Efflorescence typically occurs during the initial cure of a cementitious product. It often occurs on masonry construction, particularly brick, when water moving through a structure brings salts to the surface. As the water evaporates, it leaves the salt behind, which forms a white, fluffy deposit, that can normally be brushed off. It is not a structural, but, rather, an aesthetic concern.

It is possible to protect porous building materials such as brick, tiles, concrete and paving against efflorescence by treating the material with an impregnating, hydro-phobic sealer. This is a sealer which repels water and will penetrate deeply enough into the material to keep water and dissolved salts well away from the surface. If the source of the water penetration is not addressed efflorescence may reappear.

WIKIPEDIA ENTRY FOR EFFLORESCENCE.