Combined effects of temperature and salt solution on swelling of GMZ01 bentonite–sand mixtures

In order to dispose of high level nuclear waste during the operation of underground geological repositories, the buffer/backfill performance of bentonite–based material might be affected by the combined influence of temperature field and infiltrating groundwater thereby leading to uncertainties in the safe operation of repositories. In order to address this issue, bentonite–sand mixtures (BSM) were prepared by mixing GMZ01 bentonite with sand using ratios of 70:30 and 30:70 by weight. One-dimensional deformation experiments were conducted on the prepared BSM in NaCl solutions of varying concentrations (0, 0.1, and 1.0 mol/L) at three different temperatures (25, 60, and 90 °C). The experimental results on the BSM in NaCl solutions indicated that the swelling deformation increased as temperature rises, because osmotic swelling is the dominant swelling type under the experimental conditions. The swelling deformation of BSM could be represented by fractal e_m–p_e relation (e_m is the montmorillonite void ratio, p_e is the effective stress incorporating osmotic suction). At each temperature interval, the swelling of BSM with high bentonite content in different NaCl solutions can be expressed as a unique e_m–p_e relation; however, the swelling coefficient of montmorillonite in the e_m–p_e relationship increases at higher temperature. When the effective stress p_e exceeds a threshold, sand–particle skeletons will be formed in BSM with low bentonite content, thus resulting in deviations in the value of e_m from the e_m–p_e curve. The e_m–p_e relation offers a simple and efficacious method to estimate the deformation of BSM under combined thermal–salt effects.