Effect of mineralogy on the performance of calcined clays from Brazilian mining tailings as supplementary cementitious material in ternary cement

This study examines the impact of five calcined clays with varying mineralogy and kaolinite content on the hydration, sulfate demand, rheological behavior, and mechanical properties of cementitious systems. All clays originate from iron ore tailings and constitute an unconventional yet strategically relevant clay source, exhibiting distinct mineralogical features and including elevated Fe-bearing phases. Its proposed usage is aligned with a circular economy strategy, enabling these materials to no longer be disposed of in piles and dams, eliminating or reducing this environmental liability of the mining industry. The kaolinite content of the clays ranges from 3.6% to 82.3%, and they were used to replace 25% of Portland cement in blended systems. Hydration kinetics and phase evolution were assessed using isothermal calorimetry, TGA, and XRD at different curing ages. Compressive strength and rheological analyses were also performed. The results show that all clay-containing systems exhibit increased sulfate demand, evidenced by the earlier onset of the aluminate peak during hydration. Compressive strength, TGA, and XRD results confirmed that clays with higher kaolinite content exhibit greater pozzolanic reactivity, reflected in a faster consumption of portlandite and Strength Activity Index (SAI) values ranging from 123% to 59%. Rheological results revealed a substantial loss of workability in clay-blended systems, with yield stress up to 27 times higher and plastic viscosity up to 7 times higher than the reference paste. However, deviations from the expected correlation with kaolinite content indicate that additional mineralogical components also play a significant role in governing the fresh-state behavior. • Five calcined clays sourced from iron ore tailings, with varying kaolinite contents and mineralogical profiles, were evaluated as cement replacements in blended systems. • Pozzolanic index, isothermal calorimetry, XRD, and TGA analyses revealed that pozzolanic reactivity correlates positively with kaolinite content, as evidenced by accelerated portlandite consumption. • All clay-containing systems showed hypo-sulfated behavior due to insufficient sulfate levels after cement replacement, emphasizing the need for sulfate adjustment in practical applications. • Deviations from linear trends with kaolinite content suggest that other mineralogical phases (e.g., iron oxides, quartz) strongly influence both hydration behavior and fresh-state performance.