Efflorescence

Efflorescence is the white, powdery or crystalline deposit that sometimes appears on concrete or masonry surfaces. It forms when water moving through the material dissolves soluble salts (commonly calcium hydroxide), carries them to the surface, and leaves them behind as the water evaporates.

Efflorescence is usually a cosmetic issue rather than a structural one, but persistent efflorescence can point to a moisture intrusion problem worth investigating, especially on below-grade walls or slabs.