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.