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

Minerals

František Laufek¹  Michal Čurda¹  František Pticen¹  Magdaléna Koubová¹  Irena Hanusová²  Jiří Svoboda³  Radek Vašíček³  David Mašín⁴  Jan Najser⁴  Haiquan Sun⁴  Lenka Vaculíková⁵ 

[1] Czech Geological Survey, Geologická 6, 15200 Prague, Czech Republic;Czech Radioactive Waste Repository Authority (SÚRAO), Dlážděná 6, 11000 Prague 1, Czech Republic;Faculty of Civil Engineering, Czech Technical University, Thakurova 7, 16629 Prague 6, Czech Republic;Faculty of Science, Charles University, Albertov 6, 12843 Prague 2, Czech Republic;Institute of Geonics, Academy of Sciences of the Czech Republic, Studentská 1768, 70800 Ostrava-Poruba, Czech Republic;

关键词: bentonite;    smectite;    crystal structure;    water in the smectite interlayer;    XRD;    mineralogical changes;   

DOI  :  10.3390/min11080871

来源: DOAJ

【 摘 要 】

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.

【 授权许可】

Unknown