Study on the hydration kinetics and microstructure of the sulphate-alkali composite activated slag pastes

In this study, the phosphogypsum (PG) was successfully applied to prepare the sulphate-alkali activated slag (SAAS) pastes to reduce carbon emissions and production costs. The set-hardening properties of SAAS samples with PG were optimized using response surface methodology. Then, several in-situ measurements and several characterization methods were applied to monitor the hydration behavior and study the microstructure evolution. Furthermore, the hydration kinetics were analyzed to further reveal the new insight into the activation mechanism of the sulphate-alkali composite. The results showed that the optimized ratio was obtained at 2.7% alkali dosage (AD) and 11.8 % PG, where the initial setting time and 28d strength valued 162 min and 56 MPa, respectively. It was also found that heat flow was accelerated but mitigated, and autogenous shrinkage was gradually mitigated with increasing PG content due to its additional activation effect as sulphate. Samples with 5 % PG require a higher AD to obtain a sufficient degree of hydration. While lower AD would produce more ettringite and CASH gels with higher degree of polymerization for samples with 15% PG. Hydration kinetic analysis showed that addition of 5 % PG would make the nucleation process of CASH gels difficult. While the addition of 15% PG would increase the nucleation density by almost 10 times, the growth rate would be greatly reduced, resulting in a slower but continuous development of the microstructure. Thus, it was demonstrated that the use of PG in SAAS pastes is reasonable and has great potential.