The Transport Properties Of Water And Ions Confined In The C-S-H Nanopores: A Molecular Dynamics Simulation

Cement is the most widely utilized artificial construction materials in the world.Calcium silicate hydrate（C-S-H）,the main reaction product during the hydration process of cement particles,accounts for 60%-70%of the hydration products of cement particles and is the most crucial binding phase of cement paste.C-S-H is a porous gel with the multi-scale porous structures,in which water and ions can diffuse.Transport properties of water and ions confined in C-S-H nanopores are closely related to mechanical properties and durability of cement-based materials.In this study,based on previous researchers,molecular dynamics simulation method is utilized to investigate the water and ions transport properties in C-S-H nanopores,meanwhile,the influences of the pore size,temperature and ion concentration of the nanopore are discussed in detail.The main content of this thesis is as follows:（1）A C-S-H model based on experimental results is constructed,and the nanopore induced by loads is taken into account for the first time.C-S-H surface has good hydrophilicity,and the water molecules gathering in the vicinity of C-S-H surfaces present peculiar structural and dynamic characteristic:layer structure,high density,orientation preference,and low diffusion coefficient.Due to the lack of strong constraint from the C-S-H substrate,the surface calcium ions diffuse more rapidly than those constrained in C-S-H gel and could diffuse from the C-S-H surface into the solution.（2）The change of pore size has a tremendous impact on the arrangement of water molecules in the nanopores,especially on the structure of water molecules near the surface of C-S-H.With the increase of pore size,the diffusion coefficient of water molecules increases,while the difference between D_XY and D_Z decreases,the heterogeneity of water molecule diffusion weakens.The surface calcium ions are driven by surrounding water molecules,and the number of calcium ions desorbed from the C-S-H surface increases accordingly.（3）The transport properties of water and ions are closely related to temperature.As temperature increases,the distribution of water molecules becomes uniform,there is no obvious layer structure,high density,orientation preference in the vicinity of C-S-H surfaces.The diffusion coefficient of water and surface calcium ions increases with the increase of temperature,and the increment of D_XY is much higher than that of D_Z,which means the diffusion of water shows obvious anisotropy,many calcium ions could desorb from the C-S-H surface.（4）The increase of the concentration of sodium chloride solution in the C-S-H nanopore will significantly affect the structural and kinetic properties of water molecules.C-S-H surfaces prefer absorbing sodium and chloride ions to water molecules,which leads to decrease of adsorption amount of water molecules,and the distribution of water in the nanopore would be uniform,the diffusion coefficient of water molecules reduces with many ions in solution.（5）C-S-H surfaces tend to absorb sodium ions rather than chloride ions,and the surface structure could affect the absorptive capacity to ions.In the simulation,ions can be divided into several distinct types,the lower the ion concentration is,the faster ions diffuse,and the easier it is to distinguish the types of ions.With the increase of ions concentration,the amount of calcium ions desorbed into the solution from the C-S-H surface shows peak characteristics,while the diffusion coefficient of all types of ions gradually decreases.