Experimental Study on the Swelling Performance of Gaomiaozi Bentonite Buffer Material under Fe(II) Reducing Environment

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Abstract The release of Fe(II) from iron-based disposal canisters during the operational period of high-level radioactive waste (HLW) repositories may affect the swelling behavior of the buffer barrier, consequently impacting repository safety and stability. In this study, we employed a self-made dilatometer to measure the swelling characteristics of GMZ24 bentonite under varying Fe(II) concentrations. Our test results indicate that with increasing Fe(II) concentration, the time required for GMZ24 bentonite to swell decreases, while the final swelling pressure and swelling ratio exhibit an exponential decline. Through hydrochemical detection and mineralogical analysis, it is found that the decrease of swelling performance of bentonite is mainly due to the cation exchange between Na+ and Fe(II) in bentonite and the isomorphic replacement of Montmorillonite crystal and Fe(II) to generate chlorite. Finally, based on the double electrical layer theory, we derived a functional relationship describing the variation in swelling pressure and swelling ratio of GMZ24 bentonite as a function of Fe(II) concentration. This study provides a scientific basis for predicting and controlling the swelling behavior of buffer backfill materials in HLW repositories.