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 .
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
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 . 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  which relates diffusivity
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