Water Uptake and Swelling Pressure in a Bentonite-Based Backfill

Abstract

ABSTRACTThe partial specific enthalpy and the partial specific entropy for the water uptake of two bentonites was investigated by means of the water vapor adsorption-desorption isotherms and the heat of immersion with liquid water. From these experiments, and as confirmed by X-ray measurements, it is evident that the interlayer water shows a high degree of order. Calculations of the swelling pressure from the adsorption isotherms show good agreement with measured swelling pressures.The uptake of liquid water by the compacted bentonites under conditions of restrained swelling can be described as a non-steady state diffusion with constant surface concentration. The water uptake of two bentonites in their natural sodium and calcium forms has been investigated. The measurements were carried out in a swelling pressure apparatus with water supply from one or both sides, and in a device modelling the repository conditions with radial water supply. In this modelling experiment, the water uptake was measured and the enclosed air determined. In addition, the swelling pressure was observed and compared to earlier test results.