Study On Properties And Microstructure Of Graphene Modified Cement Asphalt Composite Cementitious Materials

With the continuous accumulation of high-speed railway operation time in China,damages such as horizontal separation and vertical cracks generated by the cement asphalt(CA)mortar layer in ballastless tracks have become increasingly significant.Graphene is a new type of nano-materials that has attracted much attention in recent years.It has super high strength,good toughness and excellent conductivity,which can greatly improve the macro performance of cement-based materials.In this paper,cement paste system(HCP),cement asphalt composite system(HCAP),and cement latex composite system(HLCP)are the research objects.The effects of sulfonic graphene(SG),carboxy graphene(CG)and graphene oxide(GO)on the macroscopic properties and microstructure of cement-based composite materials are systematically studied,and the effect of polymer latex nanoparticles on the mechanical properties and microstructure of HCAP system is preliminary explored.The compressive and flexural strength of specimens of different ages are tested by mechanical testing machine;the electrical conductivity and pressure sensitivity of the specimen are tested by the two-electrode method and the cyclic loading method;the effects of different amounts and types of graphene on the degree of cement hydration are studied by hydration calorimeter,thermogravimetric analysis and X-ray diffraction;the microstructure of composite cementitious materials is studied by mercury intrusion instrument,nitrogen adsorption and scanning electron microscope.Finally,the mechanism of the influence of layered graphene and latex nanoparticles on the macroscopic properties of cement asphalt composite cementitious system is revealed.The main conclusions of this paper are as follows:(1)The addition of SG has little effect on the early strength of each system.The low dosage(G/C=0.01% or 0.03%)of SG and CG can significantly improve the late strength,conductivity and pressure-sensitivity of each system;the high dosage(G/C=0.05%)of SG and CG have no obvious effect on the late strength,conductivity and pressure-sensitive performance,and sometimes even have a counter-productive effect.(2)The addition of SG can slightly increase the early hydration of each system.At low dosage,the presence of SG and CG can provide nucleation templates for hydration products,so it can improve the degree of later hydration of each system and refine the pore structure;while at high dosage,SG and CG are prone to agglomeration,so their influence on the degree of later hydration and pore structure is weakened or even disappeared;GO has basically no effect on the later hydration degree of each system,but it can significantly improve the pore structure at low dosage.(3)The early strength of cement-based materials is mainly determined by the degree of hydration.The addition of SG does not significantly increase the early hydration of each system,so the effect on the early strength is also not obvious.The late strength and electrical properties are mainly determined by the pore structure and the morphology of the hydration products of the composite cementitious material.At low dosage,SG and CG can significantly increase the degree of later hydration in each system and increase the content of hydration products,and then refine the pore structure of cement stone.Therefore,the macro performance of the cement material is improved.In addition,the template effect of graphene can significantly change the morphology of hydration products,resulting in the conversion of CH from hexagonal plate-like crystals to disordered amorphous state,and the increase of C-S-H gel contact sites.As a result,the microstructure of the cemented material is denser,and the mechanical properties and electrical properties are improved.(4)Due to the filling effect and film-forming effect of the polymer latex nanoparticles,S400 and SD623 at various dosages can improve the late flexural strength of the HCAP system.At low dosage,SD623 latex particles can effectively refine the pore structure of the gel,thereby slightly improving the late compressive strength of the HCAP system.At high dosage,the coverage of S400 and SD623 on the cement surface reduces the degree of cement hydration,which reduces the late compressive strength of the HCAP system.