Molecular dynamics simulation to assess the effect of temperature on diffusion coefficients of different ions and water molecules in C-S-H

Abstract

Diffusion is a particle transportation process beginning from one point of a system to another through random molecular motion. This process depends on various parameters like temperature, concentration gradient, and particle size. The objective of this article is to assess the variation of diffusion coefficients of water molecules, chloride and sodium ions against different temperatures in calcium silicate hydrates (C-S-H) through molecular dynamics simulation. A uniform sodium chloride solution is modeled between cement hydrate layers with no concentration gradient. In such a solution, temperature could affect diffusion process in a significant manner. The two most important crystalline mineral analogues of C-S-H, tobermorite and jennite, are applied in this simulation. Diffusion coefficients of different ions and water molecules are found in different temperatures. It is revealed that diffusion coefficient is higher at high temperatures. Activation energies of chloride and sodium ions transport in cement hydrates are calculated through Arrhenius law. Output values of diffusion coefficients and activation energies are compared to previous experimental and simulation results in the related literature. A multi-scale analysis is run to estimate the penetration depth of $\mbox{Cl}^{-}$ ions in cement paste through Fick’s second law.