Clay from Fourges has been selected by the Commissariat à l'Energie Atomique as a support in radioactive waste disposal studies. This material was activated by adding Na2CO3, then compacted at 60 MPa. Subsequently, its swelling behaviour was monitored at 90°C and 145°C for 330 days and at the end of this period the samples were examined by transmission electron microscopy (TEM). For this, they were embedded in a resin then sectioned with an ultramicrotome for mineralogical and chemical analyses. The initial material is essentially composed of kaolinite and smectite. Addition of Na2CO3 at room temperature induces a replacement of Ca ions by Na ions and the precipitation of finely divided carbonates on the surface of the constituents. At the end of 330 days at 90°C under a hydraulic pressure of 1 MPa, the initial particles combine and the material exerts a swelling pressure of 20 MPa. A complete reorganisation of the clay crystallites is observed without significant dissolution of the solid phases. After the same time at 145°C under a hydraulic pressure of 10 MPa, in a basic medium, the combined conditions are such that a high proportion of the clay is dissolved with formation of amorphous aluminosilicates correlated with a marked drop in the swelling pressure to 5 MPa. This work establishes the advantages of following the macroscopic properties in parallel with the microstructure variations for understanding the changes in the properties of clays.