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Can anyone tell me what this white powder might be? I was ripping up the carpet from my basement when I saw this underneath the padding and on the cement foundation. It's white and cakey.

I at first thought I would just vacuum it up, so I took a wire brush and started rubbing it to break it up into a fine powder so it could be vacuumed. I did that to a portion of it, then I noticed that there is discoloration in the cement underneath the white substance.

It seems that this white powder was probably placed there on purpose to cover up something in the cement. I don't see evidence of a crack there, nor is there cracking anywhere else in the foundation. Could it be mold related? I'm a little nervous to do much else with it until I know what it is. I don't want to undo whatever was done when it was put down.

White Substance on the right, cleaned cement left

Closed up of cleaned cement (left) and white substance (right)

Close up of Cleaned Cement. No evidence of a crack

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3 Answers 3

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Looks like effloresence to me, which itself is not harmful, but possibly a symptom, mainly of water movement; I would check perimeter walls for further signs of water infiltration, and make sure water movement outside is properly being handled, i.e. gutters, grading.

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  • This basement is known to have flooded (~ 2 inches of water) back in 2007. It was not due to any fault of the house - all the houses in the area had similar problems because of a large amount of rain and a sub-par community drainage (which has since been improved dramatically). However, it's strange that the flooding would have only caused a problem here and not elsewhere in the basement - also, not sure if the carpet was installed before or after said flooding (I bought the house in 2009)
    – rothloup
    Commented Sep 25, 2016 at 22:44
  • +1 SqlACID, my answer is duplicate. Didn't see yours whilst typing mine until page refreshed.
    – Jimmy Fix-it
    Commented Sep 25, 2016 at 22:47
  • not strange at all, the water movement after the flood would have been effected by many things, pressures exerted could be influenced by soil composition, external slope, etc, and the salts could have been slowly drawn to this area over many months as things dried out. Bottom line, nothing to be worried about, but clean it as @Jimmy Fix-it mentions, and see if it comes back
    – SqlACID
    Commented Sep 25, 2016 at 22:51
  • After reading through the link you provided, I can say that it is almost certainly efflorescence. I'm not too concerned, but I do wonder if I have a moisture problem or not. You and the link mention to check if it "comes back". My question is - how do I check? Do I leave it uncovered for a period of time? Should I lay down a moisture barrier in the affected area? In either case, how long should I wait? This could be a leftover of the 2007 flood, or it could be a recurring problem - I'd like to figure out which before I lay down my new floor. :)
    – rothloup
    Commented Sep 25, 2016 at 22:53
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    There are some simple methods of checking if your concrete slab has significant moisture content, as easy as duct taping a square piece of plastic to the floor and inspecting for condensation. concretenetwork.com/vapor-barriers/moisture-vapor-test.html
    – Jimmy Fix-it
    Commented Sep 26, 2016 at 0:30
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Efflorescence, salts rising to the surface due to presence of moisture. Common in cementitious materials where water is present.

You can wash it away with mild HCL solution but the stains will remain. Not much to worry about here. On structural masonry over long periods of time there might be a concern as it could be indicative of structural integrity problem.

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Efflorescence, salts... you'll also see it on brick faces. Rookie concrete workers. Was your basement floor poured in winter? They add more "chemicals" into the mix, keep it from freezing, keep it flowing, not hardening as fast.

Those "salts" leach up through the concrete. It is what makes an epoxy floor POP up! Fellows wit experience, can mix a batch, according to the weather, temp, humidity. New guys just follow the sign on the wall.

good luck!

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