Interfacial microstructure and bond strength of nano-SiO₂-coated steel fibers in cement matrix

In this study, a new surface modification method for steel fibers is proposed to improve the interfacial properties of similar fibers in a cement-based composite. Steel fibers were coated with a nano-SiO₂ multilayer film, which was expected to react with Ca(OH)₂ formed from cement hydration, reducing the possibility of a weak zone and creating a denser microstructure at the fiber-matrix interface. Several techniques, including scanning electron microscopy (SEM), backscattered scanning electron microscopy (BSEM), energy dispersive spectroscopy (EDS), and infrared absorption spectroscopy (IR) were used to characterize the interfacial microstructure improvement resulting from the surface modification at the micro-scale; in addition, a fiber pull-out test was carried out to evaluate the enhancement of the mechanical properties of the interface at the macro-scale. The experimental results indicate that the coated nano-SiO₂ can react with Ca(OH)₂, resulting in an increased quantity of C–S–H gel and decreased porosity at the modified steel fiber-cement interface. The dense adhesion between the modified fibers and cement matrix facilitate the establishment of a stronger interfacial bond. Both the bond strength and pull-out energy increase significantly at 3, 7, and 28 days. The results support the conclusion that the proposed surface modification method is suitable for steel fibers and will improve their reinforcement efficiency.