Technischer Bericht NTB 88-04

This report first summarizes past investigations and presently available data on highly compacted Na-bentonite (MX-80) and Ca-bentonite (Montigel). The information has been collected mainly from research work reported from Sweden and Switzerland. Then the suitability of Ca­bentonite with respect to its use as a sealing material in boreholes for low and intermediate level nuclear waste repositories is evaluated.

The comparison of the two bentonites with respect to the characteristics relevant for sealing showed that most properties differ only little. This applies particularly to the swelling pressure and the permeability which may be considered the most important properties for sealing materials. The most significant difference is in the plasticity, with MX-80 having a much higher liquid limit than Montigel. The long-term stability is similar for both materials. However, with a groundwater rich in Ca²⁺-ions, Montigel is preferred, because ion-exchange is minimized and also the tendency for illitization is decreased.

Based on the present knowledge, it can be concluded that highly compacted bentonite is a suitable material for a borehole plug. Its function will mainly be that of a highly impermeable barrier. In particular, it has been possible to convincingly establish the suitability of the Ca-bentonite Montigel in Ca²⁺-rich groundwater.