pH rise in new plaster?

[JamesW]

There are many people who experience pH rise in newly plastered pools. I think that this pH rise can be greatly reduced or eliminated by maintaining a CSI of 0.0 to +0.3 and by using a pH in the 7.7 to 7.9 range when the CSI is going to be kept slightly negative for some reason (such as using a SWG).

The initial pH rise is due to the formation and release of calcium hydroxide Ca(OH)₂. During the first 30 days, most to all of the calcium hydroxide in the surface of the plaster should become calcium carbonate CaCO₃.

Ca(OH)₂ + HCO₃^- <> CaCO₃ + H₂O + OH^-

As long as the CSI is equal to or greater than 0.0, then the calcium carbonate in the surface layer should remain passive. If the CSI is less than 0.0, then the calcium carbonate will continue to dissolve into the water, which increases the pH. This will also expose calcium hydroxide in the underlying plaster, which will also cause pH rise.

Even though plaster will continue to cure for many months, the plaster below the surface should not affect the water chemistry as long as the passivation layer at the surface is undisturbed by either dissolution or by mechanical abrasion.

The rate of pH rise should be a function of how negative the CSI is and the pH. Since the calcium carbonate is largely attacked by hydrogen ions, the concentration of hydrogen ions should be a primary factor.

CaCO₃ + H⁺ <> Ca²⁺ + HCO₃^-

For example, if the pH were 7.8 and the CSI were -0.1, you would have some pH rise due to the dissolution of plaster. If you lowered the pH to 7.2, then the CSI would be -0.7 and there would be 4 times as many hydrogen ions. The concentration of carbon dioxide would also be much greater, which will increase the rate of pH rise due to the offgassing of carbon dioxide.

Due to multiple factors, I think that the rate of pH can be greatly reduced or eliminated by maintaining a CSI of 0.0 or higher. If there is some reason that the CSI is kept slightly negative, then a pH in the 7.7 to 7.9 range should be especially helpful.

 

[chem geek]

The curing of plaster is mostly the following equation:

2Ca₃SiO₅ + 7H₂O --> 3CaO•2SiO₂•4H₂O + 3Ca(OH)₂ + heat
Uncured Pool Plaster + Water --> Cured Pool Plaster + Calcium Hydroxide

where the uncured plaster (technically, anhydrous portland cement, tricalcium silicate) absorbs water into its crystal structure and releases calcium ions that raise Calcium Hardness (CH) as well as hydroxyl ions that raise the pH. For every 10 ppm rise in CH, it takes 25.5 fluid ounces of Muriatic Acid per 10,000 gallons to compensate for pH. Or equivalently, when the plaster is curing then the requirement to add 1 gallon of acid in 10,000 gallons roughly corresponds to an increase in CH of 50 ppm.

The point you are making is how to deal with the calcium hydroxide. There are basically four startup methods as follows (much of this comes from some excellent blog posts by Kim Skinner at Pool Genius Network:

• the "Acid" startup where enough acid was added to lower the pH to 4.5 and alkalinity to zero for three days (-4.0 LSI)[/*:m:1c7d6niu]
• the "pH-Neutral" startup where one gallon of acid was added to an 8000 gallon pool, then balanced after 3 days (-0.8 LSI)[/*:m:1c7d6niu]
• the "Traditional" startup where the water was maintained in a slightly alkaline (basic) water condition (0 LSI initially, but jumps way up as pH rises a lot)[/*:m:1c7d6niu]
• the "Bicarb" startup which involves water pre-treatment with sodium bicarbonate (roughly +0.3 to +0.6 LSI)[/*:m:1c7d6niu]

The dosing for the Bicarb startup essentially adds enough bicarbonate to the fill water before adding it to the pool so that the TA + CH is equal to 500. I suppose if the CH is very low, one could increase that somewhat first, though it does not need to be increased very much.

The "Bicarb" startup results in the lowest CH increase and the least amount of plaster dust and the most solid and smooth plaster surface. As you point out, providing enough bicarbonate allows the calcium hydroxide to from calcium carbonate integrated into the plaster surface. The pH with the "Bicarb" procedure usually has the pool water be at a pH of 8.3 since the pH rises from some of the curing, but if after filling the pool with the bicarbonated water the pH ends up higher than 8.2, then it can be lowered with acid to around the 7.7-7.9 range. For the first month, the TA remains high and the pH is just kept below 8.2. After the first month, acid is added to get the TA down to the desired range and the pH ultimately reaching its target as well.

The main risk for the "Bicarb" startup is metal staining if there are metals in the fill water so one must deal with that via removers or sequestrant products (if metals are present).

 

[JamesW]

The point you are making is how to deal with the calcium hydroxide.
:
As you point out, providing enough bicarbonate allows the calcium hydroxide to from calcium carbonate integrated into the plaster surface.

What I was also trying to emphasize is that maintaining a neutral to positive CSI will prevent the calcium carbonate in the surface layer from being dissolved. If the CSI is negative, then the calcium carbonate will dissolve and cause pH rise (similar to adding sodium carbonate).

The calcium carbonate primarily only forms on the surface. Below the surface, the calcium hydroxide remains as calcium hydroxide because it is too deep for the water to react with and convert it into calcium carbonate. However, if the calcium carbonate is dissolved, then it will expose the underlying calcium hydroxide to the water and the hydroxide will be released, which will also cause the pH to rise.

So, my primary point is that if one wants to minimize pH rise from new plaster, then they should maintain a neutral to positive CSI. Adding too much acid continually eats away at the plaster causing the plaster to dissolve and further increase the pH.

My second point is that if one chooses to maintain a negative CSI, for whatever reason, then using a higher pH would cause less dissolution of plaster and less pH rise than a higher pH for the same CSI. For example, the pH rise at CSI = -0.1 and pH = 7.2 will be much greater than the pH rise at CSI = -0.1 and pH = 7.8.

The third point is that the amount of excess carbon dioxide at a pH of 7.5 is about 2.5 times as much as the excess carbon dioxide at a pH of 7.8. This should cause about 2.5 times as much pH rise due to carbon dioxide offgassing.

 

[JasonLion]

Raising PH, and thus CSI, does reduce the total PH increase, but simply keeping CSI positive isn't enough to stop PH increases. Even with a bicarbonate startup, where CSI is very positive, there is still some PH increase.

 

[chem geek]

I understand the point and yes, you will reduce the amount of pH rise by essentially limiting the plaster curing more to its surface as is done with the Bicarb startup procedure that uses a more positive CSI. However, as Jason points out, it doesn't eliminate this fairly rapid pH rise that occurs in the first weeks to month or so, though it certainly is better than the other startup methods that require a lot more acid and usually result in lower quality plaster surface.

When Kim Skinner wrote about various plaster startup methods at PGN, he had ran tests with plaster coupons and found that after 3 days the Bicarb startup had a 0-2 ppm CH rise, the traditional method had a 7-10 ppm CH rise, the pH neutral (moderately aggressive) had a 15-20 ppm CH rise, and the acid startup had a 60-80 ppm CH rise. Now I don't know how to properly scale his tank tests, but I do know that a 10 ppm CH rise would have the pH go from 7.5 to 8.4 if the TA were 100 and would need 26.8 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid) per 10,000 gallons to compensate. Another advantage of the Bicarb startup is that the TA will be higher which will help lower the amount of pH rise which in the example I just gave would only rise to 7.9 if the TA were 200 ppm.

 

[onBalance]

I agree with what has been written above, including JamesW main points. I would also like to suggest a couple of things just for clarification.
As stated above, the pH will rise slowly after a Bicarb start-up is performed, due to off-gassing of carbon dioxide, which means a little acid will be needed periodically. But an effective Bicarb start-up prevents calcium hydroxide (which is about 15%-20% of the cement) from dissolving into the pool water and does prevent the pH from rising in that manner. It is having a high TA that accomplishes that, and which is why no plaster dust develops with this start-up program.

It is a low TA tap water that allows (or perhaps facilitates) the dissolving of calcium hydroxide from a plaster surface. And then due to the low TA (which has very little buffering ability), the pH rises quickly and very high (from the dissolution of calcium hydroxide), and will be higher than would otherwise occur with water that has a high TA. When one is adding a lot of acid (the first few weeks after plastering) to lower the high pH and perhaps a somewhat high TA of 120 down to 80 ppm, the result is the same. So the problem is having to deal with a very high pH and a lot of plaster dust during the first month.

Waiting until the plaster surface has been carbonated (converting calcium hydroxide into calcium carbonate) which usually takes about one month after being filled with water, the TA can then be lowered to appropriate levels to achieve a CSI balanced water.