Efflorescence is a deposit of salt left on the surface of concrete by the evaporation of the moisture that brought the salts from within the structure. It is unsightly but generally harmless. Most efflorescence can be removed by one of the following methods:
scrubbing with a dry brush
scrubbing with water
light sandblasting and hosing with clean water.
However, some salts become water insoluble shortly after reaching the atmosphere. Efflorescence of this type can be removed with a dilute acid solution (such as one part hydrochloric acid in 9 to 19 parts water or one part phosphoric acid in 9 parts water). After treatment the area should be washed with clean water to remove all acid.
Since dilute acid solution may slightly change the appearance of the concrete, the entire area should be treated to avoid blotching. Only very weak acid solutions should be used on coloured concrete as stronger solutions may remove the surface layer, changing the colour and the texture.