What is the white residue on my brick / stone / concrete?
Efflorescence, if exuding from a crack in a plastered chimney or house wall, really needs the plaster (or rendering) to either side of the crack, removed; chipped away & the inner surface wire brushed to provide a fresh clean surface to adhere new plaster to. After plastering, leave to dry well. Then seal with 2 coats of paint etc. (keep moisture out for the future)
That is likely efflorescence. Efflorescence is a deposit of salt and minerals that are leached from within the material to it’s surface, usually by way of moisture from behind or underneath. Efflorescence usually manifests as a whitish stain on porous masonry and concrete. It is a surface stain that can be removed by several different cleaning operations, depending on your situation. Any brick supplier should have a variety of cleaning products that, upon application with a deck brush, usually do the trick.
