Carbonate Staining (Lime Run)
Lime Run is a hard white or grey surface crust that usually emanates from mortar joints or runs down from a hole or crack between brick and mortar joints. The way to differentiate between Lime Run and Efflorescence is that Lime Run does not disappear when wet.
It occurs when water deposited or collected in the wall during construction dissolves water-soluble calcium compounds in the brick or mortar. Over a prolonged period of time, the water migrates through openings in the wall surface and, as the water evaporates, the dissolved calcium reacts with the atmosphere to produce a hard calcium carbonate crust on the masonry surface.