Abstract A pelletized solution composed of COx argillite and MX80 bentonite is examined as backfill for the deep geological disposal of radioactive waste (Cigéo project). The implementation is studied in terms of installation and hydration conditions as well as their influence on the hydromechanical behavior of the mixture. In the beginning, the optimum grain size distribution (GSD) of pellets assembly is determined to minimize initial voids. A series of discrete element method (DEM) simulations on ternary samples made of spheres is carried out, investigating the optimal conditions in terms of sizes and proportions. Density maximization occurs when the maximum diameter is the dominant size, while minimum and medium diameters are following. As optimum GSD, sizes of 16 mm, 10 mm, and 4 mm in 60%, 10%, and 30% respectively are chosen. The pelletization of the selected sizes is optimized using an adapted tablet machine to carry out the pellet fabrication. A detailed analysis of influential parameters related to the machine accessories and the inserted raw materials is presented. After modifications, quasi-spherical pellets presenting a density of at least 1.95 Mg/m3 are produced. COx/MX80 pellets hydromechanical response is indirectly assessed by studying the swelling potential of COx/MX80 powdered mixtures at equivalent emplaced densities. The influence of density, stress, and MX80 content on swelling is obtained.