In my research I work on modelling of heat and mass transfer due to cement hydration in early-age concrete elements. To model non-stationary thermal and humidity fields I use coupled equations based on Fourier's law and Fick's law.

Only recently I decided to develop my model and consider co-operation of my concrete structure with soil. I thought it was necessary to model exchange of heat and moisture content between my concrete structure and soil, so those equations should be defined and solved for soil, too. For proper solution, however, I need diffusion coefficients. I found information about thermal diffusion coefficient and I learned that, as for concrete, water diffusion coefficient is dependent on actual moisture content. However, I could not find the exact functions for the second one (I only found multiple citations but could not find a full text on any paper in that topic). Could you suggest me those functions? If you have paper in that topic, I would be glad to cite it in my future work where I will use the model. Please note I am looking for moisture diffusion not ion diffusion!