In the frame of geological disposal of high-level radioactive waste, the sealing of shafts and galleries is an essential part of underground repository design. The EC project of RESEAL II has the objective to demonstrate the sealing of a borehole and a shaft with bentonite in plastic clay. Apart from a large scale in-situ test, laboratory tests are focussed on the characterisation of the hydration of a possible candidate of sealing materials, namely a mixture of FoCa-clay powder and pellets. As most techniques give only indirect information on the hydration characteristics, microfocus X-ray computed tomography (ACT) has been used to visualise and quantify the hydration of such a mixture. The mixture used here has a dry density of 1.36 g/cm3 and several ACT scans were taken during the hydration. The resulting data showed the progressive decrease of the density of the pellets, the preferential suction of the pellets and the final homogenisation at complete saturation. A side effect of the ACT scanning, probably related to disconnecting the hydration cell from the injection system during scanning, was the occurrence of fractures. These fractures allowed to measure the swelling of the pellets to be about 50% of volume increase. After drying of the sample, the original difference between pellets and powder did not reoccur. It can be concluded that the hydration of this pellet/powder mixture takes quite some time. Once saturated, no preferential paths seem to be present at the 100 Am scale. From this point of view, the 50/50 pellet/powder Foca mixture is a promising solution for the sealing of shafts of underground nuclear waste storages.