Q.: I hired a contractor to install a colored concrete patio at my home. He installed it on a 40° F day with a forecast for possible freezing temperatures that night, so I asked him to cover the slab to protect it from freezing. He feared greater damage to the surface by covering it and didn't do it, so that night I covered it. The next day, when I took the cover off, there were white blotches everywhere. The concrete supplier suggested that I try a weak solution of muriatic acid and immediately rinse it off. I tried that but it didn't help. Can I try a stronger application of acid? What can I do? I don't want to live with a permanent memorial to incompetence in my backyard!
A.: The problem is efflorescence—made worse by covering or coating with materials that don't permit the movement of water vapor. Try a 10% solution of muriatic acid or a commercial efflorescence remover. Place a small amount on a whitish area and scrub it into the surface with a stiff plastic bristle brush, and then flush it with clean water. You can repeat the process but if you see cement paste etch away from the surface to reveal sand in the concrete mix, its time to stop. You can also try sanding to remove the white coloration. But if that doesn't work, it might be best to consider a different look for your patio. A strong colored sealer, a light sandblast finish, or a thin decorative topping are options. There are also colored cement products on the market that can be sprayed on the surface to cover blemishes.