Gray (grey) blotchy staining in white pebblecrete.

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AUSpool

AUSpool

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TFP Guide
Sep 23, 2015
2,029
Brisbane, Australia.
Pool Size
20000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Astral Viron V25
Our pool has been developing a shady grey blotchy staining on our pebblecrete. Pebblecrete is a blend of cement, white in our case, and a mix of small pebbles. It’s mostly on the floor but there is a little on the walls but nowhere near as much. I’ve tried vitamin C, and rubbing with trichlor pucks with no luck.

The surface was acid washed a day after application and before filling which I think is normal here for pebblecrete.

I was reading through the Zero alkalinity treatment post from @onBalance which suggests that this staining may be due to poor application and while a zero alkalinity treatment provide some improvement the stains will return. Is there a feasible solution?

The white cement used was a Chinese import so I have no idea of possible impurities. Being mostly on the bottom is it UV dependant.

Do any of the stain removers work on this type of stain? Are there any I should avoid and what is the fait of these acids?
Citric acid.
Sulfumic acid.
Oxalic acid.
polybasic organic acids.
Ethlanediolic acid.

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This is a close up of the pebblecrete surface. The cured cement has minimal exposed surface area but I guess the problem with overdoing acid treatments is loss of cement and pebbles falling off.
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77A20412-C49A-469D-BC8D-0F2D0C2D4820.jpeg

 
Can you feel it? Does that area feel different at all?

You are correct in the acid treatments being tough on the plaster. The acid removes some of the plaster.

I am guessing you have tested and kept your water within TFP balance. Go ahead and post up your latest test results just to get that info on here.
 
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Thanks Kim and that’s a good point that I did think of. The surface is not raised, smooth like the rest of the pool. But from a close up perspective I don’t know if this staining is confined to the plaster or covers the pebbles as well. I’m thinking that it would be confined to the cement ‘grout‘ between the pebbles.

I‘ve always been within the TFP levels mostly. This has developed over the last 2 years where I think a long term trend will have more influence then the here and now. I‘ve always run a little hot with my FC, at 7 - 10ppm with my CyA at 70ppm through summer and 50ppm through winter. pH 7.6 - 8, I lower it to 7.6 when it gets to 8. Currently working with my winter targets with my temp at 16dC (60F). CH @ 350ppm, TA @ 90ppm, CyA @ 50ppm, salt @ 3500ppm, borates @ 40ppm, CSI -0.14 with pH @ 7.8 (CSI will increase to 0.03 as my pH approaches 8.0). I know I said I never bother but I was interested so phosphates at 100 - 200ppb.
 
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AUSpool said:
Do any of the stain removers work on this type of stain? Are there any I should avoid and what is the fait of these acids?
Citric acid.
Sulfumic acid.
Oxalic acid.
polybasic organic acids.
Ethlanediolic
Click to expand...

Ethanediolic acid is just another name for oxalic acid and “poly basic” organic acids are just organic molecules that have more than one acidic proton to exchange (like poly acrylic acid). None of those acids listed are great for pool water unless you plan to use them and then drain the pool. Jacks Magic #2 Stain Remover for copper and calcium scale is nothing more than sulfamic acid and it’s very problematic because the sulfamate stays in the water for a very long time while reacting with chlorine to form a weak sanitizer called N-chlorosulfamate. TFP typically tells people that use Jacks #2 to dump the entire pool because of the sulfamate “poisoning” of the chlorine. The citric and oxalic acids will work similar to ascorbic acid and produce the same result although it can take longer to re-establish FC levels. Oxalic is isn’t great to use because it will react with calcium hardness and form needle-like calcium oxalate crystals if the CH is high.

If you can get a Jacks Magic stain ID kit (or something similar down under), you can test out the area and see what works.
 
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