The effects and mechanism of aluminium sulfate on the hydration and strength development of cement paste

The effects of aluminium sulfate (AS) on the fast hardening and late-strength reduction of ordinary Portland cement were investigated by measuring the setting times, compressive strength and shrinkage, and conducting calorimetry, X-ray diffraction, thermogravimetry, scanning electron microscopy with energy dispersive spectroscopy and low-field nuclear magnetic resonance (LF-NMR). Direct evidence of the conversion of calcium hydroxide to AFt (ettringite) in cementitious systems containing AS was discovered. This indicates that there are other new pathways to generate AFt with the addition of AS besides reacting with C3A (tricalcium aluminate). The AFt generated in the above processes was the main reason for cement paste setting and hardening. The enrichment of aluminium on the surface of C3S (tricalcium silicate) affected the mid-stage hydration rate of C3S, hindering the development of the mid-term strength of the paste. The addition of AS resulted in the production of more interspersed rod-like AFt in the cement paste, which increased the proportion of air voids in the paste and caused more pseudo-expansion of the paste. The increase in the pore content and the decrease in the densification of the cement paste were the fundamental reasons for the decrease in the later strength of the cement paste after AS incorporation.