Effects of the phosphogypsum on the hydration and microstructure of alkali activated slag pastes

Abstract

Effects of phosphogypsum (PG) on hydration process and microstructure of the alkali activated slag (AAS) pastes were discussed in this study, taking the natural gypsum (NG) as a reference. Hydration process was monitored by several kinds of measurements including the isothermal calorimetry, the electrical resistivity, the autogenous shrinkage and the 1H nuclear magnetic resonance. Hydration product assemblages were analyzed by the XRD, NMR, SEM and the mercury intrusion porosimetry (MIP)test. Results showed that less than 10 % of gypsums could retard the setting process of AAS pastes. PG could enhance the late strength although the early strength was decreased, but the addition of NG would reduce the strength at all ages. Induction period and the autogenous shrinkage were both reduced by gypsum gradually. Additionally, PG conducts to the higher MCL and higher polymerization degree of CASH gels than NG. And the addition of PG would refine the pore structure and produce a lower porosity than NG. Furthermore, the comprehensive analysis showed that the setting behavior happened at the initial dissolution period. And PG would delay the acceleration period, indicating the slower development of microstructure.