Mineralogical, Geochemical and Geotechnical Study of BCV 2017 Bentonite—The Initial State and the State following Thermal Treatment at 200 °C

Abstract

Bentonites are considered to be the most suitable materials for the multibarrier system of high-level radioactive waste repositories. Since BCV bentonite has been proved to be an ideal representative of Czech Ca-Mg bentonites in this respect, it has been included in the Czech Radioactive Waste Repository Authority (SÚRAO) buffer and backfill R&D programme. Detailed knowledge of processes in the material induced by thermal loading provides invaluable assistance regarding the evolution of the material under repository conditions. Samples of both original BCV 2017 bentonite and the same material thermally treated at 200 °C were characterised by means of chemical analysis, powder X-ray diffraction, infrared spectroscopy, thermal analysis, cation exchange capacity, specific surface area (BET) measurements, the determination of the swell index, the liquid limit, the swelling pressure and water retention curves. The smectite in BCV 2017 bentonite comprises Ca-Mg montmorillonite with a significant degree of Fe3+ substitution in the octahedral sheet. Two main transformation processes were observed following heating at 200 °C over 27 months, the first of which comprised the dehydration of the montmorillonite and the subsequent reduction of the 001 basal distance from 14.5 Å (the original BCV 2017) to 9.8 Å, thus indicating the absence of water molecules in the interlayer space. The second concerned the dehydration and partial dehydroxylation of goethite. With the exception of the dehydration of the interlayer space, the PXRD and FTIR study revealed the crystallochemical stability of the montmorillonite in BCV 2017 bentonite under the selected experimental conditions. The geotechnical tests indicated no major changes in the mechanical properties of the thermally treated BCV 2017 bentonite, as demonstrated by the similar swelling pressure values. However, the variation in the swell index and the gradual increase in the liquid limit with the wetting time indicated a lower hydration rate. The retention curves consistently showed the lower retention capacity of the thermally treated samples, thus indicating the incomplete re-hydration of the thermally treated BCV 2017 exposed to air humidity and the difference in its behaviour compared to the material exposed to liquid water.

BCV bentonite hydromechanical behaviour and modelling

“…BCV bentonite (see Tables 1 and 2) exhibits a high proportion of minerals from the smectite group (Ca/Mg montmorillonite of over 60%), while other clay minerals (kaolinite, illite) are present to a lesser extent. With respect to the accessory phases, the presence of genesis-related Fecarbonates (siderite, ankerite) and Fe oxides (mainly goethite) is of particular interest [9] and [14].…”

“…This study involved the investigation of a BCV bentonite powder and a uniaxially compacted powder. The batch of the material that was subjected to experimentation was named BCV_2017; for more detailed information, see Laufek et al [14].…”

“…The water flow over at least the four latest consecutive intervals was used for the determination of the coefficient of permeability (average of the four results). The water pressure source was subsequently disconnected so as to allow for the determination of the swelling pressure (Laufek et al [14]).…”

Bentonite as a Functional Material Enhancing Phytostabilization of Post-Industrial Contaminated Soils with Heavy Metals

“…Bentonites are mudstones consisting mainly of smectites, particularly montmorillonite, which is a hydrated aluminosilicate with a layered structure [ 19 ]. Their characteristic features include their substantial specific surface area and their tendency to adsorb water in interstitial spaces, as a result of which, bentonite has the greatest tendency to swell of all clay minerals (water absorption 300–700%) [ 20 ]. The composition of bentonites varies, due to isomorphic substitutions within Al +3 ion chains, for example, with Mg +2 and Fe +3 ions, and the predominant cation, i.e., Ca +2 or Na + [ 21 ].…”

Fabrication of Adsorbed Fe(III) and Structurally Doped Fe(III) in Montmorillonite/TiO2 Composite for Photocatalytic Degradation of Phenol

“…Moreover, the significantly reduced fine particle sizes can improve the photocatalytic performance of TiO 2 [11][12][13]. Mt is a 2:1 clay mineral, with a layered structure and exchangeable interlayer cations [14][15][16], which is sufficiently distributed in nature [16,17]. Consequently, Mt has excellent adsorption abilities, especially for positively charged substances with large relative surface areas.…”