Swelling behaviors of GMZ bentonite–sand mixtures inundated in NaCl–Na2SO4 solutions

Abstract

a b s t r a c t In this study, the swelling behaviors of compacted GMZ bentonite–sand mixtures inundated in NaCl–Na2SO4 solutions are investigated and the influence of chemical solutions on the swelling behaviors of GMZ bentonite–sand mixtures as backfill/buffer material in China for high level radioactive waste (HLW) is investigated. The sand addition ratios of the bentonite–sand mixtures are 0%, 20%, 30% and 50%, and the total dissolved solids (TDS) of the NaCl–Na2SO4 (NaCl:Na2SO4 = 2:1 by mass) solution are 0.5, 1.0, 3.0, 6.0 and 12.0 g/L (pH 7.1). The specimens of bentonite–sand mixtures for swelling tests are prepared by static-compaction to various dry densities, ranging from 1.50 to 1.90 g/cm3. Test results indicate that liquid limit (LL) and plasticity limit (PL), swell time, maximum swelling pressure and maximum swelling strain decrease with the increase of TDS for GMZ bentonite–sand mixtures. All of the LL, PI and maximum swelling strain are decreased exponentially with TDS increase: very quickly as TDS 6.0 g/L. The maximum swelling pressure shows a linear reduction with the TDS increasing, but the pure bentonite indicates a high sensitivity than the bentonite–sand mixtures with 30% sand addition ratio. As NaCl–Na2SO4 (TDS = 0.5 g/L) solution was used according to the ground water, with initial dry density of 1.70 g/cm3, the maximum swelling pressure of specimens decrease exponentially while the maximum strain decrease linearly with the increase of sand addition. With 30% sand addition in 0.5 g/L NaCl–Na2SO4 solution, the maximum swelling pressure increase exponentially while the maximum strain increase linearly with the increase of initial dry density. Compared with the pure bentonite, bentonite–sand mixtures show a better tolerance withstanding the chemical attack to ground water chemistry because of the replacement of some quantity of expansive clay by quartz sand in the mixtures.