Multiple effects of nano-SiO2 and hybrid fibers on properties of high toughness fiber reinforced cementitious composites with high-volume fly ash

This article explores multiple effects of nano-SiO₂ and hybrid fibers on the flowability, microstructure and flexural properties of high toughness fiber reinforced cementitious composites. Only a little negative influences of nano-SiO₂ and hybrid fibers on the flowability are observed. SEM and MIP analysis reveal that nano-SiO₂ results in much smaller pore size in the composites. However, the porosity increases gradually with nano-SiO₂ addition. Three-point bending test results show that nano-SiO₂ increases the flexural strength of the composites with nearly equivalent deformability, but higher strength of the matrix leads to wider cracks. Due to larger volume fraction and higher modulus, hybrid fibers effectively mitigate this adverse influence on crack width and further enhance the flexural strength. The composites reinforced with 1.4% steel fiber and 2.5% polyvinyl alcohol (PVA) fiber exhibit the best flexural properties in the test. Finally, a simplified model is proposed to illustrate the reinforced mechanism of steel-PVA fibers.