Mechanical, durability, and thermal performance of concrete incorporating coffee biochar and raw and calcined montmorillonite

Introduction Montmorillonite is a natural aluminosilicate clay with potential as a supplementary cementitious material, although its reactivity in the raw state is limited. This study investigates the effect of raw and calcined montmorillonite on the performance of concrete incorporating 15% pyrolyzed coffee grounds (PCG) at 350 °C. Methods Montmorillonite calcined at 400, 600, and 800 °C replaced cement at levels of 5%–20%. Mechanical and durability properties were evaluated under acidic, saline, and thermal exposure conditions. Microstructural characterization was conducted using FTIR, XRD, and SEM, and statistical validation was performed using two-way ANOVA. Results Calcination enhanced montmorillonite reactivity through amorphization and pozzolanic reactions, resulting in improved pore refinement and matrix densification. Specimens with calcined montmorillonite at 600 °C–800 °C showed superior strength and durability performance. Discussion The combined use of calcined montmorillonite and 15% PCG biochar at 350 °C provides a sustainable approach for improving concrete performance.