Study On Adsorption And Transport Characteristics Of Water And Ions In Aluminumdoped Cement-based Materials Based On Molecular Dynamics Method

Concrete is the most widely used building material in the world,and its durability is the basis for determining the long-term performance of concrete structures.The transport and adsorption of water molecules and ions in the nanopores of cement-based materials is an important cause of the decrease in durability of concrete.As the main hydration product of aluminum-doped cement-based materials,hydrated calcium aluminosilicate?CASH?can effectively improve the chemical stability,mechanical properties and transport properties of the bonded phase hydrated calcium silicate?CSH?of traditional Portland cement.In this paper,the adsorption and transport properties of water and ions in CSH and CASH nanopores were simulated by molecular dynamics method,and the influence mechanism of aluminum phase was explored.In addition,the adsorption characteristics of water and ions in other aluminum-containing hydration products?ettringite?were also studied.The main contents are as follows:?1?Molecular dynamics methods were used to investigate the structure,kinetics and interfacial interactions of water and ions confined in CSH/CAS-H nanopores.The results show that Compared with CSH,the water molecules in the vicinity of the CASH interface exhibit characteristics:Larger bulk density and dipole moment,water molecules are arranged in order,and the inclination angle is consistent.The surface of CASH is fixed with more Na⁺and Cl^-,and the reasons for adsorption of anion and cation are different.?2?The transport behavior of water and ions in CSH/CASH nanopore was explored and the microscopic mechanism was revealed.The unsaturated transport curve of NaCl solution in CSH/CASH nanopore meets the classical capillary adsorption theory Lucas-Washburn equation.The migration of water molecules and ions in the CASH gel pores is slower than that of CSH.The introduction of aluminum in CSH resists the intrusion of water and ions.?3?The transport and adsorption properties of water and ions in the ettringite pores were studied by molecular dynamics simulation.It was discovered that the aluminum hydroxy and sulfate ions on the surface of ettringite provide a large number of oxygen atom sites to form a hydrogen bond network with the solution water molecules,which makes the water molecules near the interface show obvious characteristics:high dipole moment,orderly alignment,good angular orientation and slow diffusivity.?4?The ettringite interface has good adsorption capacity for both anions and cations.The aluminum hydroxy or sulphate of the ettringite surface provides a large number of oxygen atom sites to form a stable ionic bond(Na-O_so4\Na-O_h)with sodium ions,thereby achieving an adsorption effect.The adsorption of chloride ions is mainly caused by the charge attraction of surface cations(Ca²⁺,Na⁺)to form ion clusters.