Introduction

Moisture and diffusive transport in porous media play an important role in a wide variety of processes of environmental and technological concern, such as the degradation of building materials (e.g., mortar and concrete), the spread of hazardous wastes in the ground, oil recovery, and the containment of nuclear wastes [1]. For example, the ingress of chloride ions in an aqueous phase in concrete can lead to corrosion of steel reinforcement, while the rate of diffusion of carbon dioxide in the complementary air phase may determine the rate of carbonation of the cementitious matrix. Clearly, the diffusive transport of ions in building materials or in soils must depend on the degree of saturation of the porous medium. In this paper, results will be presented of a numerical study concerning diffusive transport in model porous media as a function of fluid saturation, taking into account fluid wetting properties. The location of each fluid phase in the pore space was obtained by numerically simulating the phase separation of a fluid mixture by the lattice Boltzmann method [2]. Upon completion of the phase separation process, each fluid phase was identified and the bulk electrical conductivity associated with each separate phase was determined, assuming that the material making up the solid was not conductive. The diffusivity was then obtained by utilizing the Einstein relation [3] which relates diffusivity to conductivity. Results are summarized on a relative diffusivity curve which describes the diffusivity of ionic species, normalized to its value at full saturation, as a function of the degree of saturation of the porous medium. It is hoped that a careful evaluation of simple but non-trivial model systems will yield insight into the problems of diffusion in partially-saturated building materials like concrete. Further such information can be easily employed in computer models which simulate the ingress of contaminants into building materials and soils by diffusion.