At night with high humidity, the garage slab gets moisture on it. When it dries, this powdery substance is left behind. It seems to then eat away at the slab itself making indentions. Does anyone know what this could be and how to fix it?

It's efflorescence. Just soluble minerals left behind by evaporating water. No big deal and normal.

If you were in Georgia the immediate answer would be pollen!

ABATTBQ11 said:

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It's efflorescence. Just soluble minerals left behind by evaporating water. No big deal and normal.

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Well no big deal unless there's a moisture problem under the slab causing foundation issues

If it's only happening after nights with high humidity, I'm assuming it's condensation and not moisture coming through the slab.

AtlAg05 said:

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If you were in Georgia the immediate answer would be pollen!

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Not enough to be Georgia pollen, you need at least 1" of powder before considering pollen.

This does not typically happen from humidity. It happens because there is a compromised vapor barrier or no vapor barrier, under the slab. The capillary force of water moving through a slab is pretty significant. We have not found a topical application that will prevent this from happening.

Hugh Stearns
Stearns Design Build

Dr. Venkman said:

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At night with high humidity, the garage slab gets moisture on it. When it dries, this powdery substance is left behind. It seems to then eat away at the slab itself making indentions. Does anyone know what this could be and how to fix it?

ectoplasm!

Your garage is infested with ghosts!

user name checks out.

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HCS said:

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This does not typically happen from humidity. It happens because there is a compromised vapor barrier or no vapor barrier, under the slab. The capillary force of water moving through a slab is pretty significant. We have not found a topical application that will prevent this from happening.

Hugh Stearns
Stearns Design Build

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Would an epoxy resin floor be susceptible to this too?

6.5 Swede said:

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Dr. Venkman said:

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At night with high humidity, the garage slab gets moisture on it. When it dries, this powdery substance is left behind. It seems to then eat away at the slab itself making indentions. Does anyone know what this could be and how to fix it?

ectoplasm!

Your garage is infested with ghosts!

user name checks out.

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Come on! How can I not get one blue star for this?
The OP's name is Dr Venkman...
ouzing powder
eating concrete?

I have this issues really bad. The etching and inventions are getting really bad. My garage floor is on its way to being an exposed aggregate.

Give this a shot. Old school technology but I've seen it work in the past for similar issues. It might be hard to find locally and have to order in.

https://www.euclidchemical.com/products/construction-products/liquid-densifiers/surf-hard

https://www.master-builders-solutions.basf.us/en-us/products/concrete-surface-treatment/concrete-curing-compounds/masterkure-hd-300wb