^aFederal Institute of Technology, Division of Foundation Engineering and Soil Mechanics, CH-8093 Zürich Switzerland

The mineral formula, the content of fixed potassium and the average interlayer charge of the montmorillonite–illite mixed layer part of four Ordovician K-bentonite samples from Kinnekulle (Sweden) were investigated. The content of fixed potassium was found to decrease from 84% to 50% with increasing depth and distance from the Permian basalt intrusion about 100 m above the bentonite beds. The interlayer charge of the mixed layer is smectitic, i.e. below 0.5 charges per formula unit. The swelling pressures of air-dried, gently grinded and uniaxially compressed specimens with dry densities between 1.5 Mg/m³ and 2 Mg/m³ range from 1 N/mm² to 15 N/mm². This is about 20% of the swelling pressure values of identically treated Ca-montmorillonite. Following ultrasonic treatment, air-drying, gently grinding and compaction, increased swelling pressures of of about 70% of those Ca-montmorillonite were obtained. The reason for this increase in swelling pressure was investigated by dispersion experiments and determinations of water vapour adsorption isotherms of ultrasonically treated and untreated material.