Colloidal properties of clays related to the erosion and sedimentation behaviour of bentonite in fractures

Smectites are selected as barriers in deep geological repositories for high-level radioactive waste repository due to their high swelling capacity and contaminant retention ability. Erosion of the clay barrier can affect repository safety and eroded particles may facilitate radionuclide migration within the host rock fractures. These processes depend on the physicochemical characteristics of the clay, on its structural properties and are affected by the chemical equilibrium established with the groundwater from the geological formation.

In order to assess the relationship between clay colloidal properties and the erosion and sedimentation behaviour in fractures, this study correlates the erosion behaviour of a compacted bentonite barrier, under repository conditions, simulating expansion and sedimentation in artificial fractures with physicochemical macroscopic properties (viscosity, turbidity,...) of different smectite suspensions. Studies are carried out under different geochemical conditions.

Results showed that sodium smectites had the highest viscosities, all showing similar behaviour: at low ionic strengths the viscosity remained constant and around 10 mM increased significantly. In spite of forming smaller particles, their expansion and sedimentation is hindered by their high viscosity. In contrast, clays with dominant bi-trivalent cations in their structure, has lower viscosity values, independent on the water ionic strength. Due to their higher particle size and lower viscosity, higher sedimentation in rock fractures is expected.

Turbidity measurements over time were done to assess sedimentation behaviour. It was observed that sodium smectites remained stable over time, even at high ionic strengths, as observed in viscosity studies. With calcium clays, turbidity decreased rapidly, indicating fast sedimentation occurred.

The study contributes to predict the erosion behaviour of the clay barrier in the fractures of the geological formation of a deep geological repository, starting from the colloidal properties of each clay.

This work was partially supported by the Swedish Nuclear Fuel and Waste Management Co, SKB (Sweden) and by the Spanish Ministry of Innovation and Science (PID2019-106398GB-I00, ARNO Project).