Sulphate Attack on Concrete

Sulphate attack is the deterioration of concrete when sulphate ions (from sulphate-bearing soils, ground water, sea water or industrial effluent) penetrate and react with the calcium hydroxide and the hydrated calcium aluminates (C₃A) of the cement paste, forming gypsum and expansive ettringite. The internal expansion causes cracking, softening, loss of strength and progressive disintegration, typically starting at edges/corners exposed to wetting-drying.

IS 456 Table 4 classifies exposure by sulphate concentration in soil/water (Class 1 to Class 5) and prescribes for each the minimum cement content, maximum w/c, and the cement type — increasingly mandating Sulphate-Resisting Portland Cement (SRPC to IS 12330, low C₃A), or Portland Slag/Pozzolana cement, plus a dense low-permeability mix. The first line of defence is always low permeability (low w/c, good compaction + curing) since attack needs sulphate transport into the concrete.