Deterioration mechanism of Portland cement paste subjected to sodium sulfate attack

External sulfate attacks on cement-based materials may cause deterioration in terms of cracking, softening and spalling. This work aimed to investigate the damage process and deterioration mechanism of Portland cement pastes exposed to sodium sulfate attack. Cement pastes were immersed in 0·031 mol/l (0·44 wt%) and 0·352 mol/l (5 wt%) sodium sulfate solutions at 20°C for up to 900 d. The surface appearance, mass change and compressive strengths of the cement pastes were investigated during the sulfate attack process. Mineral phases formed in the surface layer and the inner portion of the cement pastes were investigated with X-ray diffraction at different stages of deterioration. Scanning electron microscopy was used to further examine cement paste microstructures after immersion in 0·352 mol/l sodium sulfate solutions for 180, 300, 600 and 900 d. The results showed that the deterioration in cement pastes first occurred in the surface area and then gradually extended to the inner portions. Ettringite was the main product at the initial attack stage, while gypsum was formed at later stages. The formation of fine ettringite resulted in volume expansion of the cement pastes. Gypsum was formed as veins that were sub-parallel to the surface. Ultimately, gypsum formation was accompanied by the decalcification of calcium silicate hydrate, causing softening of the external cement paste layer.