Understanding the aqueous phase of alkali-activated slag paste under bath-curing

The chemical compositions of aqueous phases in alkali-activated slag paste and Portland cement paste were determined up to 28 days with an aim at obtaining a better understanding of stability of hydration products in the two binder systems. The saturation levels with respect to hydration products of PC and AAS are approached through thermodynamic modeling. The main findings include: (1) the effective saturation indices for portlandite in AAS system is always below 0 and sulfate-bearing phases are not stable in AAS compared to those in PC; (2) the strätlingite and hydrotalcite phases should be stable in AAS due to the high [Mg] and [Si] concentrations in the pore solution; (3) both ionic strength and alkalinity of AAS pore solution are higher than those of PC, which are responsible for severer efflorescence in AAS paste and higher conductivity of AAS pore solution. According to thermodynamic estimation, T-based C–S–H is dominant in the AAS system after 7 days, while J- and T-based C–S–H gel were presented in PC system up to 28 days. It suggests that in PC and AAS, different solid phases are formed during the hydration, which change with time, and reactions and equilibria in both binders are completely different.