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My concrete has an old unknown spill which appears in the newly warm moist weather to be forming crystals or hairs. It doesn't have direct water on it, and hasn't been damp recently.

white hairy crystal growing out of flat concrete with darkened spill area

What is the likely cause?

Is this likely to result in damage?

How should I treat the spilled area?

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That is Efflorescence. Harmless, just looks bad.

It's not a spill, just minerals from the concrete crystallizing on the surface of the concrete. This is very normal and when it is humid, not much can be done about it. Just wash it off when you notice it.

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