Influence of silica fume and thermal curing on long-term hydration, microstructure and compressive strength of ultra-high performance concrete (UHPC)

In order to enhance the hydration rate and degree of ultra-high performance concrete (UHPC) originated from the extremely low water to binder ratio, thermal curing is widely used to improve the hydration and mechanical properties of UHPC. In this study, the effect of thermal curing temperature on hydration process, microstructure evolution and long-term compressive strength of UHPC is investigated and the effect of silica fume is also studied. The results show that when silica fume is 20%, the maximum compressive strength of UHPC by 170 MPa can be obtained after 90 degrees C thermal curing for 3 d, which is increased by 26.9% comparing with 20 degrees C curing for 1 year. Correspondingly both the degree of hydration (DOH) of cement and degree of pozzolanic reaction (DOPR) of silica fume are separately increased by 5.8% and 20.6%. After thermal curing for 1 year, the DOH of cement varies within a maximum increase range of 3%, while the DOPR of silica fume increases slightly, up to 7%. In addition, the total porosity can be reduced by 64.0% under 90 degrees C thermal curing. Porosity characterized with 10 nm to 50 nm has a decisive impact on the compressive strength whose correlation coefficient can reach 0.90.