Why does new plaster that is curing raise pH?
Why does new plaster that is curing raise pH?
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The thread that Jason linked to does address this question very well. But I will try to summarize.
When the CSI is always at least +0.6 for quality new pool plaster (including quartz and pebble finishes) for about the first 20-30 days, then virtually no pH rise (increase) will be due from the curing plaster. Any pH rise would be from the outgassing of carbon dioxide (which raises the pH) when the pH is below 8.4. A high CSI prevents calcium hydroxide (which comprises about 20% of a plaster surface) from being dissolved from the plaster surface and into the pool water and affecting (raising) the pH of the pool water. Essentially, after a month has passed, all of the calcium hydroxide at surface (but not below the surface and within the plaster matrix) will have been converted into calcium carbonate. The CSI is relative to calcium carbonate, not calcium hydroxide which is much more soluble.
If the CSI is not high enough during the first month, then calcium hydroxide may be dissolved from the plaster surface and cause the pH of the pool water to rise above 8.4. If that happens, then you know why and what happened. Visible plaster "dust" is likely a sign that the CSI was not effectively high enough, or possibly from poor quality plaster.
Wow, I really do understand this. You are saying you want a high CSI for a month, but seems that the most common thing is to use an acid start-up and keep the pH in the normal range, which would certainly make the CSI lower and potentially damage the surface?
I need to go back and read all your very detailed posts on start-up.
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onBalance is advocating a bicarbonate startup, which is probably the least common startup method in practice, despite it's various advantages.
The most common approach is an acid startup which etches off the outer layer of plaster. It makes up for that fairly large disadvantage by being easy to perform and minimizing plaster dust compared to a "do nothing special" startup.
Yes, an acid start-up removes most of the calcium hydroxide on the plaster surface. Therefore, the plaster finish is no longer smooth and dense (as seen under magnification) as it was when newly finished and before filling the pool with water. That means the surface has been aged somewhat and will stain easier over time and not last as long as it would have with a better and non-aggressive water start-up procedure.
An acid start-up is so acidic and aggressive, that it prevents the dissolved calcium hydroxide in the water from becoming "plaster dust" by keeping it dissolved. The dissolved calcium hydroxide is converted into soluble calcium bicarbonate via the aggressive acid start-up.
Plaster dust is formed by the calcium hydroxide reacting with the alkalinity (bicarbonate) of the pool water to form insoluble calcium carbonate (which is what plaster dust is). An acid start-up removes the alkalinity in the water so that calcium hydroxide does not turn into plaster dust.
Thanks for the explanation.
this post and this post for more details. However, in practice 95+% of the pH rise is from carbon dioxide outgassing because the thickness of the carbonated layer (i.e. calcium hydroxide becoming calcium carbonate) is only 5-10 microns thick.
Also, with the higher TA during the bicarb start-up, the pH can theoretically rise from carbon dioxide to more than 8.4 (up to 9.0, depending on TA level), but in practice the rate of rise slows down considerably at higher pH, at least from carbon dioxide outgassing.
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Leaving aside any contractual/warranty issues, I have a general question regarding plaster startups. If the PB begins with an acid startup and then leaves it to the pool owner to take over, is there any benefit to trying to revert to a bicarbonate startup process or has the damage been locked in?
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I would assume since most cementious products take 28 days to reach their rated strength in PSI (industry considered cured), then saturating the water with bicarbonate after the acid start may be too late. I'm sure it could provide some benefit, but it may not be enough since the damage has already been done and the plaster is mostly finished curing.
I have yet to find a plaster contractor that is familiar with the bicarbonate start-up. I got a lot of strange looks when I wheeled 200 pounds of arm and hammer to the pool once it was ready to be filled.
Many of the plaster contractors perform an acid wash prior to filling to pool. I suspect this is especially hard on the fresh plaster, perhaps more so than the acid start, but it seems this has become the norm.
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