Mineralogy is an essential issue in understanding thermo-hydro-mechanical-chemical (THMC) behavior of bentonite materials. Mineralogy affects, among others, chemical composition of pore water, susceptibility for erosion, and transport of radionuclides. Consequently, mineralogy affects the designs of the buffer and backfill components.
The objective of this work was to implement and develop mineralogical and chemical methods for characterization of reference clays considered for use as buffer and backfill materials in nuclear waste disposal. In this work, different methods were tested, compared, developed, and best available techniques selected. An additional aim was to characterize reference materials that are used in various nuclear waste disposal supporting studies, e.g., the SKB’s alternative buffer material (ABM) experiment.
Materials studied included three Wyoming-bentonites, two bentonites from Milos, four bentonites from Kutch district, and two Friedland clays. Minerals were identified using x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and polarizing microscopy. Mineralogical composition was estimated using Rietveld-analysis. Chemical methods were used to support and validate mineralogical interpretation. Total chemical composition was determined from decomposed samples using spectrometry (ICP-AES) and combustion (Leco-S, Leco-C). Ferric and ferrous iron species were distinguished titrimetrically and the amount of soluble sulphate was determined using ion chromatography. In addition, cation exchange capacity and original exchangeable cations were determined. Chemical composition of fine (<2µm) fractions and poorly crystalline Fe-, Al- and Si-phases determined by selective extractions were used in structural calculations of smectite.
XRD is a basic method for all mineralogical characterization, but it is insensitive for detecting trace minerals and variations in the structural chemical composition of clay minerals. Polarizing microscopy proved to be useful in characterization of coarse fractions with regard to identification of trace minerals, estimation of particle size distribution, and characterization of crystal morphology, possible alteration of minerals, as well as mineral assemblages. FTIR not only supported mineralogical observations from XRD, but it revealed variations in clay structural compositions, and the presence of mineral impurities in purified clay fractions that were used as the basis of structural calculations.
The Wyoming-type Na-bentonites under analysis were very similar to one another and contained approximately 80 wt.% of smectite. All Kutch bentonites were enriched with ferric iron, were Al-rich, and their kaolin mineral content varied, up to 20 wt.%. Bentonites from Milos-area were Ca-rich and contained slightly more illite in fine fraction than bentonites from Wyoming or Kutch areas. Friedland clays consisted approximately 22 wt.% of smectite and 34 wt.% of illite.