Q. I recently had a new colored concrete driveway installed with a salt finish. To do that, the contractor poured salt on the surface, left it for three days, then washed it off. However, the color of the concrete currently is not uniform as we had expected it to be. It looks a lot like a leopard skin. The contractor claims the concrete needs to dry, but it was poured more than six weeks ago so that doesn't seem right. Could the salt have damaged the color?
A. Color variations are expected in any colored concrete. But from your description, and assuming that the discolorations you refer to are lighter than the main color of the concrete, it sounds like you have efflorescence on your driveway's surface. This phenomenon is a normal occurrence in concrete and happens when soluble salts and other water-dispersible materials come to the surface. The efflorescence is white, so it often is not noticeable with standard concrete, but the contrast with darker, colored concrete can make it stand out.
Several sources of efflorescence are present in the complex chemical reactions that are going on during the hydration of concrete while it hardens. The most common occurrence is from calcium hydroxide (lime), which is formed in the hydration reaction of portland cement, that is transported to the surface by water rising through capillaries in the concrete.
If addressed soon after it appears, the whiteness left on the concrete surface often can be removed easily by pressure washing or wet scrubbing. You have to rinse all the residue away, though, or it will effloresce again.
As the calcium hydroxide reaches the surface, it begins to combine with carbon dioxide in the air to produce water and calcium carbonate. Because that material is insoluble in water, the efflorescence becomes much more difficult to remove. At that point you'll likely need some chemical help—such as acetic, muriatic, or citric acid—to remove the whiteness. That requires great care and must be followed with a neutralizing rinse.
What does the salt finish have to do with all this? Perhaps nothing. That is one of the most common decorative finishes applied to fresh concrete and has been used successfully with colored concrete for years. The efflorescence you are experiencing likely would have occurred regardless of the salt finish.
Once you remove the offending efflorescence, seal the concrete surface to stop more from occurring. One option would be an acrylic sealer that will form a film coating to keep carbon dioxide out of the concrete and penetrate below the surface to plug the pores in the concrete and stop the migration of calcium hydroxide. Acrylic sealers allow water vapor to pass through while blocking the calcium hydroxide.