The maximum temperature that a geotechnical bentonite barrier in a deep geological repository for radioactive waste can withstand while maintaining its integrity and meeting safety requirements is still an open question. Therefore, an international consortium set up an in situ heater test (HotBENT experiment) at the Grimsel Test Site (GTS) in Switzerland at relevant scales and gradients with temperatures ranging from 175°C to 200°C at the heater/canister surface. After dismantling (5 and 20 years, respectively), the identification of bentonite alteration processes of (clay) minerals has to be based on the comparison of data with reference values determined before the heating started. The experiment was set up using ~150 tons of two different clays (Wyoming and BCV from the Czech Republic) provided in different batches. The bentonites were used both as compacted bentonite blocks and as granular bentonite material (GBM). The determination of representative mineralogical and geochemical bentonite reference values must be based on a significant number of samples taken from all parts of the experiment, which is presented here. Most of the compositional variability was close to the accuracy of the methods used. However, chemical, mineralogical and exchangeable cation analyses showed that different raw materials were used to produce the BCV top blocks. The Wyoming bentonite used is similar to MX80 bentonite in that it is dominated by Na-rich smectite, but the HotBENT material contains slightly more feldspar and zeolite and slightly less smectite. Overall, 55 samples were analysed from different parts of the experiment, providing a statistical basis for post-excavation investigations.

Carbon black is commonly used as a filler in the rubber industry. However, it can cause serious damage to human health and to ecosystems. Today, reducing the use of this substance via alternative materials is receiving increased attention. Clay minerals such as montmorillonite can be substituted for carbon black as environmentally friendly fillers for rubber compounds. The uniform dispersion of montmorillonite and its compatibility with the rubber matrix are the main principles for using this material in the rubber industry. To this end, montmorillonite was surface treated with various dosages (1.0, 1.5 and 2.0 wt.%) of polypropylene glycol. The surface-treated montmorillonites were investigated using attenuated total reflection Fourier-transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy and X-ray diffraction analysis. The results indicated that the treatment expanded the interlayer spaces of the montmorillonite and improved surface hydrophobicity. The untreated and treated montmorillonites were used with carbon black as dual fillers in natural rubber/styrene butadiene-based compounds. The cure characteristics, thermal stability, some mechanical properties and dispersion states were evaluated. The curing study indicated a faster optimum cure time, scorch time and increased torque difference for the rubber compounds filled with surface-treated montmorillonites. Thermal analysis of the rubber compounds illustrated that the interval between the initial and final temperature of decomposition could be increased via the treatment. Surface-treated montmorillonite samples, especially the sample containing 1 wt.% polypropylene glycol, showed improved abrasion resistance, resilience and compression set values.

Trioctahedral phyllosilicate minerals are widely distributed on the Earth’s surface, especially in soil. The mineral–water interfacial reaction of lizardite, chlorite and talc, with various structural properties (tetrahedral sheet, octahedral sheet, 1:1-type and 2:1-type interlayer domain/two-dimensional structural units), was carried out in sulfuric acid solution (1 mol L–1). The mineral samples were characterized by powder X-ray diffraction, Fourier-transform infrared spectroscopy, scanning and transmission electron microscopy and inductively coupled plasma mass spectrometry. The dissolution concentration, dissolution rate, dissolution rules and structural changes of the components during the dissolution processes of the various two-dimensional structural units were studied. The results show that the dissolution concentrations of Si and Mg in the sulfuric acid solution decrease in the following order: chlorite > lizardite > talc and lizardite > chlorite > talc. The dissolution rates of Si in chlorite and Mg in lizardite are the greatest, while talc is the most stable compared with lizardite and chlorite. With increasing interfacial reaction time and the dissolution of the ionic components of the minerals, the structure of lizardite is gradually destroyed; the structural destruction of chlorite is more obvious during the early stages of the reaction; and the structure of talc does not significantly change over the course of the entire reaction. By analysing the microtopography of the minerals, it was found that the structural failure of lizardite occurred from the surface to the interior. Chlorite had more structural defects and showed collapse of the layered structure during structural failure. The surface layer of talc decomposed by corrosion into a small lamellae structure attached to the surface, but there was no obvious structural change similar to those of lizardite and chlorite. The relationship between the evolution of composition and structure during the mineral–water interfacial reaction process with the two-dimensional structure layer type provides the mineralogical basis for studying the coupling mechanism of the migration and transformation of materials in key regions of the Earth.

As a natural clay mineral, halloysite (Hal) possesses a distinctive nanotubular morphology and surface reactivity. Hal calcined at 750°C (Hal750°C; 0.0, 1.0, 2.0, 4.0, 6.0, 8.0 wt.%) was used to replace ground granulated blast furnace slag (GGBFS; 50.0, 49.5, 49.0, 48.0, 47.0, 46.0 wt.%) and fly ash (FA; 50.0, 49.5, 49.0, 48.0, 47.0, 46.0 wt.%) for the preparation of geopolymer in this study. The effects of the replacement ratio of Hal750°C on setting time, compressive strength, flexural strength, chemical composition and microstructure of the geopolymer were investigated. The results indicated that Hal750°C did not significantly alter the setting time. The active SiO2 and Al2O3 generated from Hal750°C participated in the geopolymerization, forming additional geopolymer gel phases (calcium (aluminate) silica hydrate and sodium aluminosilicate hydrate), improving the 28 day compressive strength of the geopolymers. When the amount of Hal750°C was 2.0 wt.%, the 28 day compressive strength of the ternary (GGBFS-FA-Hal750°C-based) geopolymer was 72.9 MPa, 34.8% higher than that of the geopolymer without the addition of Hal750°C. The special nanotubular morphology of residual Hal750°C mainly acted like reinforcing fibres, supplementing the flexural strength of the geopolymer. However, excessive Hal750°C addition (>4.0 wt.%) reduced compressive and flexural strength values due to the low degrees of geopolymerization and the porous microstructure in the ternary geopolymer. These findings demonstrate that the appropriate addition of Hal750°C improved the compressive strength of geopolymers prepared using GGBFS/FA, which provides essential data for future research and supports the utilization of low-value Hal-containing clays in geopolymer preparation.

Palygorskite (Pal) shows great potential for physical, chemical and biological uses due to its colloidal, catalytic and adsorption properties. Pal mines, however, are facing the challenge of low-grade materials (5–15%), making it difficult to use Pal in emerging fields such as new materials, environmental protection and health. Therefore, there is an urgent need to develop an efficient method for separating and purifying Pal to obtain high purity levels. Hence, we have developed a dispersant-assisted rotating liquid film reactor separation strategy based on sodium hexametaphosphate as the dispersant. This strategy utilizes the double electron layer of Pal and the density difference between impurities to achieve effective disaggregation and purification of Pal bundles through the promotion of repulsive driving effects. Under optimal conditions, the purity of Pal can be increased from less than 10% to over 80%. This research presents a novel approach to the efficient refining of low-grade Pal. The crudely purified Pal’s adsorption capacity for methylene blue increased from 84.2 to 256.4 mg g–1.