Experimental investigation of the effect of high-pH solutions on the Opalinus Shale and the Hammerschmiede Smectite

The alteration and transformation behaviour of the Tertiary Hammerschmiede Smectite and the Jurassic Opalinus Shale in an alkaline solution was studie d in column experiments. The Hammerschmiede Smectite is proposed as potenti al backfill material and the Opalinus Shale as host rock for the Swiss low- level nuclear waste storage site. Over a period of 18 months, the evolution of permeability, pH and solution concentrations were measured. After the e xperiment, the columns were cut into pieces to study the mineralogical and the chemical evolution of the clays. X-ray diffraction (XRD) revealed no si gnificant appearance or disappearance of diffraction peaks at the end of th e experiments. The scanning electron micrographs of the clays revealed that both clays exhibited a precipitation zone, which extends from 0 to 2 cm be low the infiltration surface. Both clays showed significant differences in the evolution of pH and hydraulic conductivity. The solution front crossed the Opalinus Shale column entirely after only 11 weeks and the initial valu es for K+ and Na+ were conserved in the solution. For both clays, the salt concentrations in the percolating fluids minor the evolution of pH.