From ancient practice to modern innovation solving the clay mineral puzzle in brickmaking

Abstract Clay minerals are the most crucial constituents in brickmaking clays, yet the specific role of different clay minerals remain poorly understood due to their natural coexistence and formidable challenges of separation. Here, we design a kaolinite-illite-quartz-feldspar system through a multiple-stage purification strategy, enabling direct evaluation of two of the most critical clay minerals for brick making - kaolinite and illite. Precise tuning the initial clay mineral assemblage yields brick strengths up to 8.5 times the ASTM C62 Grade MW requirement, without upgrading existing production methods. Mineralogical mechanisms responsible for substantial strength enhancement are revealed. Extensive analyses of 60 brick formulations uncovered a breakdown of conventional linear relationships among different brick properties due to the temperature-dependent responses of kaolinite and illite; specific formation pathways of mullite and amorphous phases; and over-firing defects above 1200 °C due to K + -induced fluxing from illite. Notably, a previously unrecognized negative correlation between mullite and brick strength is identified, suggesting a need to reconsider the widely held assumption of mullite as a strengthening phase. This work not only presents insights for optimizing high-strength brick formulations but also opens avenues for investigating the intricate contributions of distinct starting components to key brick performance.