The in situ Engineered Barrier (EB) experiment aimed at understanding the hydromechanical behaviour of sealing materials for high-level radioactive wastes as a combination of compacted blocks and high-density pellets made of Febex bentonite. The experimental program focused on the wetting process of a heterogeneous dual-component sample consisting of pellets and blocks with technological gaps. The homogenisation tendency was investigated by determining the final (local) dry densities and pore size distributions in both zones from post-mortem analyses. Global initial dry densities between 1.36–1.44 Mg/m3 were considered for the heterogeneous mixture, wetted at constant volume and constant vertical stress (oedometer conditions). The initially high-density zone of blocks with technological gaps expanded during hydration, reaching a lower dry density at saturation than the pellets’ zone. The initially low-density pellets’ zone with a high volume of inter-pellet pores underwent compression and reached a final high dry density on saturation. The experimental results were compared with the distribution of dry density at the in situ EB experiment after dismantling, and a good agreement between the laboratory and the in situ measurements was observed.