Effect of fresh water leaching on the microstructure of hardened composite binder pastes

The microstructure of cement-slag and cement-fly ash pastes leached by fresh water for 2-3 years was investigated by MIP, XRD, TG and SEM. Results indicated that long-term fresh water leaching caused degradation of the pastes in terms of mass loss, increase in porosity and decrease in Ca(OH)2 content and Ca/Si ratio of C-S-H gel, but showed no decomposition of hydration products. The pastes containing a small amount of fly ash were not damaged severely, incorporation of 65% of fly ash deteriorated most severely, whereas the cement-slag pastes (⩽70%) showed high long-term fresh water leaching resistance.