Micro-Macro Effects in Bentonite Engineered Barriers for Radioactive Waste Disposal

This paper summarizes the findings of various investigations carried out for some time in the research group on unsaturated soils at Ecole des Ponts ParisTech. It focusses in more details on the links that can be made between nano, micro, and meso scale phenomena, and the macroscopic response of various Engineered Barrier Systems made up of bentonite, used in various concepts of radioactive waste disposal at great depth. Meso-scale observations using X-Ray microtomography usefully complete some findings established by using more standard techniques of microstructure investigation, like scanning electron microscopy and mercury intrusion porosimetry. The morphology of sand-bentonite and pellet-bentonite powder mixtures can be better understood, together with their changes during hydration. Also, nano-scale mechanisms that govern the hydration of smectites through the consecutive and ordered placement of layers of water molecules along the faces of the smectite minerals are useful for a better understanding of the macroscopic response of Engineered Barrier Systems submitted to hydration under constrained volume conditions.