Carbon fiber surface nano-modification and enhanced mechanical properties of fiber reinforced cementitious composites

It has been widely recognized that fiber-matrix interface has great significance on the performance of fiber -reinforced cementitious composites. In this research, an effective fiber modification method is proposed and its enhanced mechanical properties are observed. Through the condensation and polymerization of the tetraethyl orthosilicate under alkaline conditions, a thin SiO2 layer was formed on carbon fiber surface, which can react with Ca(OH)2 thus form calcium silicate hydrate to condense the fiber-matrix interface. In this paper, nano-SiO2 modified carbon fibers are produced and the mechanical properties of modified carbon fiber reinforced cementitious composites are investigated. Modified material exhibited 24.7% enhancement in flexural strength and up to 25.1% enhancement in tensile strength with the fiber volume fraction of 0.5%. Better post-crack behavior and toughness properties were also estimated, which were attributed to nano-modification strength-ening the ITZ through the reaction of surface nano-SiO2 with Ca(OH)2 resulting in much more amount of C-S-H gel and a denser microstructure of fiber-matrix interface. The microstructure of denser interface and porosity can be found through SEM and BSEM analysis. The porosity on the observation surface of interface transition zone of unmodified fiber and modified fiber was 4.03% and 2.43%, respectively. The change of chemical elements in the interface transition zone was analyzed by line scanning, and the Ca/Si of the modified carbon fiber was significantly lower than that of the unmodified carbon fiber.