Studies on the chemical conditions and microstructure in Package 1 of Alternative Buffer Materials project (ABM) in Äspö

The chemical and microstructural properties of MX-80, Asha and Deponit bentonites and of Friedland Clay in package 1 of the ABM project in Äspö HRL were studied after over two years of experiment. The concentration distribution of chloride and sulphate in different clays as a function of distance from the heater and in the reference bentonites, not used in the experiment, were determined. The concentration distributions of chloride were quite even. In Friedland clay and Asha the chloride concentrations in the clay decreased during the experiment while in MX-80 and Deponit the concentrations increased. Sulphate accumulated towards the hot end in the samples where the initial content of sulphate was high (Friedland Clay, MX-80, Deponit), while in Asha, where the initial sulphate concentration was low, such an accumulation did not occur.

The cation exchange capacity in different clays was determined at different distances from the heater using the complex of copper(II) ion with trietylenetetramine [Cu(trien)]²⁺ as the index cation. The CEC of MX-80 and Deponit increased slightly close to the heater. In Asha there was some indication about the decrease of CEC close to the heater while in Friedland Clay there was small decreasing trend towards the heater and the CEC was generally slightly higher than in the reference sample.

The pH measurements in the clays were performed with IrOx electrodes at a distance of 8 cm from the heater. The pH values were for Friedland Clay 6.5 – 6.6, for MX-80 6.6 – 6.8, for Asha 6.9 – 7.0 and for Deponit 6.6 – 6.9. The Eh measurements were performed with Pt and Au electrodes placed in the clays at 8 cm from the heater. The final values obtained after five months of measurement with Au and Pt electrodes were for Friedland Clay 17 mV and -231 mV, for MX-80 22 mV and -48 mV, for Asha 104 mV and 40 mV and for Deponit 76 mV and 75 mV, respectively.

The water content and bulk density were measured at different distances from the heater; the dry density and water porosity were calculated on the basis of the water content and bulk density measurements. It was obvious that the wet and dry densities were somewhat lower and water porosity higher in the cooler parts of the samples. The microstructure of the wet samples of MX-80, Deponit and Asha blocks at distances of 1 to 3 cm and 7 to 9 cm from the heater were studied with XRD. The peak places of all the clays indicated that there are two to three water layers in the interlamellar space. The basal spaces of the peaks were smaller than the evaluated theoretical basal spaces in all the studied bentonites, indicating that not all the water was in the interlamellar space of well-organized montmorillonite. Small-angle X-ray Scattering (SAXS) measurements supported the results given by XRD. By comparing the water amount based on SAXS basal space measurements with the water amount in the bentonite sample measured by drying, it could be seen that the water amount, which is not seen by SAXS, is about 25 % of the total water when the dry density is 1.4 - 1.5 g/cm³.