Optimization Design of Self-Healing Recycled Concrete Mix Proportion Using Steel Slag–Immobilized <i>Bacillus licheniformis</i>

Microbial mineralization self-repair can effectively solve the problems of pores and cracks inside recycled concrete, thereby improving its mechanical properties. In order to reasonably solve the problem of bacteria’s long-term viability in the high alkaline environment of concrete, Bacillus licheniformis was selected as the mineralizing bacteria in this study, and its alkaline resistance was domesticated; at the same time, steel slag was used as the carrier material to protect the bacteria, and the addition of alumina was used to reduce the pH of the recycled concrete. In order to explore the optimal content of different additives, orthogonal tests were carried out on four factors: steel slag content, alumina content, calcium ion concentration, and urea concentration. The significance of the variables was assessed by analysis of variance (ANOVA) and analysis of range, and the optimal combination in the orthogonal test was determined using the Tukey method. The test results showed that the optimal mix design was at 10% steel slag dosing, Ca2+ concentration of 0.6 mol/L, Al2O3 dosing of 2%, and urea concentration of 0.9 mol/L, and the interactions between multiple factors were analyzed. On this basis, the repair effect and mechanical properties of recycled concrete under the optimal mix ratio are verified. Finally, the influence of Bacillus licheniformis particle-bound slag on the microstructure and micromechanical properties of recycled concrete was deeply analyzed through electron microscope scanning and nano-indentation testing, and its self-healing mechanism was revealed.