Molecular Simulation Study On Mechanical Properties Of Nano Graphene Reinforced Cement Based Materials

Nanomaterials have excellent mechanical properties and great application potential in engineering.As a two-dimensional nano material,graphene has attracted much attention.It has excellent mechanical,electrical,magnetic,thermal and optical properties.These unique advantages make graphene have great application prospects in many fields.Graphene is composed of carbon atoms that form hexagonal carbon ring structures in an sp~2 hybrid manner and extend infinitely in the plane.This structure has stable in-plane mechanical properties and is the strongest material known by far.As the main component of construction materials,the mechanical properties of cement-based materials play an important role in the building structure.Therefore,to obtain cement-based materials with improved mechanical properties is also the goal of researchers.Researchers have carried out a series of researches on graphene/cement-based nanomaterials,but only limited to the observation of experimental phenomena.The interaction and binding mechanism of graphene and cement at micro nano level need to be explored in more detail.In this thesis,the main components of cement such as silicon dioxide and calcium silicate hydrate(C-S-H)are used as the molecular structure model,and the mechanical properties of cement-based materials mixed with graphene under tensile,shear,and compression loads are explored at the microscopic level.The deformation and failure mechanisms of the composites are compared and analyzed,and the influence of porous graphene on the mechanical properties of cement-based materials has been studied,and for more complex models involving C-S-H model,we have made modifications to the interatomic potential model so as to improve the description of the system.The conclusions of the research are as follows:(1)When porous defects are introduced into the graphene structure,the ultimate strain decreases monotonically with the increase of defects,then within a certain proportion of defects,the ultimate strain shows little changes at a constant value,while Young’s modulus and ultimate stress decrease monotonically with the increase of defects.Most importantly,both the ultimate stress and Young’s modulus are higher than those of cement-based materials over a wide range of defect concentrations,suggesting that such low-cost defective or disordered graphene structures can be used to effectively improve the mechanical strength and ultimate load carrying range of engineering structures.(2)Based on the above basis,the porous graphene was dispersed into the silicon dioxide gap to form a composite structure,and the stress-strain curve obtained by simulation demonstrate that the porous graphene can improved the mechanical properties of the composites.On this basis,this thesis systematically explored various ways to modify the mechanical properties of cement-based structures by adjusting the defect concentration,loading mode,volume fraction of porous graphene and interface roughness,etc.The results of this paper show that the addition of porous graphene can effectively improve the strength and plastic properties of cement-based materials.The observation of the local stress state at the interface of the composite structure showed that graphene exerted tensile stress on the silicon dioxide surface layer,revealing the reason for the reduction of the ultimate stress of the composite material found in this study.Also,the change of defect concentration and roughness increases the interaction between graphene and silicon dioxide interface,and the tensile and shear properties are greatly improved.The compression performance is not significantly improved,but the compression performance of construction materials is relatively strong.Therefore,the mechanical properties of composite materials have been improved in general.These results can provide a theoretical basis for designing low-cost defective graphene-reinforced construction materials.(3)As another major component of cement-based,molecular simulation of calcium silicate hydrate is still a research frontier in the academic field,and the study of its structure and mechanical model is a challenging topic.This paper adopts the molecular structure model of calcium silicate hydrate of the MIT research group as the start material.However,the interatomic potential model of this structure decreases to be negative values when the atom spacing is too close,which is unphysical.In order to solve this problem,the Buckingham function is partially modified in this paper.The simulation results of the modified potential function for calcium silicate hydrate show that the modification is effective.More importantly,the correction resolves abnormal output such as structural collapse due to non-physical function.Finally,we discuss the future prospects in simulating calcium silicate hydrate and graphene.