Editorial for the Special Issue “Application and Modification of Clay Minerals”

Clay minerals, a diverse group of hydrous aluminosilicates, are ubiquitously distributed and primarily comprise soils, sediments, and sedimentary rocks. The nanoscale layered architecture endows them with distinctive properties such as high specific surface area, superior cation exchange capacity, and unique plastic, rheological, and swelling behaviors. These underpin their widespread applications, with recent advances being particularly noteworthy. Through modification and targeted synthesis, clay-based materials can be tailored with special attributes to meet specific functional demands, and have transcended their traditional applications in ceramics and construction and entered advanced, cutting-edge fields [1,2]:

This Special Issue, “Application and Modification of Clay Minerals,” published in Materials, was conceived to address these challenges and showcase the cutting-edge research pushing the boundaries across diverse fields. The collected contributions aim to present a compelling portfolio of research that advances our knowledge on advanced synthesis routes, innovative modification, and potential applications in environmental remediation, materials science, and industrial processes. In what follows, I will briefly review all fourteen papers published in this Special Issue according to the aforementioned classification approach.

To integrate sustainability into construction materials, Fang et al. replace the scarce freshwater and river sand with seawater and sea sand to manufacture concrete. However, the abundant Cl⁻ ions pose a dual challenge, which can refine the concrete’s microstructure through the shape of Friedel’s salt phases, which also leads to the formation of damaging delayed ettringite. Increasing the Cl⁻ concentrations enhances both Friedel’s salt and delayed ettringite, while the elevation of curing temperatures or the addition of nano-metakaolin promotes Friedel’s salt and suppresses delayed ettringite, thus effectively mitigating the adverse effects of Cl⁻. In another study, the authors from the same group demonstrate the effects of Cl⁻ concentration, fly ash, and curing temperature on the microstructural evolution of two primary cement hydration products: C-S-H gels and calcium hydroxide. The presence of Cl⁻ ions increases the concentration of Ca²⁺ compounds and favors the production of C-S-H gels. Fly ash reacts with calcium hydroxide generated via cement hydration to produce C-S-H gels, and 50 °C curing can stimulate the activity of fly ash, generating a large amount of C-S-H gels. The contents of C-S-H gel and calcium hydroxide have the largest gray correlation with fly ash dosage. Among these factors, fly ash achieves the highest correlation with the amount of C-S-H gels. Maosa et al. reveal that mechanical activation is effective for preparing clay minerals to be used as supplementary cementitious materials for the formulation of low-carbon cements. Using the response surface methodology, the various grinding parameters are optimized to achieve a high degree of clay activation. The high rotation speed and prolonged grinding time promote pozzolanic activity by boosting the formation of amorphous phases from clay minerals and reducing the particle size. However, the intermediate milling parameters are sufficient to achieve the substantial degrees of amorphization and pozzolanic activity that further grant the activated clay’s reactivity. The exceedingly aggressive milling introduces impurities from the milling equipment wear, and a balance is essential.

Koroleva et al. address the impacts of different silica precursors, pH conditions, and synthesis durations on the synthesis of kaolinite group minerals, and pH exerts the most significant alteration, with the formation of halloysite with a small admixture of kaolinite at a high pH and the formation of ordered kaolinite at a low pH. Yang et al. propose a new micronization approach to disaggregate the bundles of sepiolite while preserving the original structural integrity of fibers. The sepiolite powders with a mass ratio of more than 90% and particle size of less than 22 μm are readily obtained at a steam pressure of 0.6 MPa. The steam pressure change method shows obvious advantages over mechanical extrusion in maintaining fiber integrity and activating the surface, thus showing potential applications as scavengers. Wang et al. manufacture the low-silicon X-type zeolite from lithium slag without extra Si and Al sources. The X-type zeolite has a high ion exchange capacity for Mg²⁺ of 191 mg MgCO₃/g, two times that of 4A zeolite, and a high ion exchange capacity for Ca²⁺ of 302 mg CaCO₃/g, which meets the first-grade standard of zeolite for detergent builders. With sea sand as the Si source and Al(NO₃)₃ as the Al source, Xie et al. manufacture analcime with the tunable Si/Al ratios. Impurities such as quartz and sodalite are detected when using acid-treated sea sand, while pure analcime is readily available when using alkali-treated sea sand. The adsorption capacity of Cu²⁺ amounts to 64.8 mg/g, and the adsorption kinetics follow the pseudo-second-order model.

A meso–macroporous analcime/sodalite zeolite composite is produced by Esaifan et al. by means of a hybrid synthesis process employing both a complex template method and the hydrothermal treatment of naturally abundant kaolinitic-rich clay. There are two types of zeolite particles: a hollow microsphere with an analcime icositetrahedron of 30~40 µm and a sodalite microsphere with a ball-like morphology of 3~4 µm. The composite exhibits excellent scavenging capacities for a wide range of pollutants (inorganic Cu, Cd, Cr, Ni, Zn, and Pb ions, organic dyes, phenolic compounds, and surfactants) and superior catalytic degradation activity for volatile organic compounds. Liang et al. conduct the intercalation of sericite with anionic surfactants rather than with the widespread cationic surfactants, following thermal modification, acid activation, and sodium modification procedures. Sericite, with the co-intercalation of cationic surfactant hexadecyl trimethyl ammonium bromide (CTAB) and anionic surfactant sodium dodecyl sulfate (SDS), has a layer-to-layer distance of 6.56 nm and an intercalation rate of 95.7%. Subsequently, the sericite/epoxy composite is prepared, and it exhibits enhanced mechanical properties when compared to epoxy resin, showing potential applications in oil well drilling and paints.

The definitive classification of these papers is difficult given that many encompass interdisciplinary fields. The paper of Esaifan et al. exemplifies such an overlap, and although attributed to “Advanced Materials and Nanocomposites,” it is also highly relevant to “Environmental Remediation and Sustainable Materials” and “Catalysis.” The pursuit of interdisciplinary research can lead to the discovery of new investigative avenues. Looking forward, the following research directions should be prioritized to advance the field of clay mineral science and engineering [3,4,5]:

Clay minerals are poised for a renaissance, and a transformation is underway marking a shift from bulk, low-tech applications to high-value, engineered nanomaterials. The future lies not simply in their inherent properties but in our growing ability to manipulate their structure at the nanoscale. The interdisciplinary synergy of materials science, nanotechnology, chemistry, and biotechnology with clay research is poised to unlock unprecedented opportunities, thereby elevating clay minerals to the status of foundational elements in forging a sustainable and healthy society.

Finally, I would like to extend my sincere gratitude to all the authors, reviewers, and the editorial team of Materials whose hard work and dedication made this Special Issue possible.