Over the past decade, a great deal of effort has been specifically devoted to the investigation of ion transport mechanisms in unsaturated cement systems. The topic is important because, in many cases, concrete structures exposed to ionic solutions are also frequently subjected to wetting and drying cycles. The coupled transport of moisture and ions often tends to accelerate physical and chemical degradation mechanisms and reduce the service life of the material ,  and .
Reports recently published on the subject have largely contributed to clarify some fundamental aspects of ion transport mechanisms in unsaturated concrete. Many investigations have also emphasized the intricate nature of these phenomena. If most of the difficulties related to the description of transport processes in concrete are linked to the intrinsic complexity of the material, it appears that part of them also lies with the fact that authors have used many different approaches to study these processes. For instance, the definitions of the state variables used to describe the various transport processes tend to vary significantly from one study to another. This is most unfortunate because the lack of a unified approach often contributes to confuse the issue.
This paper is an attempt to clarify some fundamental aspects of the problem. The transport mechanisms are described using a well-established mathematical procedure, the homogenization technique. The technique has been recently used to investigate the diffusion of ions in saturated systems . According to this approach, the transport equations are first written at the pore scale. They are then averaged over the scale of the material. The main advantage of the homogenization technique lies in the clear definition of the state variables.
The paper first addresses the process of moisture transport in an unsaturated porous material. For the completely coupled transport of ions in an unsaturated media, dynamical equations are required to express the moisture content as a function of time. This is achieved by averaging microscopical equations for both liquid and water vapor transport. The mathematical development yields the Richards' equation, and the moisture content and the transport coefficients are well defined.
The second part of the paper is devoted to the coupled transport of ions and moisture in the system. Here, the field quantity is the concentration of the ions within the pore solution. The homogenization technique is applied to a microscopic equation for both diffusive and convective transport. While diffusive equations already exist, reformulating the bulk equations using homogenization ensures that the transport coefficients are well defined (pore space versus microscopic quantities) and can therefore be unambiguously related to experimental quantities.