Multiscale modeling of ion diffusion in cement paste: electrical double layer effects

Abstract

Understanding the mechanism of ion diffusion in hardened cement paste is of great importance for predicting long-term durability of concrete structures. Gel pores in calcium silicate hydrate (C-S-H) phase forms dominant pathway for transport in cement paste with low w/c ratios where the electrical double layer effects play an important role. Experimental results suggest that the effective diffusivity of chloride ions is similar as that of tritiated water (HTO) and higher than the sodium ions. This difference can be attributed to the electrical double layer near the charged C-S-H surfaces. In order to understand species transport processes in C-S-H and to quantify its effective diffusivity, a multiscale modelling technique has been proposed to combine atomicscale and pore-scale modeling. At the pore-scale, the lattice Boltzmann method is used to solve a modified Nernst Planck equation to model transport of ions in gel pores. The modified Nernst Planck equation accounts for steric and ion-ion correlation effects by using correction term for excess chemical potential computed using the results from the grand canonical Monte Carlo scheme at atomic scale and in turn bridges atomic scale model with pore scale model. Quantitative analysis of pore size influence on effective diffusivity carried out by this multiscale model shows that the contribution of the Stern layer to ion transport is not negligible for pores with diameter less than 10 nm. The developed model is able to reproduce qualitatively the trends of the diffusivity of different ions reported in literature.

Anion Diffusion in Compacted Clays by Pore‐Scale Simulation and Experiments

“…This study is helpful to improve the understanding of the diffusion mechanism of monovalent anions in compacted clays. Although our pore‐scale modeling can hardly deal with heterogeneous porous materials (such as clay rocks) directly within limited computational resources, a multiscale numerical framework will be further developed, similar to the model developed for the cement paste by Yang et al (2019) to simulate ion transport in heterogeneous clay rocks. The pore‐scale modeling has shown to be a robust approach to simulate the diffusion of radionuclides in clays.…”

“…This was usually attributed to the effect of electrical double layer (EDL) when an electrolyte was in contact with the negatively charged surface of clays. The previous studies showed that the EDL had significant impacts on the diffusion of anions in compacted clays (Appelo, 2017; Soler et al, 2019; Yang et al, 2019). The effective diffusion coefficient ( D e ) of anions at a trace concentration increased with increasing ionic strength due to the decrease of EDL thickness (Idiart & Pękala, 2016; Tinnacher et al, 2016; Xiong & Jivkov, 2018).…”

Modelling of 3D microstructure and effective diffusivity of fly ash blended cement paste

“…These indicate that the predicted ionic diffusivity considering the contribution of C-S-H is 4-6 times that only considering the ionic diffusion through capillary pores in cement paste and neglecting the contribution of gel pores in C-S-H. This can be ascribed to the fact that part of capillary pores are isolated or dead-ended which are not effective channels for ionic diffusion [62]. For example, the volume fractions of connected capillary pore in virtual cement pastes with fly ash replacement ratios of 0% and 20% calculated using the burning algorithm [46] are only 0.44 and 0.40, respectively.…”

“…Gel pores in C-S-H are generally of nano-meter size and is smaller than 0 due to the increasing viscosity of pore solution in nanopores [59,60] and the surface effect, i.e. electrical double layer effect [61][62][63]. The average value of is normally regarded as around 10 -10 m 2 /s [60].…”

Effective Medium Method for Chloride Diffusion Coefficient of Mature Fly Ash Cement Paste

“…They showed that the chloride diffusion coefficient decreased with the increase of the compressive volumetric strain but increased with an increasing tensile volumetric strain. Yang et al [28] proposed a multiscale modeling technique to analyze the transport of chloride ions in cement paste and demonstrated that the contribution of the Stern layer to ion transport was not negligible for pores with a diameter less than 10 nm. Based on the reconstructed microstructure of hardened cement paste, Carrara et al [29] used a modified Fick’s law to simulate the chloride diffusion coefficient and verified the numerical method with experimental results obtained from the literature.…”