Efflorescence
Efflorescence turns up in 2 fields: concrete, waterproofing. Here is what it means to the people in each one.
Efflorescence in concrete
The white powder or crust that appears where water seeps out of concrete or masonry and leaves its salts behind.
Water dissolves soluble salts and calcium hydroxide inside the concrete, carries them to the surface and evaporates, leaving a white bloom that reacts with air to form calcium carbonate. It shows up most on walls, basements and new slabs in cool, damp weather and is usually cosmetic. Dry brushing removes fresh deposits, while a hard crust needs a mild acid wash and a fix for the water source.
"Basement wall's covered in efflorescence along the cold joint, so that's where your water is coming in."
Efflorescence in waterproofing
White crystalline salt deposits left on masonry or concrete as water moving through it evaporates.
Water dissolves salts from mortar, brick, grout or soil, carries them to the surface and leaves them behind as it dries. Efflorescence itself is cosmetic, but it maps where water is traveling through the wall, which makes it a diagnostic clue for failed flashings, blocked weeps and leaking joints. When salts crystallize beneath the surface instead, they can cause spalling.
"Look at the efflorescence fanning out under each window, those sill flashings have no end dams."