The MX80 powder/pellets mixture is one of the candidate sealing plugs used in deep radioactive waste disposal because of its swelling properties and operational advantages (lower compaction effort, reduced gaps between the rock and the seal). The present work focuses on the microscopic scale of the material, which is studied using several techniques (MIP, μ-CT and SEM observations). From MIP results, a typical bimodal distribution was found for both pellet and powder. However, a shifting of the mean size diameter of pores of the pellet was observed at lower suctions. From μ-CT observations, a heterogeneity was revealed in the internal structure of the pellet: heterogeneous density distribution of the clay minerals and presence of several high density elements.