Synergistic effects of CNTs/SiO₂ composite fillers on mechanical properties of cement composites

This paper investigated the hybridization use of carbon nanotubes (CNTs) with nano-SiO₂ in cement composites. The (CNTs)/SiO₂ composite fillers were designed and prepared by electrostatic self-assembly technology to reinforce cement composites. The mechanical properties, microstructure and hydration characteristics of cement composites incorporating CNTs/SiO₂ fillers were systematically researched. The morphology and optical microscopy results show that the CNTs inside CNTs/SiO₂ fillers tended to unwind due to the mechanical separation and steric hindrance of nano-SiO₂ particles with certain size, and its agglomeration degree in the suspension greatly alleviated over time. With the appropriate incorporation of CNTs/SiO₂ fillers (containing 0.10 wt% CNTs and 0.50 wt% nano-SiO₂), the flexural strength, compressive strength and flexural toughness of the cement mortar matrix were sharply increased by 33.5%, 36.5% and 56.0% after curing for 28 days compared to the plain sample, respectively. Microscopic observations show that appropriate nano-additives can densify and refine the hydrated microstructure, and the crack-bridging, debonding and pull-out behaviors of CNTs were all observed. Hydration analysis quantitatively reveals that CNTs/SiO₂ fillers significantly accelerated the cement hydration process by virtue of the nano-nucleating action. The reinforcing mechanisms of CNTs/SiO₂ fillers can be attributed to the proposed synergistic effects of CNTs/SiO₂ fillers, which mainly include the better dispersion stability of CNTs, the nucleating effect of nano-additives and the pozzolanic reaction by nano-SiO₂, thus positively leading to increased mechanical properties.