Understanding the aqueous phases of alkali-activated slag paste under water curing

The chemical compositions of the aqueous phases in alkali-activated slag (AAS) paste and Portland cement (PC) paste were determined up to 28 d with the aim of obtaining a better understanding of the stability of the hydration products in the two binder systems. The saturation levels with respect to the hydration products of PC and AAS were obtained by thermodynamic modelling. The main findings of this study were: the effective saturation index for portlandite in the AAS system was always below zero and the sulfate-bearing phases were not stable in the AAS system compared with those in the PC; strätlingite and hydrotalcite phases were stable in the AAS paste due to the high magnesium and silicon concentrations in the pore solution; both the ionic strength and alkalinity of the AAS pore solution were higher than those of the PC pore solution, which were responsible for more severe efflorescence in the AAS paste and the higher conductivity of the AAS pore solution. According to thermodynamic estimations, tobermorite-based C–S–H was dominant in the AAS system after 7 d, while jennite-based and tobermorite-based C–S–H gels were present in the PC system up to 28 d. The results suggest that in PC and AAS pastes, different solid phases are formed during the hydration, which change with time, and the reactions and equilibria in both binders are completely different.