||Response from Mitre 10: I think that the white deposits you`re referring to is efflorescence - a salt like deposit on the surface of the wall. This can be caused from excess moisture seeping through the brickwork. Basically, water seeps through the brick and concrete, collecting the mineral salts, then when the water reaches the surface and evapourates, the salt deposits are left behind.
The most effective treatment for efflorescence is time. The natural weathering of the surface, if maintained, will remove it. For a build up of deposits on a wall, a good scrub with a dry, stiff brush will remove the worst, but make sure you follow it with a soft brush to move the salts away from the wall entirely. The salts will stay in the pores of the brickwork and dissolve again when it is wet. Treatments, such as acid or pressure spraying etc, generally involve getting the surface, and the salts, wet again. This dissolves them and carries them into, not out of, porous surfaces. Introducing more moisture to the wall is not recommended because once the moisture has evapourated, the deposit will return.
Persistent efflorescence should be taken as a warning that water is entering the wall through faulty copings, flashings, or pipe leakage. If allowed to continue unchecked the salts carried to the face of the wall may eventually attack and cause deterioration of some bricks. The source of water causing the efflorescence should be found and stopped.