In mortar, sulfate generally originates from the calcium sulfate (gypsum, anhydrite, hemihydrate) added to control setting. Efflorescence of calcium carbonates, notably calcite, CaCO3 (formed by the reaction of Ca(OH)2 with CO2), often occurs on the surfaces of masonry or concrete elements. Efflorescence of gypsum occurs as a white foggy deposit on the surface of clay bricks. Efflorescence and wash-out of lime typically occurs on masonry units. During wetting, rain runs off the masonry surfaces and cause excessive wetting of the mortar joints. This facilitates dissolution of lime in the pore water of the mortar joints and prevents carbonation of this lime.
Most efflorescence can be washed away by high-pressure water. Acid solutions will also dissolve and remove the efflorescence from surfaces. Efflorescence can be limited or prevented through proper drainage and concrete sealers can also prevent efflorescence from reoccurring. The use of lime and GGBS in mortar mixtures would effectively eliminate efflorescence in masonry structures.