Effects of nano-SiO2 coated multi-walled carbon nanotubes on mechanical properties of cement-based composites

Abstract Interfacial microstructure and interactions largely affect the mechanical properties of multi-walled carbon nanotubes (MWCNTs) reinforced cement-based composites. To improve the interfacial adhesion of MWCNTs-cement composites, a simple and effective method for preparation of SiO2/MWCNTs nano-composites were put forward using polyvinyl pyrrolidone (PVP) as a surfactant without any oxidation treatment on MWCNTs. The morphology, microstructure and chemical composition of SiO2/MWCNTs nano-composites were characterized by electron microscopy, Fourier transform infrared spectroscopy (FTIR), Raman spectrum, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). It was found that amorphous nano-SiO2 uniformly and densely grown on the surface of MWCNTs, forming a rougher film. The chemical reactivity of the nano-SiO2 film was detected by immersion tests in Ca(OH)2 solution, where EDS and FTIR analysis confirmed the formation of hydrated calcium silicate gel (C-S-H). The effects of SiO2/MWCNTs on mechanical properties of cement mortars were investigated and compared with that of cement-based composites incorporating pristine MWCNTs. When 0.25 wt% and 0.50 wt% SiO2/MWCNTs were added, the flexural strengths of cement mortars were increased by 11.9% and 25.4%, respectively, which were more significant than the enhancement effects on compressive strengths (8.5% and 19.0%). The fracture surfaces of different samples were observed by SEM, and the Ca(OH)2 content was quantitatively determined by TG analysis, indicating that the chemical reaction of nano-SiO2 film with Ca(OH)2 generated from cement hydration process can effectively increase the C-S-H gel content at the interface between MWCNTs and cement matrix, which is the prime reason for the increase of mechanical properties.