This study provides novel insights into enhancing the self-healing capacity of cement matrix through the integration of natural Bacillus isolates derived from leached and calcified soils. The challenging and highly alkaline environment of cement matrix typically impedes bacterial activity, making the successful application of these bacteria in such conditions particularly significant. In this research, Bacillus licheniformis-Bacillus muralis co-culture was identified as highly effective in inducing calcium carbonate precipitation, a critical factor for self-healing. The selected co-cultured bacterial activity resulted in the formation of up to 2.885 g/100 mL of CaCO3, while the co-culture's effectiveness was demonstrated by the complete repair of a 0.5 mm crack within 96 h demonstrating a repair rate of approximately 0.125 mm per 24 h. Furthermore, the study showed that the bacterial co-culture could survive and remain active under varying environmental conditions, including wet-dry cycles and extreme pH levels, which are typical of construction sites. This rapid crack closure, achieved without additional protective measures for the bacteria, marks a significant advancement in the application of microbial co-cultures for enhancing the durability of cement-based materials. The study also provides a detailed analysis of bacterial behavior under various environmental stresses typical of construction sites, highlighting the robustness and practical applicability of this biotechnological approach. As the long-term output, the obtained results represent a substantial advancement in the practical application of microbial co-cultures for self-healing effect of cement-based materials.

In the context of rapid urbanization and industrialization, subterranean engineering frequently encounters geotechnical challenges, particularly when dealing with weak soil layers, such as loose silty sand. These layers are problematic due to their poor permeability and low mechanical strength. Although cement-based solidification methods are prevalent for improving soil properties, they may prove inadequate under certain extreme conditions. This study explores the solidification efficacy of graphene oxide (GO) alone, and in conjunction with silica fume (SF), on silty sand by integrating varying proportions of GO and SF into cement-based composite materials, with a focus on assessing their influence on the impermeability and mechanical properties of the solidified soil. The findings revealed that the incorporation of GO alone markedly decreased the permeability coefficient and enhanced the early bending and compressive strength of the solidified soil. Optimal impermeability and mechanical performance were attained at a GO concentration of 0.06%, attributed to GO's high specific surface area and superior adsorption capacity, which effectively filled internal soil voids and ameliorated the microstructure. When GO and SF were added together, the solidified soil's performance improved, especially at an SF content of 10%. Notably, even with reduced GO content, a significant decrease in permeability coefficient was observed, indicating a synergistic effect between the materials. The concurrent addition of GO and SF also had a positive impact on bending and compressive strength, notably enhancing the early and intermediate mechanical performance of the solidified matrix. After a curing period of 28 days, the growth trends of bending and compressive strength decelerated. Microscopic examination indicated that GO and SF addition optimized the pore structure of the solidified soil, diminishing macropores and augmenting micropores, thereby reducing the permeability coefficient and bolstering impermeability. X-ray diffraction (XRD) analysis demonstrated that although the addition of GO and SF did not alter the primary hydration products in the solidified soil, it facilitated the cement hydration reaction, leading to increased formation of hydrated calcium silicate gels and other hydration products, thereby enhancing the compactness and mechanical strength of the solid matrix.