Rebuilding using outdated methods and tearing down the buildings would have a negative impact on the environment without lowering carbon dioxide emissions or increasing sustainability. This study presents a novel approach to repair that considers environmental and sustainable factors. In contrast to conventional repair methods, the use of Bacillus subtilis as an external biological repair technique could offer a novel and sustainable solution, especially when used on alkali-activated slag (AAS) concrete. By breaking down urea into carbonate and ammonium, alkaliphile bacteria can precipitate calcium carbonate. In an environment rich in calcium, the bacteria’s opposing cell wall (CO2−3
) draws in positive calcium anions, which result in the formation of calcite crystals. The pores and crevices in the concrete are filled with these crystals. Incorporating bacteria into the fresh mixing of AAS ingredients is contrasted with using Bacillus subtilis culture in the water curing medium for pure AAS specimens. The effectiveness of both approaches was evaluated. Direct administration of Bacillus subtilis during mixing has a superior outcome regarding mechanical qualities rather than biological therapy, although their effective healing capability in closure of the crack width is similar. The enhancement in compressive and flexural strengths reached 51% and 128% over the control specimens. On the other hand, the healing rate reached nearly 100% for crack widths ranging from 400 to 950 µm. Furthermore, additional studies in this field led to some inferred correlations between the mechanical and durability aspects following healing.