Efflorescence is a white, powdery mineral deposit that appears on concrete when water carries dissolved salts to the surface and evaporates. It is a visible warning sign of moisture moving through the slab.
Efflorescence is the white, powdery, or crystalline deposit that forms on concrete or masonry surfaces when water moves through the material and evaporates, leaving mineral salts behind. It is one of the most common surface issues installers run into on slabs with moisture problems.
What Causes Efflorescence
Efflorescence happens when water dissolves soluble salts inside the concrete, wicks them to the surface, and leaves them behind as the water evaporates. It is almost always a sign of moisture moving through the slab, whether from groundwater, poor drainage, or a missing vapor barrier.
Efflorescence vs. Moisture Vapor Emission
Efflorescence is the visible residue left on the surface, while moisture vapor emission is the underlying rate at which water vapor is passing through the slab. A slab can have high moisture vapor emission with little visible efflorescence, so testing is still necessary even if the surface looks clean.
Practical Tips for Dealing with Efflorescence
Never coat over active efflorescence, since trapped moisture will keep pushing salts and vapor pressure against the new coating and cause it to fail. Address the moisture source first, mechanically remove the deposits, and run a proper moisture test before applying any coating system.
