Kim – The LSI is Reliable for Plaster Pools

Using the Langelier Saturation Index as a guide for maintaining proper pool water balance and to protect pool plaster has become a mainstay in our industry, and we believe, for good reason.

We at onBalance have conducted several LSI experiments and have determined that maintaining a balanced LSI (-0.3 to +0.5) helps prevents scaling and prevent the etching of plaster finishes.

However, one plaster industry leader suggests that even when pool water is LSI balanced, if the alkalinity level is below 80 ppm (the industry’s Ideal minimum, not Operating minimum, which is 60), the pool water is aggressive and can discolor or damage pool plaster. Not so.

Anyone that is experienced in calculating the LSI knows that a low alkalinity can be offset by raising the calcium level and/or raising pH to keep the water balanced. To demonstrate and confirm that, we conducted several plaster experiments.

For example, two quality pool plaster coupons were made and cured in balanced water for 30 days.  At that point, one coupon was placed into water that had a low Total Alkalinity of 40 ppm, well below the minimum standard. The Calcium level was set at 350 ppm and the pH was maintained from 7.8 to 8.0, which off-set the low alkalinity and achieved a balanced LSI of about -0.1 to +0.1.

The other plaster coupon was placed into water with a low Total Calcium level of 90 ppm. The alkalinity was maintained at 130 ppm and the pH from 7.8 to 8.2, which together off-set the low calcium level and achieved a balanced LSI of about -0.1 to +0.3.

After six months in the water, the plaster coupons were removed from the water containers, and the water was tested for the calcium content to determine if any dissolution and etching of plaster surface material occurred. (An increase in calcium from the water’s starting point would indicate that calcium had been dissolved from the plaster coupon, which contains the only available source of calcium in this experimental set-up).

The result? There was zero increase of calcium in either water container.

The results of this experiment are totally consistent with the concept of calcium carbonate saturation as published in the municipal water supply, as well as the swimming pool chemistry industries. The results demonstrate that even when the alkalinity or calcium level is low – below the APSP’s minimum standard – but the LSI calculation shows that the water is balanced, then the water IS balanced, and therefore, not aggressive. But understand, we aren’t recommending that maintaining low alkalinity or calcium hardness levels is the preferred way. This is just information to help pool builders and service techs confront false claims about water chemistry and various plaster problems.

The False Claim Regarding Aggressive Water

It is very unfortunate that new pool plaster surfaces sometime develop gray mottling, white spotting and streaking discoloration, flaking (spalling), craze cracks, calcium nodules (from delamination), or other such defects, all of which are attributable to poor plastering workmanship, and yet, instead of advising the plastering company that improper plastering practices occurred, one NPC inspector (incorrectly) tells pool builders, service techs, and pool owners that aggressive water causes the above problems.

As mentioned above, the blame is often directed at low alkalinity or calcium hardness levels even though the actual LSI was balanced! But more importantly, the above listed plaster problems are NOT caused by aggressive water. Cement literature, empirical evidence, and even the NPC/Cal Poly/NPIRC study results demonstrate that.

Even when pool water IS aggressive, and etches plaster, it etches it uniformly, not in a mottled, spotted, or streaked pattern unless the plaster is non-uniformly defective as well.

It is our hope that pool builders, service techs, and pool owners will not be fooled by such non-sense. But it is happening far too often. Members of the onBalance team are often contacted regarding plaster problems, and we try our best to help them avoid being a victim of false claims.