Insight on the sodium and chloride ions adsorption mechanism on the ettringite crystal: Structure, dynamics and interfacial interaction

Abstract

The adsorption of water and ions on ettringite crystal, an important hydration product in the cement material, is fundamental to the durability of cement-based material. In this study, molecular dynamics was utilized to investigate the molecular structure, dynamics and interfacial bonding for the water and ions in the vicinity of ettringite crystal surface. The aluminate octahedron and sulfate ions in the surface of ettringite provide plenty of oxygen sites to accept H-bond from the water molecules in the interfacial region. The highly solvated surface calcium ions can strongly attract the neighboring water as their coordinate atoms. The hydrophilic nature of the ettringite interface results in the dramatically different feature of the water molecules on the ettringite surface, such as high dipole moment, ordered organization, good orientation preference and slow diffusivity. Furthermore, with gradually increasing simulation time, the calcium ions, aluminate and sulfate species in the crystal surface diffuse into the solution, resulting in the dissolution of the ettringite crystal and disturbance of the ordered interfacial topology. The six-coordinated aluminum hydroxyl groups transform to the aluminate tetrahedron, as they dissolve in the solution. Additionally, the chloride and sodium ions have different adsorption mechanism on the crystal surface. While the sodium ions are associated with the oxygen atoms in aluminate octahedron and sulfate ions by NaO ionic bond and immigrate into the inner region of the dissolved crystal, the chloride ions can form the ionic pairs with the calcium ions, accumulating to cluster in the outer layer region. As compared with the chloride adsorption, the ettringite crystal has better immobilization ability on the cation ions due to the stable NaO connection with long resident time.