Performance Evaluation of a Bio-Mortar Based on CEM III Slag Cement: The Role of Bacillus subtilis Dosage and Concentration

The inherent susceptibility of cementitious composites to cracking compromises their long-term durability, a challenge that persists even with the use of sustainable blended cements like CEM III. Microbial-Induced Calcite Precipitation (MICP) has emerged as a promising bio-inspired strategy for autonomous crack repair. This research provides a comprehensive evaluation of Bacillus subtilis as a healing agent in a CEM III slag cement mortar, aiming to define the optimal parameters for its application. The study systematically investigates the influence of two bacterial concentrations (10⁵ and 10⁷ CFU/mL) and three dosages (1%, 2%, and 3% by binder weight) on the mortar's mechanical strength (compressive and tensile), durability (acid and sulfate resistance), and microstructure. The key findings identify an optimal formulation of 1% bacterial dosage at a 10⁵ CFU/mL concentration, which achieved significant enhancements in mechanical performance, including a 12% increase in 28-day compressive strength and a 10.7% increase in splitting tensile strength compared to the control. Furthermore, this formulation exhibited exceptional durability under both acid and sulfate exposure. Microstructural characterization via SEM and XRD provided direct evidence that the performance enhancement is attributed to the effective bio-mineralization of calcite, which densified the matrix by filling pores and micro-cracks. The study concludes that the controlled incorporation of Bacillus subtilis is a highly effective strategy for developing sustainable, high-performance construction materials, highlighting that a lower bacterial concentration is more efficient and providing a clear pathway for the practical application of this self-healing technology.