Degradation mechanism of seawater-mixed cementitious material subjecting to sulphate attack

This study compared the sulphate resistance of deionized water (DI)- and seawater (SW)-mixed cementitious materials based mainly on microstructural observations. The results show that, after a 450-day exposure to 50 g/L Na₂SO₄ solution, there was an increase in the mean chain length and polymerization degree of the remaining silicate chains in C-S-H gel for both DI- and SW-mixed pastes at a depth of 0–2 mm from the exposed surface. Notably, the Ca leaching of C-S-H gel in the SW-mixed paste at depths larger than 0.3 mm was slightly less than that in the DI-mixed one, despite the Ca leaching in both samples was comparable at a depth of 0.1 mm. For the aluminate phase, the mass loss and length change of the SW-mixed C₃A mortar were lower than that of the DI-mixed C₃A mortar, after immersing in 50 g/L Na₂SO₄ solution at different times. It seemed that more expansive product can be accommodated in the SW-mixed C₃A matrix. Consequently, fewer cracks deposited by gypsum were observed in SW-mixed paste than in DI-mixed paste. The above were responsible for relatively less compressive strength loss of the SW-mixed mortars suffering from sulphate attack for 450 d.