Simulation Of Mechanical Properties Of C-S-H Gels Based On Coarse-Grained Molecular Dynamics

Concrete is widely used in construction projects as one of the most important construction materials.The mechanical properties of concrete materials mainly depend on cement hydration products,and calcium silicate hydrate(C-S-H)gel is the most important hydration product and net slurry strength source during cement hydration,which controls the strength and durability of concrete to a large extent.At the macroscopic scale,theoretical and experimental studies on cementitious materials have been more detailed.At the molecular scale the C-S-H gel model exhibits delamination,showing significant anisotropy in the x,y,and z directions,which is quite different from the macroscopic isotropy,and the molecular model cannot reflect the difference in C-S-H gel density and the presence of gel pores.Most of the studies on the mechanical properties of C-S-H gels at mesoscopic scales have focused on spherical particles,while relatively few studies have been conducted on the structure of C-S-H gels composed of non-spherical particles.Recent studies have detected the selective orientation of gel particles during indentation by nanoindentation experiments on cement pastes.In order to characterize the particle orientation problem,the mechanical properties of the C-S-H gel structure are simulated and analyzed in this paper using an ellipsoidal coarse-grained C-S-H gel model based on a coarse-grained molecular dynamics approach,and the main work and conclusions are as follows:(1)Uniaxial tensile,compressive and shear mechanical behaviors of the coarse-grained CS-H gel model were simulated in x,y and z directions.The results show that the stress-strain curves of C-S-H gels in the three directions are similar,indicating that the failure mechanisms in the three directions are close and the structure of C-S-H gels tends to be isotropic at the mesoscopic scale.The coefficient of variation was used to express the dispersion of the mechanical properties in the three directions,and the analysis data showed that the coefficient of variation of Young’s modulus was much smaller than that of tensile strength,and in addition,the peak stresses in the three directions under compressive loading were larger than those under tensile loading.(2)A nanoindentation substrate model was established,and a nanoindentation simulation study was conducted on the C-S-H gel model using a virtual indenter and a diamond indenter.The whole simulation process included two stages of loading and unloading,and the load-depth curve of the nanoindentation and the deformation behavior of the substrate during the indentation process were investigated.The effects of indentation depth,loading speed and indentation radius on the nanoindentation process were investigated in depth.The results show that the load generated at the maximum displacement,the indentation modulus and the indentation hardness show an increasing trend as the indentation depth,the loading speed and the indenter radius increase.By comparing the load displacement curves,indentation modulus and hardness in three directions,it is shown that the C-S-H gel tends to be isotropic during the nanoindentation process.During the loading and unloading phases,the C-S-H gel model underwent significant elastic and plastic deformation behaviors.The particle displacement at the bottom contact of the indenter is the largest after the loading is completed,and the elastic and plastic deformation of the C-S-H gel can be clearly observed in the particle displacement diagram.(3)The C-S-H gels with defect lengths from 0 to 57 nm and random defects were modeled,and the elastic mechanical parameters of the C-S-H gels and the stress-strain relationships in uniaxial tension and shear were calculated,and the degree of influence of the defects on the mechanical properties of the C-S-H gels under different stress states was further analyzed by the stress-strain characteristic parameters.The results showed that the defects of gel pores with nanoscale structure in C-S-H gels were detrimental to their mechanical properties,and the Young’s modulus and peak stress of C-S-H gels showed a decreasing trend with the increase of the initial defect length,and the larger the defect length,the more obvious the decrease of Young’s modulus and peak stress.The Young’s modulus and peak stress of randomly defected C-S-H gels were reduced to different degrees.The Young’s modulus and peak stress values of C-S-H gels with multiple random defects fluctuated within a certain range,and the fluctuations of Young’s modulus and peak stress values of C-S-H gels under shear loading were much larger than those under uniaxial tensile loading.