Avoiding Efflorescence with the Right Concrete Mix

Efflorescence can ruin the appearance of a floor.

One of the worst things that can happen to your beautiful decorative job is to return a week later to see a white scum across the entire surface. This is especially common on stained floors with an acrylic sealer. As moisture moves through the concrete it picks up soluble salts. The sealer lets the moisture vapor pass through but not the salts which form a white blush on the concrete. Doug Bannister has written the definitive guide on efflorescence, which is available on his web site (www.thestampstore.com). "Efflorescence is the nemesis of the stainer," he says. The keys to reducing the danger of efflorescence on your projects are to use fly ash in the mix, place a good vapor barrier beneath the slab if you have control over that part of the job (a real vapor barrier, not simply polyethylene sheeting), and to use low-permeability concrete (low w/c and fly ash). Getting a good cure on the surface can also help reduce efflorescence by creating a denser surface layer, especially by using a cure and seal or curing blankets without added water. Using metakaolin in the mix can also reduce or eliminate efflorescence and can control alkali-silica reaction, too—for more about metakaolin, see "Using Metakaolin in Decorative Concrete Mixes" (Concrete Construction, August 2006).

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