A white, powdery deposit of salts that appears on brick, concrete, stone or plaster when water carries dissolved salts to the surface and evaporates.
Soluble salts from the material, mortar or ground dissolve in moisture moving through the masonry and crystallise where the water evaporates. It is a moisture symptom rather than a colour change in the material itself.
It is often mistaken for paint failure or mould; it brushes off dry and returns while the wall stays wet.
New brickwork often shows white bloom in its first winter, fading as the wall dries out.
It can make a dark brick or a coloured render look pale and patchy, and painting over it without curing the moisture fails.