Concrete Pool: Black Unbrushable Stains

[james-m]

Concrete inground plastered pool.
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TF-100 TFTestkit [/FONT]

FC	10 ppm
CC	.5 ppm
TC	10.5 ppm
CYA	35 ppm
CH	675 ppm
TA	140 ppm
PH	7.7 ppm

Pool was re plastered in April this year (i.e. 4 months ago). A couple of weeks ago I started noticing strange black stains:

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Looks like it is getting worse.

I doubt this is algae, since stains do not come of from brushing (even with a metal brush).

I have not tried to rub it with a vitamin C (btw, where can I get pool vitamin C tablets, and how exactly to try that?) to understand whether it is caused by the iron / copper binding to the plaster. The water is clear, it is municipal, and the plaster looked ok for 4 months.

I am not certain whether a 4 months plaster could start exhibiting some bad chemistry. It was supposed to be a regular diamond dust plaster, I would expect to see this problem earlier if the plaster itself was bad (but let me know what you guys think).

Thank you

 

[PAGirl]

Can you fill out a signature so we know what type of equipment you have? Have you added anything, like algaecide, that might contain metals?

Most here just use regular vit. C tablets, I believe, to check.

Your calcium looks a bit high. Have you plugged your readings into pool math to see where the CSI is at?

 

[Vickery]

Just get a bottle of vitamin C from the drugstore and rub one on the stain. Try several different spots. The ones I have used stay fairly solid and can be rubbed like a piece of chalk.

 

[YippeeSkippy]

One of those stains looks just like a footprint in a boot to me

Cheap-o Vit C chewable tabs.. you can put a handful in a sock and rub them over the stain.

 

[duraleigh]

The most correct way to do the vitamin C tablets is to put 10 or more of them in a sock and then crush them up (like with a hammer)....or crush them before you put them in a sock. You want the most amount of contact with the stain you can get and crushing them will give you that,

I assume the stains are rough to the touch, right?

You have borderline test results on TA, pH and CH. Those have probably been higher in the past and the stains are likely calcium scale (with some dirt embedded to make them dark).

Let us know what more you find out with the vitamin C and touching the stains for roughness.

 

[james-m]

thanks for all the replies!

@PAGirl: 1.5hp pump, 35K concrete pool, pentair 300 chlorinator. Have not added algaecide. CSI is 0.66.

@Vickery: Thank for the tips, I'll get the tablets from the pharmacy and will try.

@duraleigh: Thanks for the detailed "sock instructions". Stains feel no different than other white plaster. What's interesting is that they are getting better, and then go back to be black. I can see some other in new spots. "stains are likely calcium scale": could it mean that this is not caused by copper then?

I'll try to rub vitamin C against them and report back of course.

 

[Swampwoman]

Does the "get better" then "back-to-black" pattern happen to correspond to your ph level? Metal staining generally is less when ph is kept around 7.2 ppm.

While doing vit c test, you may also wish to test with dry acid, eg ph down. If the dry acid works better, they may be copper stains.

That color can also be magnesium stains, rarer than copper but possible particularly if you're on well water -- are you on well water?

Scale can combine with metals to make tougher stains, so try to keep your csi slightly negative to help.

If dry acid works best, you may wish to consider a spot treatment of either Jack's Stain #2 OR a spot treatment of Muriatic Acid with a tool like this one, but don't go crazy since your plaster is new: Amazon.com: Purity Pool UAW Underwater Acid Wash Set: Patio, Lawn Garden

 

[JamesW]

If the plaster was originally white, I think that you have copper staining. The general plaster color has a turquoise hue that is characteristic of copper. Black and grey stains are different compounds of copper in different oxidation states.

Do you have a heater?

Did the builder do an acid startup?

Any idea how copper could get in the water?

Do you have pictures of the old plaster?

 

[james-m]

@Swampwoman: my PH level is rather high (fluctuates between 7.7 and 7.8), not sure if this difference could be attributed to the "stains travel". I do not use well water (it's from a public sewer). Thanks for the "spot treatment", I am reading about both options.

@JamesW: yes, the plaster was originally white, here is a better picture of the stains (without pump running, so there is no water distortion):



Do you have a heater?

No.

Did the builder do an acid startup?

If I understand what you mean: they did an acid wash and then a bonding coat before plastering

Any idea how copper could get in the water?

No.

Do you have pictures of the old plaster?

Sure, here is what the surface looked like last year before the replastering:



--
I did try to rub a sock with vitamin C tablets against the stains, but I did not notice any effect.

Again, thanks a lot for all your help and suggestions.

 

[JamesW]

Try leaving a vitamin c tab on a flat spot that is not black or gray.

I think that you have copper staining.