In the French concept of deep nuclear wastes repository, the galleries should be backfilled with excavated argillite after the site exploitation period. After several thousands of years, the degradation of the concrete lining of the galleries will generate alkaline solute (pH N 12) that will diffuse through the backfill. The object of the paper is to describe the influence of such solute diffusion on the properties of compacted argillite. Since it is planned to introduce additives (bentonite, calcareous sand or lime) in the remoulded argillite to backfill the deep galleries, such mixtures were also studied. Saturated-portlandite water was circulated through compacted samples for 3, 6 and 12 months at 20 or 60 °C. The microstructure before and after fluid circulation was determined with mercury intrusion porosimetry. The results showed that the 50% sand-50% argillite mixture microstructure was slightly affected by the alkaline fluid circulation. Conversely, the microstructure of the other tested materials changed dramatically. The fluid circulation provoked an increase of the macropore void ratio by almost 3 times for the pure argillite, almost 1.5 for the MX-80 and argillite mixture and 2 for the lime-treated argillite. This particular behaviour could be attributed to the dissolution of soil minerals in such a high pH environment. Hence, it is likely that hydraulic and mechanical properties of these mixtures would be dramatically affected by alkaline solute diffusion over a long period of time.