Harnessing kaolinite’s layered structure in solid-state reaction

Abstract

The optimization of kaolinite [Al₂Si₂O₅(OH)₄] as a raw materials for solid-state reactions plays a crucial role in the formation of high-quality ceramics. The earliest discover in this field was the production of thin-walled yet remarkably strong Chinese porcelain, which primarily made from kaolinite-rich clay. Despite kaolinite’s poor plasticity and low dry strength, its exceptional hand formability was achieved through the practice of slurrying and storing of clay with urine in large pits in traditional Chinese porcelain-making techniques. This “ageing” process, along with a focus on “kaolinite’s layered structure”, led to the intercalation of urea—a component produced by urine decay during “ageing”—between kaolinite layers, inducing their expansion. As a result, rigid kaolinite platelets were transformed into more thinner ones, and some forming wrinkled aggregates, ultimately enhancing plasticity and facilitating precise hand shaping. When used as a raw material, kaolinite undergoes calcination in the presence of other compounds, initiating solid-state reactions that contribute to ceramic formation. In industrial applications, where cost-effectiveness is prioritized, these reactions are carried out in the presence of by-products. Additionally, its layer expansion has never been optimized for solid-state reactions. Thus, kaolinite’s layered structure should be further “harnessed” in ceramics research to maximize its potential for future applications. This perspective also offers a pathway to elucidating kaolinite’s solid-state reactions, particularly that through the solid-state reactions of CaAl₂Si₂O₈ and BaAl₂Si₂O₈ polymorphs. In this review, recent studies—beginning with the first major optimization of kaolinite’s layered structure for solid-state reactions in 2021, following its foundational discovery in 1963—are introduced. Kaolinite intercalation chemistry emerges as a promising approach to optimizing kaolinite as a raw material for solid-state reactions, with particular attention given to layer expansion to disrupt the stacking order, the role of impurities in reaction dynamics, and the adsorption at edge surfaces, as well as the influence of morphology and crystallinity.