An experimental investigation on the hydromechanical behaviour of pellet-powder mixtures, candidate materials for sealing the galleries in radioactive waste disposal concepts, was described. Imbibition experiments were performed using a specially-designed column, with square cross-section and glass sides allowing pictures to be taken by a camera. Relative humidity and swelling pressure were measured at different elevations of the column. Three mixtures with an identical pellet volume fraction and various powder volume fractions were used to study the influence of heterogeneities of density on the hydromechanical behaviour of the mixture under repository conditions. Results demonstrated that hydration of the material was dominated by water vapour flux, with pellets and inter-pellet pores not at hydraulic equilibrium. In mixtures with low powder content, the evolution of swelling pressure was controlled by contact forces between pellets. The material progressively lost the initial granular structure upon hydration. Note that hydration was made by imbibition, instead of injecting liquid water at high pressure, to avoid unrealistic liquid water flow rates in the initial large inter-pellet voids. In addition, sensors four times larger than the pellets were used to measure a representative value of swelling pressure.