Ageing effects in a compacted bentonite: a microstructure approach

Abstract

It is suspected that the as-compacted state of heavily compacted clays used as possible engineered barriers for nuclear waste disposal is time dependent, and that further change may occur in the material, even at constant water content and density. This paper presents an investigation of the time-dependent microstructure changes of an MX80 clay bentonite compacted at various dry densities and water contents. Microstructure investigation is based on mercury intrusion pore size distribution measurements and scanning electron microscopy (SEM). Results obtained by other researchers by using X-ray diffraction at low angles are also used. Statically compacted samples of MX80 were kept at constant volume and water content for various periods of time (1, 30 and 90 days) prior to mercury intrusion and SEM micro-structure investigation. A significant change in micro-structure with time was observed, characterised by a decrease in the inter-aggregate porosity and an increase in the very thin porosity not intruded by mercury (r < 3·7 nm). The former observation is related to the filling of inter-aggregate larger pores by exfoliation, and the latter observation is related to changes that occur in the aggregates within the smectite particles when suction is reduced. These changes are interpreted in the light of an investigation carried out on similar samples using low-angle X-ray diffraction. Changes inside the aggregates are governed by the progressive placement of interlayer water molecules inside the particles together with the subdivision of particles, which gives rise to an interparticle porosity that develops inside the aggregates. It is suspected that the as-compacted state of heavily compacted clays used as possible engineered barriers for nuclear waste disposal is time dependent, and that further change may occur in the material, even at constant water content and density. This paper presents an investigation of the time-dependent microstructure changes of an MX80 clay bentonite compacted at various dry densities and water contents. Microstructure investigation is based on mercury intrusion pore size distribution measurements and scanning electron microscopy (SEM). Results obtained by other researchers by using X-ray diffraction at low angles are also used. Statically compacted samples of MX80 were kept at constant volume and water content for various periods of time (1, 30 and 90 days) prior to mercury intrusion and SEM micro-structure investigation. A significant change in micro-structure with time was observed, characterised by a decrease in the inter-aggregate porosity and an increase in the very thin porosity not intruded by mercury (r < 3·7 nm). The former observation is related to the filling of inter-aggregate larger pores by exfoliation, and the latter observation is related to changes that occur in the aggregates within the smectite particles when suction is reduced. These changes are interpreted in the light of an investigation carried out on similar samples using low-angle X-ray diffraction. Changes inside the aggregates are governed by the progressive placement of interlayer water molecules inside the particles together with the subdivision of particles, which gives rise to an interparticle porosity that develops inside the aggregates.