Uncovering the role of micro silica in hydration of ultra-high performance concrete (UHPC)

This study aims to clarify the role of micro silica in hydration of UHPC. The behavior of two different types of micro silica in UHPC was investigated with regard to their filler effect and pozzolanic reaction by using analytical techniques; XRD, TG, ²⁹Si and ²⁷Al NMR spectroscopy, and MIP. The micro silica with high pozzolanic activity led to the higher level of Al-substitution for Si in C-S-H and the denser structure, thus increasing compressive strength, whereas that facilitating filler effect limited the consumption of Ca(OH)₂ even after high temperature curing and led to the higher amount of AFm phase. In the UHPC showing high filler effect, additional hydration of C₃S and C₂S occurred at later ages, increasing the fraction of Q¹ site in C-S-H and decreasing the porosity in the pore diameter region below 10 nm. Consequently, a significant increase in the compressive strength of this UHPC was achieved.