Sustainable bio-foamed concrete bricks: optimising compressive strength with rice husk ash, coal bottom ash, and Bacillus tequilensis

The production of sustainable bio-foamed concrete brick (B-FCB) by incorporating a microorganism in the presence of sustainable materials has emerged as advanced step in bio-concrete. The study aimed to optimise the compressive strength of B-FCB incorporating Bacillus tequilensis (B. tequilensis) and alternative materials like rice husk ash (RHA) as cement substitute and coal bottom ash (CBA) as sand substitute. The study employed full factorial analysis to explore the relationship among bacteria concentration, RHA and CBA in B-FCB with density of 1800 kg/m3 and its effects on the compressive strength. The findings revealed that the suggested B-FC recorded high workability compared to control FC. The optimum compressive strength was achieved with B. tequilensis concentration of 3 × 105 cell/ml, 10 RHA, and 10 CBA with a 60 increase compared to control FCB. Meantime, increasing CBA to 50 under the same condition resulted in a 25 increase in the compressive strength. Increasing RHA to 50 under different ratios of CBA and B. tequilensis resulted in a significant reduction of the compressive strength by 67–77. Moreover, using B. tequilensis along with RHA and CBA did not enhance the compressive strength. ANOVA results illustrated that the RHA and CBA*B. tequilensis interaction significantly influenced the compressive strength. The microstructure analysis showed that the healing process of the specimen’s pores occurred through the mechanism of crystallinity formation of calcium carbonate (CaCO3) for B-FCB containing 10 RHA and 10 CBA. High ratio of CBA has a negative effect on the activity of B. tequilensis. The results indicated that using 10 RHA and 10–50 CBA in B-FCB enhances the compressive strength. Additionally, utilising sustainable materials in B-FCB contributes to the production of environmentally friendly bricks. © 2025 Informa UK Limited, trading as Taylor & Francis Group.