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I live in the countryside of Western Europe by the beach with a temperate climate. After a period of unusual humidity at 84%, I noted this white fluff in the basement floor:

White fluff in basement floor

Close-up of white fluff

The first picture is about 40 cm and the second 10 cm.

The basement is usually closed and keeps humidity. Temperatures average 18-20ºC, and humidity is now 77%. I assume it's a fungus since the floor was clear before and it seems to collapse when I rub my foot on it.

What is this species, and how can I prevent it?

Update: Following @anongoodnurse's suggestions, it's definitely a salt. I scraped the stuff, poured water, and it shriveled instantly. Here are photos before and after:

White fluff before water White fluff after water

Here is a close-up of another piece, where the shards look like a crystal:

close-up of white fluff

It did not smell indeed, and I did not taste it. I opened the basement door to ventilate the area, and I understand that it would accelerate the evaporation of the salty water.

I don't know if this growth has happened before, e.g. because of a leak in a pipe, or is a one-off occurrence, e.g. because of the unusual humidity in the last few weeks. I will check if the basement has any pipes.

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  • $\begingroup$ What is that floor made of? Looks like square vinyl tiles. Has it been raining a lot during this period of high humidity? $\endgroup$ – Bryan Hanson Nov 3 at 13:46
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    $\begingroup$ The crystalline appearance makes me suspect that that is a mineral efflorescence rather than a biological growth. See for example: ceramictilefoundation.org/blog/… $\endgroup$ – tyersome Nov 3 at 19:54
  • $\begingroup$ The floor is either ceramic or stone tiles, not vinyl. $\endgroup$ – miguelmorin Nov 3 at 21:00
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    $\begingroup$ @miguelmorin - Yes. Scrape a bit up (it should feel a bit sandy, not mossy) and see what a drop or two of water does. Mold doesn't dissolve in water, it floats. If water seems to dissolve some of it, add more. If it all dissolves, it's probably a salt, and since you live near the beach, it might be mostly salt, i.e. NaCl, with trace minerals thrown in. If it dissolves and you're brave, you can taste a tiny bit then spit and rinse. Fun! $\endgroup$ – anongoodnurse Nov 3 at 21:40
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    $\begingroup$ Salt water is probably entering the basement through the porous grout; as it evaporates, it leaves salt crystals behind. More water = saltier solution = bigger crystals as the water evaporates. If it turns out to be salt, you might ask over on DIY what to do about it. If you have metal pipes under the floor, it can't be good. $\endgroup$ – anongoodnurse Nov 3 at 21:44
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Might as well make this an answer.

...it seems to collapse when I rub my foot on it.

As @tyersome stated, it appears to be salt, not mold. Salt crystals would definitely collapse easily underfoot, and should be mostly quite dry, whereas mold would be... maybe squishy (?) but likely unpleasant in some way. Mold also smells, which you haven't mentioned, whereas salt is odorless.

To test, scrape a bit up (salt crystals should feel a bit sandy, not mossy) and see what a drop or two of water does. Mold doesn't dissolve in water; it floats. If water seems to dissolve some of it, add more. If it all dissolves, it's probably a salt, and since you live near the beach, it might be mostly salt, i.e. NaCl, with trace minerals thrown in.

If it dissolves and you're brave, you can taste a tiny bit then spit and rinse. While salt isn't toxic, I can't vouch for what was in the grout.

My guess is that there may be a low spot or two in your basement where salt water is entering through the porous grout; as it evaporates, it leaves salt crystals behind. More water = saltier solution = bigger crystals as the water evaporates.

If you have a magnifying glass, you should see crystals in the undisturbed deposit. If instead it looks soft and hairy, it is mold. I can't think of a white mold off the top of my head (well, Candida albicans, but that smells yeasty and is definitely not crumbly). Mold/yeast needs a substrate (food) to grow on, and wet grout isn't very nutritious. Mildew, sure, but mot big fluffy mold.

I don't like this reference because it's not scientific, but it explains efflorescence well.)

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