Development of methods for on-line measurements of chemical conditions in compacted bentonite

This work describes the development of an on-line method for studying the chemical conditions in compacted water-saturated bentonite. The equipment includes a squeezing cell, measurement electrodes, a reference electrode, and a data-logging system. IrOx wire was used for the pH measurements and Au and Pt wires for the redox measurements. The electrodes were made in-house at VTT. The reference electrode was a commercial leak-free electrode. The measurement electrodes were installed in the bentonite, while the reference electrode was placed in water outside the clay. The contact between the reference electrode and the measurement electrodes was established via the solution in a tube.The work included first a test and calibration of the electrodes in various standard solutions. The performance of the electrodes was subsequently studied in compacted MX-80 in squeezing cell experiments. The squeezing cell initially contained MX-80 mixed with different solutions at a dry density of 0.8–0.9 Mg/m3. The pore water was then slowly squeezed out from the clay until the final density of 1.5 Mg/m3 was reached. The measurement electrodes were then placed in the clay and the potential measurements were started. At the end of the experiments the electrodes were removed from bentonite and recalibrated. The pH and Eh in the squeezed pore waters were determined for comparison.The results obtained so far have led to the conclusion that it is possible to perform pH measurements in compacted bentonite by using IrOx electrodes. The electrode can survive the swelling pressure inside compacted water-saturated clay without losing its ability to function as a pH electrode. It is also possible to perform Eh measurements in compacted clay by using Au and Pt electrodes.