Efflorescence

Efflorescence is the white, powdery or crystalline deposit that appears on the surface of brick masonry, plaster or concrete when soluble salts (sulphates, chlorides, carbonates of sodium, potassium, calcium, magnesium) present in the bricks, sand, cement or ground water are dissolved by moisture, migrate to the surface, and crystallise as the water evaporates. It is primarily an aesthetic defect but persistent dampness driving it can also cause sub-florescence (salt crystallising within pores), which spalls plaster and decays masonry.

IS 1077 limits the efflorescence rating of building bricks (tested by standing them in water and observing deposit) to a maximum of 'moderate' for general work and 'slight' for facing/exposed work. Control measures: use low-salt bricks + clean sand + potable water, provide damp-proof courses + waterproofing to cut the moisture source, and remove existing bloom by dry brushing or dilute-acid washing rather than just painting over it (which traps the salts).