Answer: Efflorescence usually occurs when water saturates masonry walls and dissolves water salts within the mortar and brick. These salts are deposited on the surface of the masonry as the water evaporates. There are several possible source of moisture, including water penetration or condensation from moist air.
Salts also deposit below the surface. This action sometimes occurs when coatings on the surface of the masonry penetrating sealers restrict evaporation and the migration of salts to the surface. Spalling can occur when these salts crystallize below the surface.
The problem is particularly severe when there are excessive sulfates in the brick units. Efflorescence that contains sodium sulfate can exist in two crystal states: anhydrous (without water) and hydrous (with water). In its hydrous state the crystal occupies more space than its anhydrous state. Spalling of the brick and mortar can occur if the salt crystals cycle between these two states.
Sodium sulfate can occur in brick units that are not fired at high enough temperatures or if insufficient quantities of barium carbonate were added to the raw materials. Barium carbonate is often added to control soluble sulfates in the units. Barium precipitates the soluble sulfates in the presence of moisture.