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I'm not sure what it is, but I've cleaned it with mould remover and it comes back after a few weeks.

Enter image description here

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  • 1
    How long have you lived in the property? It's hard to tell from the photograph, but that looks like chemical residue. Oct 15, 2023 at 9:58
  • 2
    Do you own or rent?
    – FreeMan
    Oct 15, 2023 at 11:41
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    What's the wall made of? What is on the other side?
    – Criggie
    Oct 15, 2023 at 20:54
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    Can you clarify if the status of the white substance we see in the picture is its "natural" status, i.e. it looks like this by itself, or if this is after you tried to clean it? Oct 16, 2023 at 8:07
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    Is it damp? Have you tried something like "Do It Yourself Mold Test Kit" before using mold remover? And what kind of remover are you using? Chlorine based?
    – Mołot
    Oct 16, 2023 at 8:49

1 Answer 1

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Hard to be certain, but it looks like it might be efflorescence, which is caused by water passing through the wall and then evaporating on the surface. The minerals contained within the water are left behind, usually looking like a white powder on the surface.

The Wikipedia article suggests some countermeasures. My local home center sells an efflorescence remover that is basically diluted stearic acid. This dissolves the white stuff, but to address the problem permanently you need to eliminate the water.

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    Not disagreeing at all, but wondering, if it is efflorescence, why does the residue have such a non-organic shape? The shape of the residue looks human-made, not organic. Oct 16, 2023 at 2:13
  • @EndAnti-SemiticHate previous human efforts to clean it?
    – stannius
    Oct 16, 2023 at 2:35
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    Are you sure it's efflorescence? When I search for images of efflorescence it gives results that look quite different (the image in the question looks almost like a powder [not unlike the residue you describe], but the image search shows little non-uniform bubbles). Search results here.
    – stevec
    Oct 16, 2023 at 5:03
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    I do not think this is efflorescence at all. Usually you will find it at the bottom of walls, where the other side is underground or very close to ground, and most definitely it usually does not occur in the shape of sponge streaks. Oct 16, 2023 at 8:06
  • I've retracted my up vote: The question asks how to remove it. The answer says what it is. There may be removal instructions at the link, but there are no instructions here. Without that, this is simply a link-only answer. Please summarize at least some of the instructions on how to remove it - we all know that links die and who knows what the next edit to the Wiki page will do to it...
    – FreeMan
    Oct 16, 2023 at 16:26

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