Efflorescence appears as a whitish crystalline deposit that's formed by soluble bases or salts migrating from the interior of the concrete to the surface. Calcium Hydroxide, Ca(OH)2, is the main contributor to efflorescence. It is a normal byproduct of cement hydration, and is not soluble in water.
Efflorescence is most common in damp winter months when lower temperatures and slow evaporation rates allow greater migration of Ca(OH)2 to the surface of the concrete. Higher density concrete is less likely to develop efflorescence. Attention to mix design, curing, and finishing procedures also reduces the possibility of efflorescence.
Efflorescence is less noticeable on exposed- aggregate surfaces with light-colored aggregates. Etched, sandblasted, and retarded surfaces are more likely to show efflorescence. Acid washing using hot water and high pressure is the most likely to cause efflorescence.
Tips for minimizing efflorescence: use a cement low in water-soluble alkalis; use a minimum total cement content consistent with mold or form stripping requirements; use an air entraining agent and superplasticizer; consider using fly ash; always use washed sand and aggregate; use clean mixing water; and consider high-pressure steam curing.