The white powdery substance that commonly appears on masonry is a salt residue called “Efflorescence”. When water flows through the masonry it picks up salts from the mortar and leaves them on the masonry surface during evaporation. It can be removed but more importantly it is an indicator that water is penetrating through the masonry frequently. Sometimes water enters from the outside; sometimes the efflorescence is caused from improperly vented furnaces or other heating appliances.
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