**Multi-scale study water and ions transport in the cement-based
materials: from molecular dynamics to random walk**

W Zhang and DS Hou and HY Ma, MICROPOROUS AND MESOPOROUS MATERIALS, 325, 111330 (2021).

DOI: 10.1016/j.micromeso.2021.111330

The transport properties of water and ions play a key role in the durability of cement-based materials. However, little work has been done to study the diffusion behaviors of water and ions in a multi-scale view. Therefore, the current paper presented a multi-scale molecular dynamics fed random walk (MD-RW) simulation scheme for the purpose of obtaining the diffusion coefficients of water and ions in the calcium- silicate-hydrate (C-S-H) gel. At nanoscale, MD simulations were used to obtain the diffusion coefficients of water and ions in the C-S-H with nanopore size ranging from 0.5 nm to 5 nm since pore size has a crucial effect on the diffusion behavior of water and ions. Then these results were utilized as the input parameters for random walk simulation to scale the confining effect up and derive the effective diffusion coefficients of water and ions in the C-S-H gel at mesoscale. The results acquired from mean square displacement, atomic intensity distribution function and radial distribution function show that water molecules and Cl ions would form Ca-Cl, Ca-O ion-clustering and hydrogen bonds with C-S-H substrate which will restrict the transport of water molecules and Cl ions. In addition, with the size of gel pore increases from 0.5 nm to 5 nm, the diffusion coefficient of water increases from 0.016 to 1.85 x 10(-9) m(2) s(-1) and the diffusion coefficient of Cl ions increases from 0.002 to 0.75 x 10(-9) m(2) s(-1). In the random walk simulation, the diffusion coefficients of water and Cl ion were derived as 1.01 x 10(-12) m(2) s(-1) and 1.2 x 10(-13) m(2) s(-1) in the FCC packing model and 1.55 x 10(-11) m(2) s(-1) and 1.77 x 10(-12) m(2) s(-1) in the BCC packing model respectively, which reaches reasonable agreement with the previous studies results. Unlike traditional reverse methods which back-calculate these coefficients from macroscopic experimental results and hypothesized microstructure, this study presents the first bottom-up method deriving the transport properties of C-S-H gel directly from the physicochemical characteristics and microstructure, which has relevant significance for the study of concrete resistance to ion erosion and provides a basis for designing durable concrete.

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