Microstructural and physicochemical characterization, mechanical and durability performance with statistical evaluation of montmorillonite-impregnated burnt red soil bricks incorporating pyrolyzed coffee and tobacco biochars

The growing demand for sustainable construction materials has increased interest in utilizing agricultural waste as functional additives in fired clay products. However, limited information is available regarding the influence of pyrolysis temperature on the engineering performance, durability, and physicochemical characteristics of biomass-modified burnt red soil bricks. This study investigated the effects of pyrolyzed coffee grounds (PCG) and pyrolyzed tobacco grounds (PTG) produced at 300°C, 350°C, and 500°C and incorporated at replacement levels of 5%, 10%, 15%, and 20% into burnt red soil bricks containing 10% montmorillonite. Mechanical performance was evaluated through compressive, tensile, and flexural strength tests, while durability was assessed using water, chloride, and sulphur penetration measurements. Microstructural and physicochemical characterization was conducted using FTIR, XRD, DSC, Raman spectroscopy, and SEM analyses. Statistical significance was evaluated using analysis of variance (ANOVA) followed by Tukey’s honestly significant difference (HSD) post-hoc test at a 95% confidence level. The results demonstrated that biomass type, pyrolysis temperature, and replacement level significantly influenced both mechanical and durability properties (p < 0.05). Among all investigated mixtures, PCG350−10 exhibited the optimum performance, achieving compressive, tensile, and flexural strengths of 11.8 MPa, 2.8 MPa, and 3.9 MPa, respectively, compared with 8.5 MPa, 1.5 MPa, and 2.2 MPa for the control specimen. Similarly, water, chloride, and sulphur penetration depths decreased from 38 mm, 32 mm, and 35 mm in the control brick to 12 mm, 10 mm, and 11 mm, respectively, representing reductions of approximately 68–69%. Tukey HSD analysis confirmed that PCG350−10 was statistically superior to the control and most alternative formulations. Characterization results revealed progressive thermal transformation of biomass constituents with increasing pyrolysis temperature and indicated that the 350°C condition produced a favorable balance between carbon stabilization, structural integrity, and pore refinement. SEM observations further demonstrated a denser and more homogeneous morphology for the optimum mixtures compared with the control and high-replacement specimens. Overall, the findings demonstrate that incorporating 10% pyrolyzed coffee grounds produced at 350°C significantly enhances the mechanical performance and durability of montmorillonite-modified burnt red soil bricks while providing a sustainable pathway for agricultural waste valorization and resource-efficient construction materials.