Strength and microstructural characteristics of conventional concrete incorporating rice husk ash and bacteria (Bacillus pasteurii and Bacillus subtilis)

To address the dual challenges of high cement costs and carbon emissions in the Nigerian construction industry, this study evaluates a sustainable high-performance concrete developed by combining agricultural waste (Rice Husk Ash, RHA) and bio-based self-healing agents. This synergistic approach uses the pozzolanic activity of RHA to densify the cement matrix while bacteria provide an autonomous mechanism to seal micro-cracks. The study investigated RHA replacements at 0%, 4%, 8%, and 12% by mass, combined with Bacillus pasteurii and Bacillus subtilis at dosages of 0–30 mL. Results indicated distinct optimal performances: the RHA08BS30 mix (8% RHA + 30 mL B. subtilis) achieved a peak compressive strength of 36.42 MPa, while RHA12BP20 (12% RHA + 20 mL B. pasteurii) reached 35.86 MPa. XRD revealed higher peak intensities for calcite in bacterial mixes, confirming biomineralization, while SEM showed improved pore densification. These findings demonstrate that optimizing bacterial species with specific RHA dosages significantly enhances mechanical properties and microstructural integrity, offering a scalable, sustainable pathway to reduce cement dependency.