Efflorescence is a powdery deposit of salts which forms on the surface of bricks and mortar. It is usually white but efflorescence may be yellow, green or brown.
Salts enter the wall from various sources. New bricks seldom contain soluble salts but mortar and concrete have relatively high soluble salt contents. Ground waters that are naturally salt-bearing, can be drawn into base brickwork. A faulty damp proof course or a damp course bridged by mortar will allow the salts to migrate up the wall. Render which has been applied over a damp proof course can also allow salt to migrate up the face of the brickwork.
The amount of efflorescence that occurs is directly related to the amount of water in the bricks, and their drying time. The more water in the bricks, and the longer it is there, the more chance salts will have to dissolve in it and be brought to the surface as the bricks dry out. Water allowed to enter uncovered cavity walls during construction is likely to cause efflorescence so brickwork must be protected from water entry during construction.
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.
Efflorescence can be minimised by laying dry bricks and by speeding up the drying process after the bricks have been laid by providing good ventilation. The salts that cause efflorescence are soluble in water. Hosing with water will cause the salts to dissolve and be re-absorbed into the brickwork, and then reappear when the brick wall dries out again.
Acid or alkaline treatments are not recommended as they increase the salt content of the wall.
The best method is simply brush off the deposit with a stiff dry bristle brush after the wall has dried out. Then sponge the surface with a damp synthetic chamois or high suction sponge. Use very little water and rinse sponge frequently in fresh water.