Effect of thermal aging at 165 °C–200 °C on bentonite performance

The engineered barrier system in the deep geological repository will encounter varying design loads (hydraulic, mechanical, thermal and chemical) over time. The bentonite buffer may remain dry for long periods of time locally, where access to water from the host rock is low, and consequently, heat may build up increasing the buffer temperature leading to dehydration of smectite. The temperature for complete removal of interlayer water on the smectite is within >150 °C and depends on among others exchangeable cation, smectite type and confining pressure. Although dehydration reactions can be considered reversible, complete dehydration may lead to irreversible alterations. The objective of this work was to determine the effect of thermal aging on bentonite buffer performance. Bentonite from Wyoming was thermally aged at 165 °C, 180 °C or 200 °C for 4 weeks, 8 weeks or 20 weeks in a dry, unsaturated state. Thereafter, bentonite was cooled down to room temperature and its properties and performance were evaluated determining the mineralogy by X-ray diffraction (XRD), cation exchange capacity (CEC), swelling index, and performing swelling pressure and hydraulic conductivity tests. Swiss repository conditions were applied for this work using granular bentonite mixture (GBM) and a target dry density of 1450 kg/m ³ and Opalinus Clay synthetic porewater (TDS of 16.92 g/l) in swelling pressure and hydraulic conductivity experiments. Mineralogical changes observed include dehydration of gypsum and change in clay suspension color indicating small changes in iron oxide mineral phases or contents. No impact on the swelling pressure was noted due to thermal aging, but thermally aged samples had a factor of 1.8 higher hydraulic conductivity compared to references. In addition, up to 4 % decrease in apparent CEC, and up to 25 % decrease in swelling index and faster settling of clay suspensions were observed to occur with thermal aging time and temperature. One potential explanation for observed changes is partial fixation of exchangeable cations and consequent changes in particle size due to heat exposure.