Can a degrading marcite pool surface contribute to the calcium level increasing?

[Lux Man]

I haven't visited for awhile, as you can see I am in the industry. This past year I have directed dozens of pool owners to this site because it is the single most informative and accurate site on the internet.

Here is my scenario- this is an indoor pool the water was replaced on 7/1/13. I balanced it for CAH at 300ppm. It has fill water coming in which measures 20 ppm. The pool evaporates approx. 1/4" per day. It's 20 x 30 x 4 (18,000 gal) therefore, I have calculated that it has essentially evaporated 85,500 gallons since 7/1/13. I believe, if my thinking is correct that I should have seen the CAH increase by 120 ppm bringing it up to about 420 ppm however it measures much closer to 620 ppm. The reason for my concern is because the pool does have signs of staining beginning to show. Prior to 7/1/13 it have major stain issues, it was at that time that I encouraged the customer to install a water treatment system which has all but eliminated any staining. We did drain and acid wash the pool prior to re-filling it on 7/1/13. The CAH when I took over the pool in 2012 was 850 with a TDS of 2000+. A co-worker had been handling the pool this year but I am back on it for the winter. I had been unaware of the issues beginning to creep back in until this fall. BTW- it is a bromine pool.

As a side issue this pool has a ever increasing PH problem. Every single week it takes 1 gallon of muriatic acid to bring it back into the 7.4-7.6 range. I have tried running the ALK as low as 70-80 and as high as 130-140 it seems to make no difference. I have also wondered if the degrading surface can possibly effect the PH.

So the over-all question is what effects can a degrading surface have on water chemistry, if any?

 

[chem geek]

If the surface was composed of some calcium carbonate, then if that were dissolving into the water it would raise pH, TA, and CH. If you add acid to compensate for the pH, then you still have some TA rise and of course the CH remains higher. For an initial dissolving that increases CH by 10 ppm, the TA also increases by 10 ppm and the pH rises (how much depends on the TA level, but if at 100 ppm it goes from 7.5 to 7.9. After adding acid to lower the pH back down, the CH is still 10 ppm higher, but the TA is only about 5 ppm higher.

So are you seeing that over time both the CH AND the TA are rising? If so, then that would indicate dissolving of calcium carbonate. However, that normally shouldn't happen if the water is saturated with calcium carbonate.

I presume you are sure that no one was using a Cal-Hypo shock product, even to activate more bromide to bromine.

 

[Lux Man]

Only the PH rises over time. The ALK drops because I am putting acid in it so then I have to compensate for that as well. No one puts anything in the pool other than myself and we do not use Cal-Hypo.

I did read on the net that marcite has calcium hydroxide in it and that is does leach into the pool if the surface is degrading. I read it in a report done in Florida which was trying to address the issues pertaining to marcite. Would calcium hydoxide show up as calcium using the Taylor reagents? I'm thinking maybe I stumbled upon the culprit- maybe...

 

[Lux Man]

It looks like calcium hydroxide is basically lime.

 

[chem geek]

When plaster is cured it produces calcium hydroxide, but during curing the water is supposed to be saturated (or even somewhat over-saturated) with calcium carbonate and the calcium hydroxide as a result should get converted to calcium carbonate in place. That "seals" the plaster. Marcite is another name for pool plaster.

The following are possibilities for what happens with degrading plaster through various mechanisms and for other situations that have either CH rise or the pH rise. In the following for the acid amounts (to keep pH from rising) I assume a 10,000 gallon pool and the acid amounts are gallons of full-strength Muriatic Acid (31.45% Hydrochloric Acid). I am using a reference CH increase of 100 ppm for the comparisons (for those situations where CH would increase).

............................................................ ...................... CH . Acid ... TA
GOOD: Plaster Curing in Place ...........................................0 .... ? ........ 0 ..... (TA drops when you add acid, but how much is needed is unknown)
BAD: Plaster Calcium Carbonate Dissolving.................... +100 .. 1.0 ... +48
BAD: Plaster Curing and Calcium Hydroxide Dissolving ... +100 .. 2.0 ....... 0
OK: Evaporation and Refill .......................................... +100 ... ~0 ...... ? ..... (TA change depends on fill water TA content; CH rise is from fill water)
OK: Carbon Dioxide Outgassing ........................................ 0 .... ? ........ 0 ..... (TA drops when you add acid, but how much is needed is unknown)

Technically what happens in each situation is the following.

GOOD: Plaster Curing in Place

Plaster curing is mostly the following which produces soft calcium hydroxide:

2Ca₃SiO₅ + 7H₂O ---> 3CaO•2SiO₂•4H₂O + 3Ca(OH)₂(s) + heat
Tricalcium Silicate + Water ---> Calcium Silicate Hydrate + Calcium Hydroxide + heat
Soft Uncured Plaster + Water ---> Hard Cured Plaster + Soft Slaked Lime + heat

If your water is properly saturated (or over-saturated) with calcium carbonate is as the case during a bicarbonate startup and should be the case one month after any kind of startup, then the following occurs in-place in the plaster:

Ca(OH)₂ + HCO₃^- ---> CaCO₃(s) + H₂O + OH^-
Calcium Hydroxide + Bicarbonate Ion ---> Calcium Carbonate + Water + Hydroxide Ion
Soft Slaked Lime + "TA" ---> Hard Limestone + Water + "High pH"

So the net effect of properly curing plaster is the following which I re-write in terms of carbon dioxide instead of bicarbonate to show that this process looks exactly the same as outgassing (removal) of carbon dioxide from the water:

2Ca₃SiO₅ + 3CO₂(aq) + 4H₂O ---> 3CaO•2SiO₂•4H₂O + 3CaCO₃(s) + heat
Tricalcium Silicate + Carbon Dioxide + Water ---> Calcium Silicate Hydrate + Calcium Carbonate + heat
Soft Uncured Plaster + Carbon Dioxide + Water ---> Hard Cured Plaster + Limestone + heat

BAD: Plaster Calcium Carbonate Dissolving

CaCO₃(s) ---> Ca²⁺ + CO₃^2-
Calcium Carbonate in Plaster ---> Calcium Ion in water + Carbonate Ion in water
Part of Plaster ---> "CH" + "TA"

In practice at normal pool pH it is bicarbonate that is mostly formed so the pH rises:

CaCO₃(s) + H₂O ---> Ca²⁺ + HCO₃^- + OH^-
Calcium Carbonate in Plaster + Water ---> Calcium Ion in water + Carbonate Ion in water + Hydroxide Ion in water
Part of Plaster + Water ---> "CH" + "TA" + "Lye"

BAD: Plaster Curing and Calcium Hydroxide Dissolving

This is the same as plaster curing shown earlier except that the calcium hydroxide produced goes into the water instead of being converted to calcium carbonate:

2Ca₃SiO₅ + 7H₂O ---> 3CaO•2SiO₂•4H₂O + 3Ca²⁺ + 6OH^- + heat
Tricalcium Silicate + Water ---> Calcium Silicate Hydrate + Calcium Ion in water + Hydroxide Ion in water + heat
Soft Uncured Plaster + Water ---> Hard Cured Plaster + "CH" + "Lye" + heat


In your situation, 85,500 gallons of evaporated water in a pool that holds 18,000 gallons would have the CH rise from evaporation and refill of (85,500/18,000)* 20 = 95 ppm CH. I don't know how you got your calculation of 120 ppm.

Since you are seeing the CH rise but not the TA after adjusting for pH, then that does indicate calcium hydroxide from the plaster dissolving into the pool but for that to happen you'd have to degrade some part of the plaster surface and have the calcium hydroxide paste come out. Do you see any volcanoes at the bottom of the pool or drips on the side of the pool? Does the pool water have a low (negative) CSI? It doesn't sound like it does with the numbers you've given, but the symptoms do seem to be that of dissolving calcium hydroxide into the pool. Is the acid being poured too quickly in one place? Maybe the acid additions are stripping the calcium carbonate from the plaster letting calcium hydroxide underneath come through. Either that or the water treatment system has some terrible side effect. What exactly is the water treatment system being used?

 

[Lux Man]

Sorry I have been awol, tore a calf muscle playing tennis so I spent 10 days or so laying around and I wasn't thinking about this pool.


Regarding the CH rise of 120 v.s. 95 you are correct. Regarding volcano's and drips, yes I have those. Regarding putting the acid in one spot, yes to some degree. I don't put it in the same spot every time but I do put it near areas where I see some staining taking place. I can understand that while it may help the staining it may also be exasperating the surface degrading issue. Regarding the CSI it is going to vary from .2 to .6 . It's going to be around .2 in the days just after I have treated it to .6 when the PH has risen back up to 7.8-7.9 range. The fill water going in is PH 7.6, ALK 10, CH 20