(Morrisburg, Ontario, Canada)
Q. I just repoured a 11'X14' X 3" room in my basement on Sunday. It seems to be hard but there are a lot of places where it seems like wet chalk. If I walk on it it leaves a faint shoe print. Will that harden up? It's been 48hrs now.
A. It sounds like Efflorescence to me. Efflorescence is caused when soluble salts and other water dispersible materials come to the surface of concrete and mortars. It's induced by low temperatures, moist conditions, condensation, rain, dew, and water added to the surface of fresh concrete to assist troweling.
It can occur very soon after exposure to moist or cool conditions or gradually, especially when it comes from within the concrete or from the subgrade.
If the subgrade was moist and no vapor barrier was used or if there was a lot of water used to mix the concrete, then efflorescence is very common.
The "wet" part could be excessive bleed water from the concrete mix or if the subgrade was saturated with standing water, this water will bleed up through the concrete and remain on the surface until it evaporates.
If the concrete is hard you should be able to wipe it up or wash it off.
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