Efflorescence is the white, powdery, sometimes crystalline deposit that appears on brick, block, or concrete surfaces after moisture has passed through them. It forms when water moving through masonry dissolves naturally occurring salts in the material, carries them to the surface, and then evaporates — leaving the salts behind as a visible crust. It's cosmetic in itself, but it's also a reliable signpost that water is actively moving through the structure somewhere behind it.

It shows up in a lot of the situations already covered in this hub: on walls affected by rising damp, around cracks where polyurethane injection has stopped an active leak, and on newly cast concrete water tanks as residual construction moisture works its way out. In new concrete, some efflorescence in the first year is common and often resolves on its own as the structure dries out fully; persistent or worsening efflorescence years after construction usually signals an ongoing water source that needs investigating.

Treating the salt deposit itself — brushing or acid-washing it off — does nothing for the underlying moisture problem and it will simply return. The correct sequence is to identify and stop the water source first (a failed DPC, a leaking joint, a cracked pipe), let the structure dry out fully, and only then address the surface staining and any plaster or render damaged by the process. Skipping straight to a cosmetic fix is one of the most common false starts in damp remediation.