Efflorescence is the residue that is left behind after water is removed. The residue is mostly salt-based and harmless. Different types occur in the construction industry, including concrete, brick, and masonry. Efflorescence remover typically is available to prevent unsightly build-up on surfaces.
In concrete, efflorescence is caused when the water used to mix the original concrete powder begins to dissolve the salt that naturally occurs in the mixture. As the water evaporates over time, the salt is drawn to the surface of the concrete and leaves the residue behind. Most concrete is gray or white so the residue usually is not as obvious.
Efflorescence on bricks, however, usually is more noticeable because the white powdery residue can stand out against the reddish color of some bricks. The water used to mix the grout that is used between the bricks often is where the residue originates. Grout efflorescence usually can be removed by using a stiff brush with water. If the residue is more pronounced over a larger amount of space, another option can be pressure washing.
Masonry efflorescence is also caused by water evaporation which leaves crystallized salt deposits behind. New construction may not reflect the problem but after several weeks, a new construction bloom may occur as the water makes its way to the surface. Often damp conditions such as ongoing rain or hairline cracks in the masonry can make the occurrence more severe.
Cleaning for severe conditions that are not solved by hand-scrubbing or pressure washing might require chemicals. The most common chemical used to treat the problem is muriatic acid. This treatment usually will wash away the efflorescence, but it also takes off a thin layer of the surface; it typically is used only in extreme conditions. The acid is diluted in water then spread on the affected surface. As soon as the residue is eaten away, baking soda or ammonia is spread on the surface to neutralize the acid and prevent erosion.
While total prevention is not possible, measures are often taken to reduce the likelihood and severity of efflorescence. The most common method is to add silicon sealers to concrete and masonry mixtures to seal the surface. This is accomplished when the silicon reacts with the lime in the mix which results in calcium silicate. Calcium silicate acts as a barrier and prevents the water from evaporating at the surface, which in turn prevents the salt residue from being left behind.