Effects of silica-fume surface modification on polypropylene fiber-matrix interaction in cementitious composites

The fiber-matrix interface plays a pivotal role in determining the mechanical performance of fiber-reinforced composites. However, achieving optimal interfacial bonding remains challenging, particularly for synthetic fibers like polypropylene (PP), due to their inherently smooth surface and low chemical affinity to cementitious matrices. To further improve PP fiber-matrix interface properties, the current study evaluates the effectiveness of three novel surface treatments using silica-fume surface coating methods developed at the University of Victoria—adhesive dry coating, adhesive wet coating, and a non-adhesive heat coating. The impact of these surface modifications on fiber-matrix adhesion was assessed through contact angle measurements, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS/EDX), and micro-hardness testing. Mechanical performance was further evaluated using single-fiber pull-out, flexural, and compressive strength tests. SEM, EDX, and micro-hardness results confirmed that silica-fume from the coatings reacts with portlandite, significantly improving the interfacial transition zone (ITZ) within the first 40 μm from the fiber edge. Samples treated with adhesive dry coating and adhesive wet coating exhibited 96% and 31% higher overall energy absorption in pull-out tests, respectively, compared to untreated fibers. Furthermore, all coating methods nearly doubled post-crack energy absorption in flexural tests. Samples containing fibers treated with non-adhesive heat coating demonstrated the most pronounced effect among the surface treatments, resulting in 857% higher energy absorption (compared to untreated fiber samples) under pull-out loading, leading to fiber fracture failure. In these samples, the failure mechanism involved post-peak softening followed by strain hardening to a perfectly plastic response until fiber fracture. Based on mechanical and microstructural evaluations of the present study, non-adhesive heat coating method emerged as the most effective surface treatment, demonstrating superior fiber-matrix bonding and enhanced composite performance.