Molecular Dynamics Study On Ion Adsorption Of Calcium Silicate Hydrated And Aggregate Surface

Calcium-Silicate-Hydrate（C-S-H）,as the main hydration product of ordinary Portland cement,plays the role as the glue that binds the original cement particles together.It is also the source of strength for the cement concrete material.C-S-H is composed of ordered or disordered calcium silicate layers,and has multi-scale porous structure.The adsorption and diffusion behavior of ions and water molecules in C-S-H porous solution will affect the mechanical properties and durability of cement-based materials.Based on previous studies,the crystal model between C-S-H and different aggregates（SiO₂,CaCO₃,MgCO₃）is constructed by molecular dynamics,the diffusion behavior of ions and water molecules at the interface between C-S-H and aggregate crystal in Ca（OH）₂ and CaSO₄ solution was simulated.The adsorption mechanism of ions on the surface of C-S-H and aggregate was studied.The results can be used to explore the source of adsorption capacity of C-S-H gel in concrete structure and improve the mixing efficiency.The main contents are as follows:（1）The C-S-H crystal model based on the experimental requirements was constructed,and the solution of Ca（OH）₂ and CaSO₄were filled in the nanopores respectively.The transport model of ions and water molecules in the C-S-H nanopore was established.The results show that the adsorption of Ca²⁺on the pore surface of C-S-H gel is stronger than that on hydroxyl and sulfate ions.The presence of sulfate ion can inhibit the binding of Ca²⁺with non-bridged oxygen on the silicon chain in C-S-H crystal.High temperature can promote the adsorption of calcium on the surface of C-S-H gel.With the increase of temperature,calcium ion not only adsorbs at the structure interface,but also penetrates into the crystal structure.The hydrophilic property of the silicon oxygen tetrahedron in the crystal structure will attract a large number of water molecules in the solution to gather at the crystal interface.（2）Based on molecular dynamics method,the interfacial adsorption of C-S-H and SiO₂crystal surfaces in different solutions was simulated.The results show that:in Ca（OH）₂solution,the surface of C-S-H and SiO₂ crystal gel pore is negatively charged,and the adsorption capacity for calcium ion is different;in CaSO₄solution,the adsorption phenomenon of crystal interface is less obvious due to the strong binding capacity between sulfate ions and calcium ions.Due to that the silane alcohol group（Si-OH）at surface of SiO₂ crystal and C-S-H is hydrophilic,water molecules will gather at the interfaces,but the degree of aggregation of water molecules at SiO₂ crystal surface is stronger than at C-S-H crystal,especially in CaSO₄ solution.（3）Calcite（CaCO₃）and magnesite（MgCO₃）are the two most common carbonate minerals in carbonate crystals.In Ca（OH）₂ and CaSO₄ solutions,due to the strong hydrogen bond between water molecules and carbonate crystal.Water molecules in carbonate crystal surface appear aggregation and the distribution density is higher than in the C-S-H interface,there is a significant stratification phenomenon and has a stable adsorption layer.With gradually away from the surface of carbonate crystal,the density of water molecules decreases,which has obvious characteristics of diffusion adsorption layer.（4）The adsorption and transport properties of ions on the surface of C-S-H and CaCO₃crystals were studied.Calcite（1 0 4）crystal surface negatively charged and carbonate ions are positively charged.Therefore,calcium ions and the surface of C-S-H attract each other by electrostatic action,resulting in a large number of calcium ions in the solution is adsorbed bound to the surface of CaCO₃ crystal.Hydroxide and sulfate ions are gradually exposed,and C-S-H crystal surface calcium ions attract each other and adsorbed on the crystal surface.Because of its strong adsorption,sulfate ions are combined with calcium ions in the solution,weaking CaCO₃ crystal surface adsorption capacity of calcium ions.（5）The adsorption and transport properties of ions on the surface of C-S-H and MgCO₃crystals were studied.At the surface of magnesite（1 0 4）are electrostatically bound to the anions in the solution,resulting in a large number of hydroxide ions and sulfate ions in the solution being adsorbed in the vicinity of the interface of the MgCO₃ crystal.Because of their own negative charge is greater than the hydroxide ions,sulfate ions adsorbed in the region closer to the surface of the MgCO₃ crystal.Due to the electrostatic repulsion away from the MgCO3 crystal surface,calcium ions in the solution gradually close to the C-S-H crystal,combined with each other between the surface of the bridge tetrahedron non-bridging oxygen adsorbed on the C-S-H crystal surface.