Q: In our garage, the concrete foundation, or lower part of the wall that meets the floor, is quite powdery and flaking off. Is this safe? Should we paint it? How do we prevent the concrete from flaking off?
A: Not to worry. This condition is natural, safe and common. It doesn’t look good, but it won’t hurt you or your house. The white powdery substance you see is called efflorescence.
In Bill’s Boise, Idaho, rental he had a similar problem, but on a larger scale. At one point he decided to build a guest bedroom in the basement. The chosen room had an exterior concrete wall that doubled as the foundation.
All but about 6 inches of the wall was below grade. There was an assortment of rosebushes and other shrubs, as well as an irrigation system on the perimeter of the house. The wall was abloom with efflorescence. A quick brush of the hand dislodged powdered effluence, leaving an unsightly mark. The cause of this mess was water migrating from the flower beds through the wall.
We got to work. We cleaned up the wall, sealed and painted it. You wouldn’t confuse the concrete with a smooth plaster job, although we could have gone the extra mile and done that. When we were done, Bill had a perfectly functional guest bedroom.
Any material containing Portland cement can produce efflorescence because of the water-soluble salts it contains. Portland cement, when mixed with water, is the glue that binds the aggregate and lime together to form concrete. There is approximately 140 pounds of Portland cement in each cubic yard of concrete.
Efflorescence is caused by the capillary action of water moving through concrete mix, dissolving water-soluble salts in the cement and landing on the surface. The water evaporates and the salt residue remains. In your case, salts in the cement are migrating from the earth under the slab or around the concrete footing, leaving the white powder.
Usually it’s relatively easy to remove efflorescence compared with some other stains. These water-soluble salts can be removed by emulsifying them in water and scrubbing them with a wire brush, then rinsing them away with clean water.
Heavy accumulation or stubborn deposits of white efflorescence salts can be removed with a solution of one part muriatic acid to 12 parts water and scrubbing. Beware, muriatic acid is real acid and can hurt you. Be careful, follow label directions and suit up. Long-sleeve shirts, trousers, shoes and socks are a must.
Also, wear eye protection and rubber gloves. Review first aid instructions on the package so you know what to do if a stray drop of the stuff gets into an eye or other mucous membrane.
After acid-washing, slabs should be rinsed thoroughly and neutralized with baking soda (sodium bicarbonate) or an equivalent. Acid residues can harm plants. If you don’t neutralize and rinse the reaction products of acid on concrete, since all are soluble calcium and iron salts, they can cause more efflorescence.
To sum it up: The powder is not dangerous, it’s pretty easy to get rid of and we suggest you paint the area with an elastomeric paint to help slow this natural process.
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