SWELLING PRESSURE AND MICROSTRUCTURE OF AN ACTIVATED SWELLING CLAY WITH TEMPERATURE

Clay from Fourges has been selected by the Commissariat a l'Energie At omique as a support in radioactive waste disposal studies. This materi al was activated by adding Na2CO3, then compacted at 60 MPa. Subsequen tly, its swelling behaviour was monitored at 90 degrees C and 145 degr ees C for 330 days and at the end of this period the samples were exam ined by transmission electron microscopy (TEM). For this, they were em bedded in a resin then sectioned with an ultramicrotome for mineralogi cal and chemical analyses. The initial material is essentially compose d of kaolinite and smectite. Addition of Na2CO3 at room temperature in duces a replacement of Ca ions by Na ions and the precipitation of fin ely divided carbonates on the surface of the constituents. At the end of 330 days at 90 degrees 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 obser ved without significant dissolution of the solid phases. After the sam e time at 145 degrees C, under a hydraulic pressure of 10 MPa, in a ba sic medium, the combined conditions are such that a high proportion of the clay is dissolved with formation of amorphous aluminosilicates co rrelated with a marked drop in the swelling pressure to 5 MPa. This wo rk establishes the advantages of following the macroscopic properties in parallel with the microstructure variations for understanding the c hanges in the properties of clays.