Effect of elevated temperatures on saturation degree and microstructure of Bavarian bentonite, Friedland clay, and Opalinus clay

Abstract

Water saturation of compacted specimens of Bavarian bentonite, specimens of intact Friedland clay and drilled-out specimens of shaly facies of the Opalinus clay was carried out at target temperatures of 35–150 °C and a back pressure of 7 MPa. Mercury intrusion porosimetry (MIP) indicates that increasing temperature-induced expansion of mesopores accompanied by a decrease of their void ratio and an increase of macropore void ratio in bentonite, whereas no temperature-induced changes were observed for the mesoporous structure of Friedland and Opalinus clays. Gravimetric determination of water contents revealed an apparent oversaturation for bentonite at all studied temperatures. An approach based on the assumption of increased density of tightly bound water, which was successfully applied in previous bentonite studies at temperatures below 100 °C, could not explain this observation. A modification of this approach, which assumes that the decrease of density of weakly bound water due to thermal expansion is partially compensated by the increase in salinity due to mineral dissolution, was still unable to provide a realistic explanation for the oversaturation observed at 150 °C. The only successful explanation is that the true grain density of bentonite is greater than its apparent grain density of 2.69 g/cm³, measured with a helium pycnometer in the dry state, and equals approx. 2.80 g/cm³. The presence in the dry state of bentonite of smectite layers with basal spacings below the threshold basal spacing for helium diffusion, which is estimated to approx. 1.05 nm, is proposed to explain this result. If this result is confirmed, the consequence for modelling the thermal-hydraulic-mechanical-chemical behaviour of clay-based geotechnical barriers in geological repositories for radioactive waste would be that the porosity is greater and the degree of saturation lower than when calculated on the basis of grain density in the dry state.