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Our kitchen has one of the main walls of our house which is a thick concrete wall. There is nothing behind the wall, just some empty space and then the neighbor's house. I just painted the wall with Flexa indoor water-based color, as grey.

Now, as you can see in the pictures, after a couple of months we see some green spots on the wall in different places.

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My question is, what are these green spots? They don't look like mold or mildew.

Update

  • I just tested our cleaning sprays on the wall to see if it happens because of some spray drops but it was not the case.
  • This place is not reachable for our kids.
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  • 1
    Looks like some kind of deliberate marker
    – user253751
    Mar 5, 2021 at 14:17
  • Is this in an area where children might have splashed something on it like lime Kool-aid or colored with a marker?
    – UnhandledExcepSean
    Mar 5, 2021 at 14:18
  • They are not in a reachable place for our kids and we have many of these spots.
    – S.Yavari
    Mar 5, 2021 at 14:18
  • 3
    That is a bold comment. I have a neighbor kid over here last week manage to get orange soda pop spray on everything within 4 feet. :)
    – UnhandledExcepSean
    Mar 5, 2021 at 14:20
  • What is the actual wall surface? Obviously that's not bare concrete. Also, what's the scale of those photos?
    – isherwood
    Mar 5, 2021 at 14:25

1 Answer 1

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Looks like copper corrosion products - so if there is a bit of copper embedded inthe wall, this sort of staining to the surface is quite plausible. WHY there's copper there to cause staining, I can't tell you.

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4
  • Maybe someone was pressure-treating wood in the area and for one reason or another it splashed?
    – MonkeyZeus
    Mar 5, 2021 at 19:23
  • Pressure treating is not a DIY operation and most of the DIY preservatives don't have copper, IME. I'm thinking "copper nails" but having a hard time figuring why anyone would have put copper nails in a masonry wall, so I'm back to I Don't Know but Looks Like Copper...
    – Ecnerwal
    Mar 6, 2021 at 12:56
  • I certainly did not mean to imply industrial pressure treatment. I was implying the DIY route with a Copper Naphthenate application. I wasn't sure if "preserving wood" is the proper explanation of using such a product so I figured "pressure-treating" would be more universally understood.
    – MonkeyZeus
    Mar 8, 2021 at 16:38
  • Copper sulfate is sometimes used in fungicide for house plants. It's a stretch, but that could be an explanation.
    – Thegs
    Apr 1, 2022 at 17:36

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