Reprinted with permission from Kim Skinner of On Balance:
The concept of the “Zero Alkalinity” process is to make pool water aggressive enough (by adding acid) to dissolve and remove stains or scale from cement-based plaster surfaces, including quartz and pebble swimming pools. Unfortunately, this process may remove some of the plaster surface material.
Assuming a total alkalinity of 100 ppm, it requires 4 gallons of muriatic acid (31.45% strength) per 20,000 gallons of pool water to reduce to zero alkalinity, and to a pH of 4.5. That acid treatment over a 7 day period can dissolve and remove about 8 pounds of plaster material (equivalent to 50 ppm of calcium carbonate) from a fully cured surface. So is that good or bad for a plaster pool?
The reality is that 8 pounds of plaster material dissolved from a 20,000 gallon pool is not significant and would barely be seen under magnification. If stains are removed that improve the aesthetic appearance of older pools, then that can be an acceptable program. If the acid process only removes dirt and scale deposits and not calcium from the plaster surface, well, all-the-better.
It is an unfortunate fact that some new white plaster pools develop smooth, but blotchy and darkened gray mottling discoloration within a few months of plastering. Also, some new dark colored plaster pools develop a whitish and streaky discoloration soon after plastering. Sometimes it is an immediate plaster dust problem that sticks on the plaster, or a porous surface issue and loss of color pigment during the first few months. Those two issues are often caused by improper plastering practices and/or the failure of plasterers to ensure and supervise the proper filling of the pool with balanced tap water. The chemical start-up program may not be at issue.
Representatives of the National Plasterers Council (NPC) actively promote the Zero Alkalinity program as a “solution” for the above problems. However, instead of recommending 4 gallons of acid, they advise adding 10 gallons of acid to 20,000 gallons of pool water, more than double the amount needed to reduce to zero alkalinity. That amount of acid can lower the pH below 3.5, lower the LSI to a negative (-) 5.0, and dissolve about 20 pounds of plaster material from the surface. (Twenty pounds of plaster material is equivalent to 120 ppm of calcium carbonate dissolved from a plaster surface which then increases the calcium hardness of the pool water).
Yet, if adding all that acid doesn’t improve the appearance of the plaster well enough, they advise to double the dose of acid for another week! The plaster apologists suggest that removing 40 pounds of plaster material is okay when trying to correct the above plaster defects. That is too much; it is detrimental to the plaster surface, a disservice to the pool owner, and it destroys the evidence of the real cause of the plaster defect.
It results in an etched surface that was originally smooth, which stains easier and quicker by dirt and metals after the acid program has been performed. When the pool becomes stained and aged within a year after a Zero Alkalinity treatment, who is going to be blamed? Whoever is maintaining the pool water chemistry, not the plasterer.
To further undermine service techs, the plaster industry advocates that pool water with a slightly negative LSI (-0.1 to -0.3, which is within APSP standards), is detrimental and that it will cause everything from craze cracking to gray mottling to plaster spalling (delamination). Yes, even the same gray discoloration that the so-called Zero Alkalinity process attempts to remove. Obviously, that is contradictory and completely false. No wonder they claim that pool plaster will only last about 7 years. But the fact is plaster will last 20 years if plastered and maintained properly.
On Balance, January 2013
Mike the Poolman
Pool Service & Repair in Folsom, CA since 1995