Experimental study on the swelling pressure of compacted bentonite under high temperatures above 100 °C

Abstract

Disposal and storage of high-level nuclear waste in deep geological repositories requires understanding the behavior of compacted clay at high maximum design temperatures (> 100 °C); however, the effect of temperature variations on the swelling pressure of compacted bentonite has generally been examined within a restricted temperature range of 20–90 °C. Consequently, this experimental investigation uses specially designed temperature-controlled apparatus to determine the swelling pressure of compacted Na-bentonite across a temperature span of 10–160 °C. The poro-thermal and chemo-thermal effects on the swelling pressure of bentonite were studied using three dry densities (1.20, 1.35 and 1.50 Mg/m³) and three electrolyte concentrations (de-ionized water, NaCl 0.1 M and 0.5 M solutions). A linear relationship was observed between the normalized swelling pressure (the ratio of swelling pressure at a specific temperature to the swelling pressure at 25 °C) and temperature under the tested temperature ranges, suggesting that the swelling pressure measured at relatively low temperature ranges (below 100 °C) could be used to estimate the high temperature (above 100 °C) swelling pressure based on extrapolation. The swelling pressure increased with temperature, and higher dry densities and lower electrolyte concentrations amplified this effect. The complex interplay of dry density, pore fluid concentration, and temperature on the swelling behavior of compacted bentonite was explained based on the competition between the interlayer and interparticle swelling pressures.