Heterogeneous transport · Stochastic modelling
Gas Diffusion in Heterogeneous Cement Paste
Main claim
A two-state fluctuating-diffusivity model connects microscopic heterogeneity in cement paste with oxygen transport beyond a single effective diffusion coefficient. Gas molecules can alternate between fast and slow environments inside cement paste, so their spread need not follow the simplest diffusion picture.
Problem
Gas transport in cement paste is commonly summarized by one effective diffusion coefficient. That average is useful, but it does not describe how a molecule encounters pores and solid phases with very different mobilities, or whether rare, long displacements occur more often than a Gaussian model predicts.
Approach
We represented the heterogeneous pore environment with a two-state fluctuating-diffusivity model. A gas molecule switches stochastically between faster and slower states. Model parameters were estimated from a colloidal picture of calcium-silicate-hydrate microstructure, then used to calculate oxygen diffusivity and the full displacement distribution.
What we found
The model gives a reasonable account of reported oxygen diffusion coefficients while predicting non-Gaussian motion at intermediate scales. The displacement distribution has heavier tails than a Gaussian with the same mean-square displacement. The duration of this behavior depends on the characteristic size of the heterogeneous structure; at longer scales, ordinary effective diffusion is recovered.
Why it matters
The result separates the mean rate of transport from the probability of unusually long molecular motion. That distinction may matter when gas ingress is used to assess carbonation, corrosion, and the long-term reliability of concrete structures. It also provides a compact framework that can be extended to other chemical species and more than two mobility states.
Keywords
cement paste, gas diffusion, fluctuating diffusivity, non-Gaussian diffusion, heterogeneous transport.
Paper
, “Gas diffusion in cement pastes: An analysis using a fluctuating diffusivity model,” Construction and Building Materials 407, 133411 (2023).