Technischer Bericht NTB 86-14

Wasseraufnahme und Wasserbewegung in hochverdichtetem Bentonit

Water uptake by the bentonites MX-80 and Montigel was investigated according to the classical method of determination of the heat of immersion and the adsorption-de­sorption isotherms. In addition, the layer expansion of the montmorillonite was measured as a function of the water content. The evaluation of the adsorption isotherms according to Dubinin-Radushkevich and the stratification distances determined by X-ray confirmed gradual water uptake. Up to 10 % water content (calculated for pure, dry montmorillonite), the water is adsorbed as a monolayer and, up to 20 %, as a bimolecular layer around the interlayer cations. The partial specific entropy could be determined from the approximative calculation of the partial specific enthalpy from the heats of immersion and of the free enthalpy from the adsorption isotherms. From this it is evident that the interlayer water shows a high degree of order. In this condition, the mobility of the water molecules is considerably lower than in free water. From the adsorption isotherm and the layer expansion observed, it can be assumed that water can appear in the pore space only from approximately 25 % water content (calculated for pure montmorillonite). The spaces outwith the interlayer space and the surfaces of the montmorillonite particles are considered as pore space. If free swelling is prevented and with dry densities greater than 1.8 Mg/m3 for the highly compacted bentonites, water uptake causes a drastic reduction of the original pore space so that practically all the water is in the interlayer space.

Calculation of the swelling pressure from the adsorption isotherms gives a good approximation of the measured swelling pressures. A montmorillonite surface of ca. 750 m2/g for both bentonites can be derived from a Dubinin-Radushkevich analysis of the adsorption isotherm. Water uptake into the compacted unsaturated bentonites (over ca. 1.7 Mg/m3 dry density) can be described as diffusion with a diffusion coefficient of the order of magnitude of 3·× 10-10 m2/s.