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efflorescence or what

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Yes, that is extreme, but I have seen efflorescence like that. Here, it will usually be in a crawlspace with a thin concrete skim coat. Moisture evaporates and leaves those calcium(?) crystals behind. Nobody ventures down there so after a long period of crystal buildup, you see plumes like that.

There is a moisture problem under that floor. Perimeter drainage around the footings is not working. If there is a vapor barrier under the concrete, it has gaps or holes in it. But if the drainage was correct, a few gaps would be irrelevant.

Make sure the downspouts are not delivering water to the footings. It used to be standard practice here to run downspouts into the perimeter drain system. All very well until the flow gets restricted with sludge, plus the crap from the eave troughs.

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I see that sort of thing once or twice a year, but the thickest accumulation is usually about 1/8", never anywhere near as dramatic as what's in your photos.

It's caused by very slow movement of water through the slab. The water carries soluble minerals with it and when it reaches the surface, the water evaporates, leaving the minerals behind. If liquid water were coming through the concrete, it would wash the minerals into a crust somewhere, so you've just got vapor coming through.

Of course, start with the basics: keep the gutters clean, and direct the downspout water well away from the house to reduce the saturation of the soil immediately under the house. After that, I usually advise installing a sub-slab drainage system. If that's not possible, then remove the vinyl tiles, thoroughly clean the floor, and leave it unfinished. Then install a dehumidifier. If they occasionally sweep the floor, they'll probably never notice the problem again because it'll be happening on such a slow scale.

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