Fracture toughness improvement of multi-wall carbon nanotubes/graphene sheets reinforced cement paste

•MWCNTs or graphene were uniformly dispersed into cement pastes.•MWCNTs increased fracture toughness of cement paste significantly.•Appropriate amount of MWCNTs or graphene improved microhardness of composite.•Pore refinement by MWCNTs or graphene strengthen cement. Different amounts of multi-walled carbon nanotubes (MWCNTs) and graphene were homogeneously dispersed into cement paste by applying efficient surfactant and ultrasonic treatment. Subsequently, their influences on the compressive and fracture toughness of cement paste were studied contrastively. Results indicated that the introduction of MWCNTs and graphene can soar the mechanical properties of the composites, and the fracture energy and fracture toughness of cement paste were increased by 54.1% and 42.6% respectively with the addition of 0.05 wt% MWCNTs (by weight of cement). Similarly, the fracture energy was promoted by 37.0% with the same amount of graphene. On the basis of micro analyses, the bridging effect of MWCNTs and the high bonding strength between graphene and cement hydration products were observed, which made it possible to enhance load-transfer efficiently. Furthermore, the porosity and pore size distribution of the composites were refined when adding moderate amount of MWCNTs and graphene, and the microhardness of the composite was also improved.