Research On The Transport Of Water And Ion In Cement-based Materials Based On Molecular Dynamics

The transport of water and ions is responsible for the durability degrade of cement-based materials.While macroscopic studies on the transport of water and ions were quite comprehensive,few researches focused on the mechanism investigation at micro-scale.Hence,based on the molecular dynamics method,the structure,dynamics and interfacial bonding of water and ions were systematically investigated to illustrate the transport and adsorb mechanism for ions and water in cement-based materials.Calcium-Silicate-Hydrate?C-S-H?is the main hydration product of Portland cement and this paper mainly focused on the transport of water and ions in its gel pore.In addition,the transport of water and ions in the surface of other important hydration products was also investigated,including ettringite,calcium hydroxide and Friedel salts.The detailed content of this thesis are listed as following:?1?Based on the molecular dynamics method,the structural characteristics of water in C-S-H were studied.The results showed that the water molecules confined in the interlayer region and the water molecules adsorbed on the surface of C-S-H gel are quite different from the capillary water:the larger packing density,the increased dipolar moment,and more orderly organization for interfacial structure,and more pronounced orientation preference.These structural properties,which are distinguished from the capillary water,are mainly attributed to the H-bond interaction between interfacial oxygen atoms and water molecules.It also indicates the hydrophilic nature of the C-S-H surface.?2?The interlayer water,surface adsorbed molecules and capillary water in C-S-H exhibit different dynamic properties.The results showed that by the mean square displacement,the diffusion coefficients of three types of water molecules were calculated:the diffusion coefficient of the interlayer water is 0.003×10^-9m² s^-1,the diffusion coefficient of surface water is 1.2×10^-9m² s^-1,the gel pore water diffusion coefficient of 2×10^-9m² s^-1.The mobility reduction of interlayer water and surface water mainly results from the strong interfacial H-bonds.?3?The structure and dynamics properties of 0.5mol/L sodium chloride solution in C-S-H were simulated.The results showed that the adsorption capacity of sodium ions on the C-S-H surface is stronger than that of chloride ions and the adsorption of sodium ions is more stable.Furthermore,while the sodium adsorption on C-S-H surface results from the stable Na-O bonding,the chloride adsorption is from the Ca-Cl ionic pairs.By comparing the diffusion coefficient,the mobility of the water molecules,chloride ions and sodium ion is ranked in the following order:water molecules>chloride ion>sodium ion.?4?The transport and adsorption of water molecules in four hydration products were compared.The results showed that the surface of calcium hydroxide has stronger hydrophilic properties and hydrophilic properties of tobermorite and ettringite are weaker.Friedel salt has no obvious hydrophilic properties.?5?The transport and adsorption of sodium chloride solution in four hydration products were compared.The results showed that the ion adsorption capacity of the four products is quite different,and the adsorption capacity of chloride ions is ranked in the following order:Friedel salt>calcium hydroxide>ettringite>tobermorite;the order of the adsorption capacity of sodium ions is:tobermorite>calcium hydroxide>Friedel salt>ettringite.