Gypsum formation mechanisms and their contribution to crystallisation pressure in sulfate resistant hardened cement pastes during early external sulfate attack at low sulfate concentrations

Gypsum formation mechanisms during external sulfate attack were studied on Portland cements of various C₃A content exposed to low sulfate concentrations. Though ettringite is acknowledged as the main expansive phase during external sulfate attack, the simultaneous formation of gypsum can cause expansion of laboratory samples prepared with SR Portland cement. The comparison of free expansion, expansion pressure, sulfur distribution, microstructure and thermodynamic modelling was used to identify three different gypsum formation mechanisms: (i) gypsum formation in mesopores of the hardened cement paste matrix (>1500 mg l⁻¹ SO₄²⁻), (ii) gypsum formation in new macroscopic cavities caused by matrix expansion (>1500 mg l⁻¹ SO₄²⁻) and (iii) replacement pseudomorphs of gypsum after portlandite (metastable, ≤ 1500 mg l⁻¹ SO₄²⁻). Only the first mechanism is likely to contribute to expansion of non-constrained samples. The second and third mechanism are not able to exert significant expansion pressure and are therefore unlikely to cause damage under field conditions.