Efflorescence on concrete
A white, chalky or crystalline deposit on the surface, often appearing after rain or in the first months of a slab's life. It can look alarming on coloured or stamped work, where it mutes the colour.
VerdictCosmetic — does not compromise the slab
What causes it
Efflorescence is dissolved salt carried to the surface by moving water. Water inside the concrete dissolves calcium hydroxide and other soluble salts, migrates to the surface, evaporates, and leaves the salt behind. It is a moisture symptom rather than a concrete defect — which is why it recurs on slabs with a moisture source underneath and stops on slabs that dry out.
How it is repaired
Fresh efflorescence often brushes off dry, and mild cases respond to water and a stiff brush. Persistent deposits need a proprietary efflorescence remover or a dilute acid wash, which must be neutralised and rinsed thoroughly — acid etches the surface and strips sealer, so it is a last resort on decorative work. The deposit will return unless the moisture path is addressed.
How to prevent it
The fix is drainage and vapour control, not surface treatment. A 10 mil vapour retarder under interior slabs per ASTM E1745, positive drainage away from exterior slabs, and a penetrating sealer all reduce the water movement that carries the salt.