Solute transport solved with the Nernst-Planck equation for concrete pores with 'free' water and a double layer

For calculating ion transport in electro-migration experiments in concrete, the pore space is divided between free pore water and a double layer that compensates the surface charge of minerals lining the pore. The concentrations of counter-ions and co-ions in the double layer are averaged in a Donnan layer, which allows to solve the flux equation of the individual ions. A numerical model is developed for transient states, with the harmonic mean of the chemical activity coefficients and the physical parameters to adapt cell sizes in zones with high concentration gradients. Diffusion coefficients of the ions are corrected for temperature and ionic strength with an empirical equation with three parameters optimized on published transport numbers and specific conductances. The current and the Cl- breakthrough is calculated in an electro-migration experiment with a concrete sample. The surface charge is positive in the experiment, enhancing and reducing the diffusivity of anions and cations.