Sulfate attack in concrete and mortar

Sulfate attack typically happens to ground floor slabs in contact with soils containing a source of sulfates. Sulfates dissolved by ground moisture migrate into the concrete of the slab where they react with different mineral phases of the hardened cement paste.

The attack arises from soils containing SO2−
4 ions, such as MgSO4 or Na2SO4 soluble and hygroscopic salts. The tricalcium aluminate (C3A) hydrates first interact with sulfate ions to form ettringite (AFt). Ettringite crystallizes into small acicular needles slowly growing in the concrete pores. Once the pores are completely filled, ettringite can develop a high crystallization pressure inside the pores, exerting a considerable tensile stress in the concrete matrix causing the formation of cracks. Ultimately, Ca2+ ions in equilibrium with portlandite (Ca(OH)2) and C-S-H and dissolved in the concrete interstitial water can also react with SO2−
4 ions to precipitate CaSO4·2H2O (gypsum). A fraction of SO2−
4 ions can also be trapped, or sorbed, into the layered structure of C-S-H. These successive reactions lead to the precipitation of expansive mineral phases inside the concrete porosity responsible for the concrete degradation, cracks and ultimately the failure of the structure.

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