Influence of thermal loading on index and physicochemical properties of Barmer bentonite

The concept of the deep geological repository (DGR) has been widely accepted for the safe disposal of high-level nuclear waste (HLW). Compacted bentonite/bentonite-sand mixture is considered as buffer material in the deep geological repository due to its favorable physicochemical and hydro-mechanical properties. The bentonite buffer is expected to be exposed to high temperature for a long duration because the heat released from the waste canister and due to the geo-thermal gradient inside the deep geological repository. The temperature from the waste canister can go initially as high as 200 °C and probably reduce to ambient temperature upon the decay of the waste material. This long duration temperature variation induces a thermal loading on the bentonite buffer, affecting its index, physicochemical and hydro-mechanical properties. Hence there is a need to critically examine the effect of the induced thermal loading on the long-term performance of the buffer as this study will be quite useful for the preliminary understanding before fixing its design values for the use as a part of engineered barrier system (EBS) in DGR. In the present study, two bentonites (Barmer 1 (B1) and Barmer 2 (B2)) from Barmer district of Rajasthan, India, was subjected to four temperature levels, i.e., 60 °C, 110 °C, 150 °C and 200 °C for a time duration of 3, 7, 15, 21, 28 days. Changes in the physicochemical properties of both bentonites were observed and reported. The decrease in the Atterberg's limit, swelling, specific gravity, optimum moisture content, pH and exchangeable monovalent cation and increase in maximum dry density was recorded with an increase in temperature as well as with the duration of heating.