Preliminary results on the material composition of Ca-bentonite from the Kardzhali deposit, Southern Bulgaria

Abstract

This paper presents a preliminary investigation of Ca-bentonite from the PropastDobrovolets deposit, Kardzhali (Bulgaria) with respect to its potential application in engineered barrier systems (EBS) for radioactive waste isolation. Three forms (raw, powdered and a fraction of 2 μm) of non-activated bentonite were analysed using geochemical analytical techniques and geotechnical testing. The results indicate that the studied materials are dominated by dioctahedral montmorillonite (72–85%), with minor quantities of cristobalite, oligoclase, and calcite. The chemical composition confirms their classification as Ca-bentonites, with a SiO₂/Al₂O₃ ratio of 3.59. The powdered bentonite, characterized by a higher clay fraction (59%), exhibits enhancedphisycal properties, including a higher liquid limit (267%), specific surface area (92 m²/g), and water absorption capacity (391%) compared tothe raw material. SEM observations reveal a characteristic honeycomb microstructure with edge-to-face contacts, indicative of a well-developed swelling matrix. The analyzed Ca-bentonite shows properties that meet the requirements for EBS applications. It contains a high amount of swelling clayminerals, which provide high plasticity, swelling and strong sealing ability. Therefore, it could be considered potentially suitable for use as both a buffer and a sealing material in an engineered barrier system (EBS) for radioactive waste disposal.