Rapid pH increase with SWCG

Bperry

Gold Supporter
Aug 20, 2020
1,012
Knoxville, TN
Pool Size
27000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
CircuPool RJ-60
I’m curious h
I have similar plaster particles in my robot bag as well but my plaster is 20 years old now. Yours should not be failing at 3 years. Now I’m scared to replaster.
I’m curious how hard it would be to replaster it myself in basic white plaster. I’ve done some concrete, drywall, and stucco work so not totally unfamiliar with it but if there is really some expertise in mixing it to avoid early failures like this it makes me hesitant until I understood what those issues are.
 

onBalance

TFP Expert
LifeTime Supporter
In The Industry
Jul 25, 2011
1,218
Utah
I agree that this is a difficult one to figure out, but it is possible that the rising pH could be caused by calcium hydroxide being dissolved where the small plaster spots (that I see in the one photo) are deteriorating. It doesn't take much of that to affect the pH, and to a lesser degree the alkalinity and calcium increasing slightly.
I don't see much that can be done to prevent this from happening. I am curious to know if the spotting problem is everywhere in the pool?
 
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flippinGeo

Bronze Supporter
Oct 8, 2016
81
Maryland
I agree that this is a difficult one to figure out, but it is possible that the rising pH could be caused by calcium hydroxide being dissolved where the small plaster spots (that I see in the one photo) are deteriorating. It doesn't take much of that to affect the pH, and to a lesser degree the alkalinity and calcium increasing slightly.
I don't see much that can be done to prevent this from happening. I am curious to know if the spotting problem is everywhere in the pool?
It is quite widespread. On all the steps and horizontal surfaces. There is also a substantial surface change at/around the returns. I assume this is from chemical dosing, although I'm pretty careful with dosing, especially MA and CaCl.

I had mentioned in one post my ignorant mistake of using stainless brush during curing. I plead ignorance because our home (and pool) came with a stainless brush. I had only had vinyl previous so I figured a stainless brush was just standard with plaster. Oops. That has ended up being an expensive lesson. Only thing I can figure is that my stainless brushing had removed the finished surface, exposing the substrate. Now the water is just having an easy time breaking it down.
 

JamesW

TFP Expert
Mar 2, 2011
28,256
The alkalinity and calcium are rising at about the same rate, which seems to point to calcium carbonate or maybe calcium hydroxide.

The calcium has increased by 150 with no calcium being added.

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JamesW

TFP Expert
Mar 2, 2011
28,256
What is the current calcium?

Maybe allow the CSI to sit at about +0.3 to see if that helps.

Just be on the lookout for scale, which can develop quickly under the right (or wrong) conditions.
 

JamesW

TFP Expert
Mar 2, 2011
28,256
An increase of 150 ppm of calcium in 20,000 gallons is 25 lb of calcium carbonate.

That's a lot of calcium carbonate, but if it is being removed evenly, then it is less noticeable.

The amount of plaster dissolved includes cement and aggregate.

If we assume that only the cement is dissolving and the aggregate is not dissolving, then the total amount of plaster is higher than 25 lbs.

If the cement/aggregate ratio is about one part cement to 1.5 parts aggregate, the total amount of plaster dissolved will be about 62.5 lbs, which is 25 lbs cement and 37.5 lb of aggregate.

If the aggregate is calcium carbonate, then some will probably dissolve.

The total amount of plaster dissolved to increase the Calcium by about 150 ppm is about 25 to 62.5 lbs.
 

onBalance

TFP Expert
LifeTime Supporter
In The Industry
Jul 25, 2011
1,218
Utah
I was not aware that the calcium level has increased by 150 ppm. That is more than I think would be relevant to plaster being deteriorated and dissolving from small spots. I guess other possible reasons for the calcium level to increase has been ruled out? 25 lbs. of plaster is the correct number as the equivalent of 150 ppm of calcium.
 

flippinGeo

Bronze Supporter
Oct 8, 2016
81
Maryland
I was not aware that the calcium level has increased by 150 ppm. That is more than I think would be relevant to plaster being deteriorated and dissolving from small spots. I guess other possible reasons for the calcium level to increase has been ruled out? 25 lbs. of plaster is the correct number as the equivalent of 150 ppm of calcium.
At this point, yes, I believe we've ruled everything else out. No CaCl has been added since July 29. On the 30th I measured CH at 450. I didn't measure much from that point because typically I only measure CH monthly. Rapid pH rise has plagued this season so before I left for vacation on Sept 11 I brought pH down to 7.2, TA 60. Upon return on Sept 19 (9 days) my pH was 8.2, TA 120. At that point @JamesW suggested looking at CH. That is when the daily test began. CH was 490 (+40 since last CaCl dose). Since then it has continuously risen with no related chem additions. And it noticeably jumps any time I brush, or now use my robotic cleaner with brush head. As of 10/21 I was up to CH 580.

My pump cuts on at 10A (EST), I'll go out around 12 today to test again and include CH.

Those calcs you list for potential pounds of dissolved plaster seem really substantial. As you said it, if it is widespread it may be less noticeable, but I wouldn't think I've pulled +25lbs of plaster/aggregate out of this thing. I suppose it could be in the water column, or in the filter? Filter pressure is still good.
 

Newdude

Gold Supporter
TFP Guide
Jun 16, 2019
11,229
NY
And I know salt on concrete for deicing also promotes deterioration
This is true, but for a *whole different* discussion. That discussion is for sidewalks that get salted for de-icing at a concentration many times above sea water. Your pool is many times below the concentration of seawater. (10%…….. and most ‘chlorine pools’ hit 5%+ on their own).
 

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flippinGeo

Bronze Supporter
Oct 8, 2016
81
Maryland
Is evaporation and adding tap water also ruled out?
Plaster that becomes dissolved would not be in the filter.
So evaporation/adding well water is not typically an issue for me. We are pretty humid most of the summer and/or we get enough rain to keep within 1" of mean water line target. I've only added water from the well 2 times this season and each time, I estimate < 500 gals. The last time water was added was just before vacation on 9/10 to raise it above mid skimmer line in case of evaporation while away.

I try very hard to not use the well water in the pool because it is holds a pH typically between 4-5, and we have Sodium Chlorides from road deicer runoff (>1000 ppm).
 

JamesW

TFP Expert
Mar 2, 2011
28,256
The dissolved plaster is measured as calcium hardness and alkalinity increase.

Calcium carbonate increases the TA and CH by the same amount.

Since the TA and CH seem to be increasing at roughly the same rate, that points to calcium carbonate or possibly calcium hydroxide or possibly a mixture of both.

In addition to the dissolved part, you also have some aggregate that is not dissolved.

So, the 25 lbs is only the dissolved part that you don't see.

The undissolved aggregate is what you do see.

So, the total plaster dissolved is 25 lb. plus the undissolved aggregate, which will be somewhere between 0 and 37.5 lb.

A quartz or pebble aggregate will not dissolve at all.

A marble aggregate is calcium carbonate and it will dissolve some but not completely.

Since you are seeing aggregate, the total amount of plaster is more than 25 lbs.
 

flippinGeo

Bronze Supporter
Oct 8, 2016
81
Maryland
Ok. Finally closing this headache of a pool. I pulled the salt cell and there is no visible scale or buildup. I would have thought this think would’ve been caked with the issues I’ve been having with CH. Anyway, the cells are nearly brand new looking. Do I hold on the acid soak? Seems prudent to not expose the cell to acid if not needed.

Second question is why don’t I have scale?
 

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Newdude

Gold Supporter
TFP Guide
Jun 16, 2019
11,229
NY
The plates are never shiny. As long as there is no toothpaste on them, a dull sheen is clean. (y)

If and when you do need to soak, it will fizz like a kids science experiment. As soon as the fizzing is done, stop and rinse well.