<p>This study investigates a sustainable approach for producing cement-free and self-hardening construction materials by upcycling brick dust and fly ash, which are commonly discarded industrial wastes. The materials were activated through Microbially Induced Calcium Carbonate Precipitation (MICP) using <i>Bacillus subtilis</i>. This process promoted in-situ calcium carbonate formation and improved particle bonding. Six types of specimens were prepared, including three untreated conventional and three bacteria-treated composites. The self-hardening behavior and performance of the materials were evaluated through compressive strength, permeability, water absorption, weight change analysis, HCl fizz tests, and microstructural observations over curing periods of 7, 14, and 28 days. The results showed that bacteria-treated specimens achieved higher compressive strength and exhibited substantial reductions in permeability and water absorption compared to untreated specimens. SEM analysis revealed reduced porosity and a denser matrix in the treated composites, consistent with bio-mineralization. Among the materials studied, fly ash demonstrated the most pronounced improvement, followed by brick dust and the mixed composite. The findings indicate that MICP can act as a potential binding mechanism in waste-based systems without the use of cement or synthetic binders. This work presents an eco-friendly pathway for waste valorization and supports circular economy practices by converting low-value industrial by-products into low-carbon construction materials.</p>