Influence of steam curing and nano silica on hydration and microstructure characteristics of high volume fly ash cement system

In order to find a suitable way to improve the early strength of cement-high volume fly ash (CHVFA) system, nano silica (NS) was introduced. Two curing regimes with the same curing time was conducted: one is under standard curing (20 ± 2 °C, RH ≥ 95%) for 3 days, and the other is under steam curing (65 ± 2 °C, RH ≥ 95%) for 12 h and then standard curing for 60 h. The effect of NS and curing regimes on properties of CHVFA was systematically investigated. Pore structure, hydration products, and microstructure of interfacial transition zone (ITZ) were studied to reveal the mechanism behind the improvement in compressive strength. Results show that addition of NS can increase the strength of 3 days, regardless of the curing process. The reason for this increase is not only because of the accelerated hydrations of both cement and FA, but also due to the decrease in porosity and total pore volume as well as the improvement in the microstructure of ITZ. An interesting finding is that steaming curing can exert more efficient effect on increasing the strength in the presence of NS, which means that NS seems more effective to enhance the strength under steam curing, indicating an obvious synergistic effect between NS and steam curing in CHVFA system. Such results suggest effective way to improve the early strength of CHVFA paste, and the addition of NS can also promote the use of FA in precast concrete, with economic and environmental benefits.