Enhancing the PVA fiber-matrix interface properties in ultra high performance concrete: An experimental and molecular dynamics study

•Nano-silica enhances the interface properties of UHPC.•Molecular dynamics unravel the interaction mechanisms on the UHPC.•Sol-gel method has a relatively high coating efficiency. The interface properties largely determine the improvement effect of fibers on the cement-based matrix. In this work, nano-silica particles are employed to treat PVA fibers using a sol–gel method, aiming at enhancing interface properties in ultra-high-performance-concrete (UHPC). Based on a comprehensive analysis of SEM-EDS, TG, and Raman spectra, this modification measure has a relatively high coating efficiency and the nano-silica can be evenly distributed and stably adsorb on the surface of PVA fibers. Furthermore, by molecular dynamics simulations, the interaction mechanisms on the fiber-cementitious matrix interface are unraveled. The Ca (from matrix surface) -O (from silica) coordinations lead to chemical bondings, substantially stronger than H bonds formed on PVA/C-S-H interface. As a consequence, the pull-out of SiO2 modified fibers consumes significantly higher energy than pristine PVA. In conclusion, the coated nano-silica particles not only increase the surface roughness of PVA fibers, which correspondingly improves the physical friction and interlocking effect, but also react with Ca-containing phases in the cementitious matrix, forming new calcium silicate hydrate gels and filling in the cracks. These two mechanisms help increase the UHPC compressive strength by around 10% and the flexural strength by over 26%.