That chalky white residue on concrete, brick, or block is efflorescence — mineral salts left behind when water moves through the masonry and evaporates. It's usually cosmetic, but it's also a clue that moisture is moving through the material.
Why it happens
Water dissolves natural salts inside the concrete or mortar, carries them to the surface, and evaporates, leaving the salt behind. It's common on new masonry and on surfaces exposed to moisture, like basement walls and the base of brick.
Remove it
Light efflorescence often brushes off dry. Heavier deposits need a stiff brush and water, or a specialized efflorescence cleaner for stubborn cases. Avoid harsh acid unless necessary and done carefully, as it can damage the surface.
Address the moisture
Because efflorescence is driven by water moving through masonry, recurring deposits point to a moisture source — poor drainage, leaks, or a missing moisture barrier. Sealing the masonry and fixing the water source prevents it from returning.