Bentonite interaction with saline high-pH solutions

Bentonite and cement materials are essential components in constructing a deep geological repository. Degradation of cementitious materials produces leachates of high pH. Such an alkaline plume, if reaching the bentonite buffer, is likely to induce mineralogical and chemical changes in bentonite over long times and may jeopardise the set safety function of the buffer. Laboratory experiments were started with the objective to gain data of the buffering capacity, possible alterations, especially dissolution of montmorillonite when bentonites are in contact with high-pH solutions. Batch experiments were carried out in an anaerobic glove-box at 24±1 °C. Two different bentonites, MX-80 and Deponit CA-N (Ca-bentonite), were leached with three types of simulated cement waters (pH 9.7, 11.3 and 12.0) and one saline groundwater simulate (pH 8.3) as reference. Each bentonite sample (20 g) was leached with several batches of leaching solution. For each renewal of the leaching solution the phases were separated by centrifugation, the reacted solution withdrawn, and the chemical composition analysed. The high-pH experiments (11.3 and 12.0) have continued to show an initial decrease in the pH-values after each renewal of the leaching solution, albeit less dramatic than in the beginning, while the other two experiments (pH 8.3 and 9.7) have shown rather unaltered pH-values. In general, slightly lower pH-values were observed in the Deponit CA-N samples than in those of MX-80. Now one experimental set of three parallel ones has been stopped and the bentonite phases are characterised to be compared with the results of the initial bentonite materials