Interactions of sodium chloride solution and calcium silicate hydrate with different calcium to silicon ratios: A molecular dynamics study

摘要

Calcium silicate hydrate (C-S-H) is the main hydration product of cement-based materials, which fundamentally affects the durability of concrete, such as chloride ion permeability. Since the structure and properties of C-S-H are closely related to the calcium to silicon ratio (Ca/Si), in this article, tobermorite and jennite models are combined to represent C-S-H with different Ca/Si ratios and their interface of the mixed phase. Atomic density distribution, ionic binding mode and Cl ion dynamic states were employed. The results show that water molecules link the intralayer calcium ions to become the inner layer complex, and establish H-bond network together with the non-bridging oxygens and hydroxyl groups on the C-S-H surface to become the outer layer complex. Na+ ions replace free Ca2+ ions on the C-S-H surface to be adsorbed, and the latter can form Ca-Cl clusters by capturing Cl ions after desorption. The diffusion coefficient of Cl ions is larger in the low Ca/Si ratio C-S-H, and the adsorption amount of Cl ions is larger in the high Ca/Si ratio C-S-H. In the interface of the mixed phases, Cl ions tend to be adsorbed closely on the high Ca/Si ratio surface and away from the low Ca/Si ratio surface. Finally, a Ca/Si ratio gradient microstructure design for cement-based materials is proposed to prevent chloride ion penetration. (C) 2020 Elsevier Ltd. All rights reserved.