Efflorescence: the white powder on concrete, brick and block

Concrete, mortar, brick, block and pavers all contain soluble salts. When water soaks in — from rain, from the ground, from a gutter overflowing onto the same spot every storm — it dissolves some of those salts and carries them outward. The water evaporates at the surface. The salt stays behind as a white film.

So efflorescence is not really a cleaning problem. It is a moisture problem that happens to show up as a stain.

White deposits get mistaken for each other. Efflorescence is crystalline: it crumbles into powder and dissolves if you put water on it. White mold is fuzzy and smears when you rub it.

The white tide-lines on a garage floor or along the bottom of a driveway in spring are usually de-icing salt tracked in on tires through the winter. That one rinses away and stays away until the next winter.

Block foundations and poured basement walls sit in wet ground for half the year, and water wicks up through them constantly. Every freeze and thaw opens the pores a little further. The band of white a foot or two above grade on so many foundation walls here is exactly that water finding its way out.

New work does it too. Fresh concrete and new pavers often bloom white in their first season as leftover salts from the cement work their way out. That kind, called primary efflorescence, usually fades on its own over a year or two as the supply runs out.

Blasting efflorescence off puts a lot of water into the masonry, and water is what carries the salt out in the first place. The surface looks clean when the job is done. A few dry days later the salt has come back up, sometimes heavier than before.

Heavy or crusted efflorescence is removed with a dedicated efflorescence cleaner — a mild acid that dissolves the salt — rather than with force. It is a separate treatment from a wash, and like rust it is worth pointing out on the call so you know up front whether it's part of the visit.

Cutting off the water. Clean the gutters so they don't overflow onto the same spot. Extend downspouts so they empty away from the foundation. Grade soil and mulch so they slope away from the house rather than toward it. Once the masonry stops getting soaked, efflorescence stops forming.

On pavers, a breathable sealer applied after the efflorescence has finished coming out — not before, or it gets trapped under the sealer — cuts down how much water gets in.