The fact that “plaster dust” forms in newly plastered swimming pools when filled with balanced tap water is evidence that balanced pool water can “attack” (meaning “is aggressive to”) and dissolve certain cement compounds from a pool plaster surface.
Even “scale forming” water, with a Langelier Saturation Index (LSI) as high as +0.5 to +1.0, can attack (dissolve) soluble calcium hydroxide and calcium sulfate, which comprises about 20 percent of a plaster mix. Calcium chloride, an accelerant, generally added to the mix by the plasterers, is actually the most soluble compound of all and is readily dissolved by balanced water. Therefore, let’s understand that 0.0 (balanced) and slightly positive LSI water, is “aggressive” to those cement components of fresh plaster.
It is calcium hydroxide dissolved from a new plaster surface that is converted into “plaster dust.” After a couple weeks, the surface becomes carbonated (i.e., remaining calcium hydroxide is chemically converted to calcium carbonate), and the general dissolution of calcium hydroxide in the plaster surface stops. (The above issue is why onBalance promotes the “Bicarb” [high alkalinity] start-up method for new plaster pools. It prevents the dissolution of calcium hydroxide and the formation of plaster dust by effecting the hydroxide-to-carbonate conversion immediately and in-surface).
Of course, balanced pool water is not aggressive to, and does not dissolve calcium carbonate, calcium silicates, and calcium aluminates which comprise about 80 percent of new pool plaster. The important thing to understand is that the LSI predicts solubility and precipitation of calcium carbonate only, not calcium hydroxide.
Even calcium nodules (efflorescence) that form on cracks and delamination sites are evidence that balanced water can leach soluble compounds from within pool plaster.
When severe micro-cracking or localized areas of weak, porous, and deteriorating soft spots on plaster surfaces (due to improper plastering practices), allow pool water to penetrate behind the carbonated surface of cured plaster, even properly maintained and balanced pool water will begin to “attack” or dissolve away more soluble cement compounds that are still present sub-surface. This creates increased porosity in those specific areas of a plaster surface, which then eventually causes the plaster in those areas to turn whiter in color in comparison to the larger smooth dense areas surrounding the white porous spots, highlighted cracks or discolored streaks.
White soft spots or streaks are sometimes claimed to be “spot etching” or “etching deterioration” by plasterers, and only occurs when the water has been aggressive. The truth is that the severe combination of adding calcium chloride to the mix, excessive amounts of water while troweling, and late hard troweling by the plasterers, is what leads to “spot etching” and a defective plaster surface.
Also, if the water has indeed been maintained in an aggressive condition, then the spotting will definitely appear much sooner, because aggressive water will dissolve the soluble cement compounds in those defective areas far quicker than balanced water would. But that doesn’t mean the aggressive water caused the defect – it merely exploited a defect faster. In that case, there will also be evidence that the dense plaster surface surrounding the spots has been etched. Absent that, it wasn’t aggressive water that contributed to the white spotting discoloration problem.
The simple empirical evidence that aggressive water doesn’t cause white spotting is demonstrated by the fact that acid washes (the most aggressive treatment of all) and “zero” alkalinity (no-drain acid wash) treatments don’t result in white spotting on good quality plaster. And of course, spotting discoloration has been observed in balanced pools with no evidence of etching anywhere.
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