Fracture toughness enhancement of cement paste with multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) have been incorporated into cement pastes to investigate the effect on compressive strength and fracture toughness. Cement-based composites have been prepared from Portland cement with various amounts of common MWCNTs or MWCNTs with a carboxyl group (MWCNTs-COOH), ranging from 0% to 0.1% by weight. Nanotubes were uniformly dispersed into cement paste by applying ultrasonic energy in combination with the use of surfactants. Results indicated that fracture toughness of composites increased by the addition of common MWCNTs, while compressive strengths showed no significant increase. However, MWCNTs-COOH significantly improved fracture energy, fracture toughness and compressive strengths of cement pastes. The porosity and pore size distribution of composites were measured by Mercury Intrusion Porosimetry, and the microstructure of samples was analyzed using a Scanning Electron Microscope. The number of micropores with diameters of 25–50 nm was found to reduce significantly with the addition of MWCNTs, especially MWCNTs-COOH, and the nanotubes bridged the cement particles.