Calcium Buildup On Tiles

[tkalla2000]

Hello, I just took on service on a pool that has been running for about 2 weeks. The tile on the negative edge has allot of what looks like calcium buildup on it. I ran some tests on the water and came up with this.

TA - 130

CH - 300

PH - 8.0

TEMP - 92F

SALT - 2900

According to my calculations that puts it about 0.85 on the saturation index.

I lowered the PH to 7.6 and the TA to 70. I figure that will put the saturation index to 0.2, will this stop this buildup?

most of the buildup seems to be around the grout.

I this buildup coming from deposits in the water or is it leaching out from the grout?

Also is the CH level ok for a new plaster pool?

[chem geek]

Most likely the buildup is due to the over-saturation of calcium carbonate which, as you calculated, gives a very high saturation index. I'm not sure how you get to a +0.85 since my calculations, assuming CYA of 50 ppm (you didn't list the CYA level), would be +0.49 which is still too high. You can use The Pool Calculator which would give +0.54 with your numbers, again assuming the CYA is 50 ppm.

Lowering the pH and the TA should help, but with a new plaster pool the pH will tend to rise as will the CH so you'll be needing to adjust it more frequently. Also, this sounds like a saltwater chlorine generator (SWG) pool and the pH will tend to rise because of that as well. Lowering the TA helps, but so does using 50 ppm Borates in the pool (added either with a combination of separately added 20 Mule Team Borax and Muriatic Acid or with Boric Acid such as Proteam Supreme Plus). The negative edge adds to aeration which will tend to make the pH rise -- this isn't a very good combination with an SWG pool. Finally, having the CYA at 70-80 ppm helps reduce chlorine loss from sunlight which should let you turn down the SWG ontime and that will reduce the rate of pH rise.

Since lowering the TA reduces the amount of carbon dioxide outgassing that raises the pH, that's the best approach unless you see the CH climbing rapidly in which case you'll need to dilute the water at some point to lower the CH.

I assume you know how to lower the TA which is not done with acid alone, but with a combination of aeration and acid at lower pH as described in this link.

Richard

[tkalla2000]

Thank you very much. I will give that a try and see if the buildup stops.

[Mark357]

You note that this is a new pool. As new plaster is curing it gives off Calcium Hydroxide which combines with the Carbonates and Bicarbonates (Alkalinity) in the water to form Calcium carbonate (Scale) and Hydroxide ions (pH increaser). As shown here:

Ca(OH)2 + CO3-- <-> CaCO3 + 2OH-

Calcium hydroxide + carbonate <-> Calcium Carbonate + hydroxide.

Or

Calcium hydroxide + bicarbonate <-> Calcium Carbonate + water + hydroxide.

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

If the water is not kept low in pH, Alkalinity and Calcium during the first 15 days scale will get on everything. If the SI is high during the plaster curing phase the high levels of calcium carbonate will act like a precipitating agent to force supersaturated Calcium Carbonate out of the water.

My calculations for the SI are +0.54 and + 0.70 using two different methods. It is important to stay between 0.0 and 0.20 for the first 3 months. After the first three months the range is 0.0 to 0.3. You can add a "Stain and Scale" control chemical to help.

The aeration caused by the fountain and the aeration caused by the salt cell cause some of the carbon dioxide to be outgassed. Carbon Dioxide is lost as it goes over the fountain this immediately causes three effects:

1) The pH is increased. This lowers the solubility of Calcium Carbonate and the equilibrium between carbonate and bicarbonate shifts towards carbonate.

2) The increase in the carbonate concentration due to the pH increase creates more calcium carbonate.

3) The solubility of Calcium Carbonate is proportional to the level of dissolved Carbon Dioxide in the water. As the carbon dioxide level drops, the water can hold less calcium carbonate and the Calcium Carbonate precipitates out of solution creating scale.

The same thing happens in a salt cell due to the effects of the aeration causing the loss of carbon dioxide and the reduction of hydrogen at the cathode given by this equation here:

2H2O + 2e- --> H2 (g) + 2OH- which creates hydroxide ions which raises the pH even further.

The other challenge that you are dealing with is the warm water temperature at 92 F. When calcium is dissolved in water heat is given off (exothermic), therefore its solubility is inversely proportional to temperature. Also, Carbon Dioxide is less soluble in hot water. At 92 degrees F water can only hold about 90% of the dissolved Carbon Dioxide as water at 84 degrees F. And, as water is heated, the equilibrium between bicarbonate ions and carbonate ions is shifted toward carbonate. There is not much you can do about the temperature of the water.

I would recommend that you avoid using any chemicals that contain Calcium, like Calcium Hypochlorite. Try to get the Calcium below 270.

Reference(s) (General) http://www.advancedaquarist.com/issues/mar2002/chem.htm

[Mark357]

It is also important to note that the salt should not have been added for the first 30 days. The salt can damage the fresh plaster. You should check the salt cell for scaling as it is almost definitely scaled.

I would like to give special recognition and thanks to Richard, aka Chem Geek, for all of the help he has given to so many people. He is a super-smart and really good person.