Uncovering the role of hydrophobic silica fume (HPS) in rheology, hydration, and microstructure of ultra-high-performance concrete (UHPC)

This article aims to investigate the effect of hydrophobic silica fume (HPS) on the rheological behavior, hydration performance, compressive strength, and microstructure of ultra-high-performance concrete (UHPC). Those characteristics of UHPC containing HPS or silica fume (SF) were explored by analytical techniques: rheology, X-ray diffraction, thermogravimetric, Fourier-transform infrared spectrum, X-ray photoelectron spectroscopy, Si-29 nuclear magnetic resonance spectroscopy, mercury intrusion porosimeter, and scanning electron microscopy-energy dispersive spectroscopy. This study demonstrated that UHPC containing HPS showed a lower viscosity and yield stress and developed higher compressive strength and flowability than that of UHPC with SF. Furthermore, the HPS with higher pozzolanic activity consumed calcium hydroxide crystals and formed C-S-H gel, which refined the pore structure, improved the compressive strength, and decreased the porosity. Moreover, HPS changed the gel molecular structure, increased the Al substitution in the silicate network and mean chain length of C-(A)-S-H, and deceased Ca2p-Si2p energy separation values, which led to a higher degree of polymerization in the C-(A)-S-H.