Swelling pressure and microstructure of an activated swelling clay with temperature

Abstract

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.

On the high stress compression of bentonites

“…X-ray plots also show peaks corresponding to quartz, calcite, and plagioclase. The Fourges clay (Tessier et al 1998) is a Ca 2+ clay composed of 80% mixed layer smectite-kaolinite, 4% free kaolinite, and 6% quartz, with the remainder being composed of calcite (1.4%), goethite (6%), and some hematite and gypsum. When Na + activated, this clay is known under the name of FoCa7 clay.…”

Fault weakening caused by smectite swelling

“…Because of a low thermal structure of the Japan Trench, smectite-illite transition is suppressed within 100 km distant from the trench Kameda et al 2012), suggesting that the swelling of smectite may have a potential to weaken the wide area of the shallow plate interface. It should be noted, however, that when the temperature increases with depth, the swelling ability of clays may be reduced due to dissolution and collapse of smectite particles (Tessier et al 1998;Schleicher et al 2015). In addition, when the effective normal stress increases with depth, the interlayer water can be removed from the interlayer space, resulting in the deswelling.…”

Soft ground improvement with solar-powered drainage

“…At constant soil/void ratio, the elevated temperature induced higher permeability of soft Bangkok clay normalised by permeability at room temperature (258C) as shown in Figure 1 ( Abuel-Naga et al, 2006a). Moreover, the absence of evidence of mineralogical change upon heating the saturated finegrained soils up to 908C, was attributed by Hueckel (2002) to the temperature-induced mechanical behaviour changes to the dehydration and rehydration of clay caused by heating or cooling, respectively, at temperatures and pressures below the boiling point of water (Tessier et al, 1988).…”