Mineralogical and geotechnical characterization of two German bentonites from Westerwald and Bavaria

This study investigates the fundamental mineralogical, physico-chemical, and geotechnical properties of two naturally occurring Ca/Mg bentonites from Germany, along with their Na-saturated sub-fractions. The bentonites — Secursol UHP® from Westerwald and Calcigel® from Bavaria — are commercially available and used in different forms, including granular and pillow types, for geotechnical purposes and multiscale testing of the Sandwich sealing system for hydraulic sealing in underground waste repositories. The dioctahedral smectite content in Secursol UHP and Calcigel is 79 % and 61 %, respectively. Smectite in both was identified as montmorillonitic beidellite with significant octahedral iron and no interstratification. Smectite in Secursol UHP shows a slightly higher layer charge and broader particle size distribution, while Calcigel smectite has a 25 % larger average basal surface and slightly greater stack thickness. Geotechnically, both are highly plastic clays with almost identical consistency as received; however, Calcigel exhibits a higher liquid limit, greater water uptake, and better dispersity in deionized water despite its lower smectite content, lower total specific surface area, and relatively coarser particles. These discrepancies are attributed to the inherited fabric of Secursol UHP hindering smectite expansion.