Molecular Scale Simulation Of Mechanical And Heat Transfer Properties Of Cement-based Materials

The microstructure and properties of cement-based materials are a complex and very important issue,which requires in-depth research.In the process of research,the content should be kept rigorous and comprehensive.Currently,there are still many different research methods for C-S-H gel,which require researchers to continuously explore and accumulate.The application of molecular dynamics to design cement-based materials on the nanoscale is conducive to improving the physicochemical properties of cement-based materials in engineering applications.Moreover,the application of molecular dynamics simulation and macroscopic performance test will greatly promote the research and engineering application of cement-based materials.At present,most of the studies on the molecular scale of C-S-H gels only focus on the mechanical properties of the single-crystal configuration of c-s-h class crystals,with significant anisotropy.In addition,the rationality and accuracy of the molecular force field for the study of the thermal conductivity of C-S-H have not been confirmed.Molecular dynamics is one of the basic methods to study the atomic scale of cement-based materials.C-S-H gel is the main hydration product of cement,which determines the macroscopic properties of hardened cement-based materials.The focus of this study is to establish a C-S-H gel model and to simulate the mechanical behavior of cement slurry as a layer composite material of hydrated and unhydrated phases at the nanometer level by using molecular dynamics method.The main purpose of these models is to reveal the effects of mechanical properties and thermal conductivity of cement slurry.This paper describes in detail the C-S-H gel molecular structure model and the common types of molecular dynamics field,which is the key factor to determine the accuracy of simulation,and discusses the relationship between the C-S-H gel molecular dynamics model and the mechanical and thermal properties.(1)Analyze the different mechanical properties of C-S-H gel in x,y and z directions.Under the action of tensile load,the tensile strength in x direction is greater than that in y direction and far greater than that in z direction.The failure pattern in the x and y directions is different from that in the z direction.The z direction is more prone to damage due to the connection between the x and the y direction of the silicon chain,which plays an important role in the tensile properties of the C-S-H gel.By simulating the tensile C-S-H gel model,the mechanical data are in good agreement with the results of nano-indentation experiment,which can be used for further simulation research.Under the action of compression load,the compressive strength in y direction is greater than that in x and z directions.In addition,the comparison of tensile strength and compressive strength in the three directions shows that the compressive strength in the three directions is generally greater than the tensile strength.Therefore,we better explain the mechanism of brittle materials' compressive strength and tensile strength from the microscopic level.(2)Establish the possible structure of C-S-H gel in nanoscale,and the mechanical properties of this randomly distributed model in x,y and z directions were analyzed.Through the model of stretching and compression,we find that the mechanical properties of the random distribution model established this time are quite different from those of the previous model.Firstly,although the tensile strength of the three directions is lower than the tensile strength of the previous model,the difference between them almost disappears,which is more in line with the isotropic properties of the actual cement concrete mechanics.(3)Analyze the thermal conductivity of C-S-H gel in three directions.By simulating the heat transfer of the initially established C-S-H gel model,we found that the temperature distribution curves in the three directions all presented a stable state,but the thermal conductivity obtained by calculation had some differences.Then,the thermal conductivity simulation of the randomly distributed model shows that the thermal conductivity is no longer very different from that of the single cell model,and the thermal conductivity is reduced compared with that of the single cell model.