Hierarchical porosity of bentonite-based buffer and its modification due to increased temperature and hydration

The use of the hierarchical pore structure in a bentonite-based mixture as a part of a nuclear waste repository's engineering barrier is proposed. The pore structure was observed in an experimental mixture composed of milled Ca-bentonite (85 vol.%), quartz sand (10 vol.%), and graphite (5 vol.%), which had been subjected to long-term (44 months) combined effects of increased temperature (up to 90 °C) and hydration during the Mock-Up-CZ experiment. Although there were negligible changes in the mineralogical composition (a slight increase of illite content, and minor conversion of montmorillonite to beidellite), the studied material underwent significant changes in the hierarchical pore structure.The parameters of the pore space were examined by adsorption techniques (CO2 and N2), as well as by the intrusion technique (mercury porosimetry). Detectable pore radii ranged from about 0.4 nm to 58 μm (micro-, meso-, macropores, and coarse pores). The observed pore categories were attributed to the presence of solid particles and their arrangement.The smallest pores exhibited a typical radius of 0.65 nm (a range from 0.4 to 1.6 nm), a total specific surface area of 50 m2/g for the initial material. Mesopores showing radii 10–20 times higher were found within aggregates of clay mineral particles. Their specific surface area was roughly similar to that of micropores.Simultaneous heating and hydration decreased the specific surface area of the micropores close to the source of heat (i.e. in the direction of increasing temperature). There was a slight increase of their volume in moderately heated areas (50–70 °C) and a decrease in both the less heated (30–40 °C) and highly heated areas (over 70 °C). The same process increased the specific surface area of mesopores by 1–18%. The maximum increase of this parameter was observed in the samples exposed to a lower temperature (30–40 °C). The volume and specific surface area of macropores and coarse pores significantly decreased (by 20 and 40% respectively) when compared to the pre-experimental material but the typical radius of macropores was increased by a factor of about 2 or 3 in the zone of maximum temperature. This fact contributed to increased hydraulic conductivity observed by Pusch et al. (2007).