High-density bentonite pellet mixtures have been proposed as candidate materials for filling technological voids in the deep geological disposal repository of high-level radioactive waste (HLW). Water vapour transfer plays a major role in the function formation of the unsaturated engineering barrier system, due to the existing high thermal gradients generated by the decay heat released from the canister. In this work, series of suction controlled swelling pressure tests, swelling deformation tests and compression tests were conducted on GMZ bentonite pellet mixtures. Results show that, for the specimen hydrated by the vapour equilibrium technique, no collapse was observed on the evolution curve of swelling pressure. However, for that wetted by the osmotic technique, a collapse of swelling pressure was observed after the first peak value. For specimens hydrated with suction-control, a collapse on the evolution curve of swelling strain was only observed under high vertical stresses. In addition, the unloading curves of specimens exhibited a linearity at the high suction stage and a bi-linearity at the low suction stage. The swelling pressure determined by the swell-consolidation method was higher than that determined by the constant-volume method.