High-pH plumes arising from cementitious leachates are known to alter the mineral assemblage of clayey formations selected to play the role of natural barriers for radioactive waste repositories. Dissolution of constitutive minerals such as smectite and precipitation of secondary phases such as feldspar and zeolite have been reported [1, 2]. These mineral changes are suspected to have a significant effect upon diffusive transport properties, either enhancing or decreasing the mudrock ability for radioelement confinement. For instance, experiments performed under alkaline conditions have shown a decrease in water diffusion fluxes and an increase in cation fluxes . This study was designed to provide better understanding of the phenomena that govern diffusion processes during the transient phase between site and alkaline conditions. Experiments involving the use of "through-diffusion" cells were performed to reproduce the effect of an alkaline plume through different types of clayey materials from the Tournemire experimental platform (France). Different kinds of alkaline fluids and sliced rock materials were investigated for the diffusion of both water and major cations. Monitoring of the pH and the concentrations of the major cations (sodium, potassium, calcium and magnesium) and investigations of the solids (-DRX, isotopy, microscopy) before and after alteration by the diffusion experiments were performed for assessing the role of mineral alteration on diffusion processes.
 A. Bauer, B. Velde, Clay Minerals, 34 (1999) 259-273.
 R. Mooser-Ruck, M. Cathelineau, Applied Clay Science, 26 (2004) 259-273.
 T. Melkior, D. Mourzagh, S. Yahiaoui, D. Thoby, J.-C. Alberto, C. Brouard, N. Michau, Applied Clay Science, 26 (2004) 99-107.