In comparison, maintaining an LSI of -0.3 for six months may dissolve about 1.5 pounds of calcium material. In other words, it takes at least 10 years of consistently maintaining an LSI of -0.3 to equal the same uniform etching effect of adding 10 gallons of acid for one week. Note: It is very difficult to constantly maintain a negative LSI because aggressive water seeks to balance itself as it liberates calcium from the plaster surface, and the pH generally rises.
The NPC/Cal Poly (NPIRC) plaster study was used to mislead the industry regarding aggressive water chemistry. Reviews by two Ph.D. chemists indicated that the (fabricated) conclusions don’t match the results and data. Those studies actually demonstrated that deterioration and spotted discoloration occurred in balanced (non-aggressive) water and in only four months. That implicates poor workmanship or materials as the problem, not aggressive water.
The Portland Cement Association and the American Concrete Institute do not blame rain (aggressive water) for causing rapid discolorations and defects on cement/concrete surfaces. Instead, they have identified improper workmanship and material issues as causes. Rain is about 1,000 times more aggressive than an LSI of -0.3 (which occurs often in pools), and yet, does not cause similar defects or discolorations (as mentioned above) on cement sidewalks and driveways, even after several years.
For many years, thousands of pools across the country have been intentionally (and unintentionally) maintained with slightly aggressive water to prevent calcium scaling, and they do not develop plaster discolorations or defects.
There are petrographic (forensic) plaster studies showing that improper and “short-cut” practices cause plaster discoloration and deterioration problems, some of which only become visible after pools are filled, and sometimes even after months. See this link: http://www.poolhelp.com/home/onbalance-research/consulting/
The Bottom Line
Maintaining slightly aggressive water (LSI -0.1 to -0.3) is acceptable. It does not cause rapid plaster deterioration and discolorations problems on quality applied pool plaster. The plaster problems mentioned above are prevented by following good workmanship standards. Right now, there are none. Good standards would provide accountability, slow down the plastering process, and give pool owners a quality and beautiful plaster finish that lasts 20 years.