The Effects Of Graphene On The Mechanical And Electrical Properties Of Cement-based Materials

In recent years,the construction cement consumption of China has reached 70% of the world's total cement use,and nearly 2 billion cubic meters of concrete is used for engineering construction every year.The traditional cement-based materials currently widely used have many defects,such as high brittleness,low tensile strength and poor tensile toughness,which will adversely affect the long-term safety and durability of the structure.The common solution is to add macroscopic fiber reinforcement materials such as steel fibers,carbon fibers to cement-based materials.However,the ductility of these fibers is not high,and they can only limit the development of macroscopic cracks,but cannot control the generation and development of initial cracks from the microscopic scale.Graphene is an ideal two-dimensional nano-reinforcing material with a huge specific surface area and excellent physical and chemical properties,which is expected to effectively improve the mechanical properties of cement-based materials.Therefore,it is of great significance to explore the reinforcing and toughening effects and mechanisms of graphene on cement-based materials for the development of new concrete materials.Since the surface of graphene is inert,it is difficult to effectively disperse into water or other solvents.Therefore,different types and dosages of surfactants(SAAs)were used to prepare graphene dispersions with different ultrasonic energy in the paper,and their dispersion effects were evaluated using UV-visible spectroscopy and centrifugal deposition.Experimental results indicated that the effect of surfactants on the dispersibility of graphene was SDBS(sodium dodecyl benzene sulfonate)> SDS(sodium dodecyl sulfate)> SP(2651F polycarboxylate superplasticizer)> CTAB(hexadecyl trimethyl ammonium bromide)> TX100(polyethylene glycol tert-octylphenyl ether).When the mass ratio of graphene to SDBS was 1:6,a uniform and stable dispersion solution could be obtained after applying 270 kJ ultrasonic energy,and no obvious precipitation was observed after centrifugal precipitation test.Secondly,the surfactant SDBS and ultrasonic were used to prepare uniform and stable graphene aqueous dispersions,and the effects of different amounts of graphene on the mechanical properties of cement paste were analyzed.Experimental results indicated that appropriate amount of graphene could effectively improve the mechanical indexes of early age of cement paste,while excessive amount of graphene would adversely affect the mechanical properties of cement paste.The 7-day compressive strength,flexural strength and tensile strength of cement paste with addition of 0.025 wt% graphene were increased by 14.9%,23.6% and 15.2% respectively,and its 28-day compressive strength,flexural strength and tensile strength were increased by 10.0%,16.4% and 2.4% respectively.SEM,MIP and XRD analysis showed that graphene could be dispersed in cement hydration products and bridge the hydration products together,reducing the porosity of the structure and improving the pore structure distribution.In addition,microhardness tests showed that graphene can also improve the micro-vickers hardness of cement paste.Thirdly,the effects of MWCNTs and graphene on the fracture mechanical properties of cement paste were compared and analyzed.The results indicated that both MWCNTs and graphene could improve the mechanical properties and fracture toughness of cement paste.The addition of 0.05 wt% MWCNTs improved the fracture energy and fracture toughness of paste by 54.1% and 42.6% respectively,and the addition of 0.05 wt% graphene increased the fracture energy and fracture toughness of paste by 37.0% and 11.0% respectively.Microscopic analysis showed that the bridging effect of MWCNTs could effectively improve the load transfer efficiency in the matrix and control the development of microcracks,and the two-dimensional fold morphology of graphene could effectively wrap the hydration products and improve the integrity of the matrix.Finally,the effects of different water content and temperature factors on the resistivity of graphene cement paste were explored,and the pressure-sensitive properties of cement paste under cyclic load were analyzed.The results indicated that the resistivity of cement paste decreased obviously with the increase of graphene content,water content and temperature.In the dry state,the resistivity of 2.0 wt% graphene cement paste was only 24.75% of the control group.The resistivity sensitivity to temperature decreased with the increase of graphene content,and there was a functional relationship between the resistivity and absolute temperature.The pressure sensitivity test indicated that graphene could effectively improve the pressure sensitivity of cement paste,and the resistance change rate of the 0.08 wt% graphene cement paste could reach 5.92%.