Efflorescence is a very common and somewhat mysterious ailment effecting masonry construction including brick, concrete, and mortars. It occurs when water moving through a sidewalk or other structure brings salts to the surface that are not commonly bound as part of the cement stone. As the water evaporates, it leaves the salt behind, which forms a white, fluffy deposit, that can normally be brushed off but sometimes create more lasting stains.
The resulting white deposits are referred to as “efflorescence” in this instance. In this context efflorescence is sometimes referred to as “salt petering.” Since primary efflorescence brings out salts that are not ordinarily part of the cement stone, it is not a structural, but, rather, an aesthetic concern.
Here’s a resource for what causes efflorescence as well as how to prevent and remove it.