Microstructure features in the behaviour of engineered barriers for nuclear waste disposal

Engineered barriers made up of bricks of compacted swelling clays are considered as potential barriers for the isolation of high activity nuclear waste at great depth. To better understand their coupled hydro-mechanical response, the microstructure of engineered barriers has been studied by various authors by using most often scanning electron microscope observations and mercury intrusion pore size distribution measurements (MIP). These studies confirmed that the microstructure of compacted bentonites was made up of aggregates with two classes of pores: inter-aggregates pores and intra-aggregate pores. Further investigation conducted by using X-Ray diffractometry at low angles conducted more recently provided deeper insight into the hydration mechanisms that occur during hydration. This paper presents some results obtained by various authors by using these techniques. It shows how the hydration mechanisms occurring at the level of the clay particles inside the aggregates help interpreting existing MIP data. Some conclusions about the water retention properties of compacted bentonites with or without swelling allowed are given. Some consequences on the water transfer properties of compacted bentonites are also drawn. In both cases, the clogging of inter-aggregate pores due to particle exfoliation has a significant effect.