Microbial self-healing of cracks in 3D-printed cementitious materials: Use of recycled sand and bentonite as bio-carriers

A bacterial self-healing system for 3D printing of mortars to mitigate shrinkage cracking and improve the durability of printed elements is presented. In this study, the impacts of incorporating Bacillus subtilis (BS) cultures and calcium lactate (CL), optimizing their concentrations, and using biocarriers (natural sand (NS), recycled sand (RS) and bentonite (Ben)) for the biomineralization process were evaluated. The optimal system was subsequently implemented in mortars for 3D printing and evaluated under real exposure conditions (weathering). The results revealed that a bacterial concentration of 1 × 10 8 cells/ml and a CL content of 4% resulted in the greatest precipitation of CaCO 3 , achieving an adequate balance between metabolic activity and mechanical performance. The combination of RS and Ben as biocarriers significantly improved the bacterial survival and crack sealing efficiency rates because of their high porosity levels and nucleation capacities. The developed self-healing mortar (Mix-6–3D) exhibited adequate rheological properties for 3D printing, along with a reduction in porosity and increases in mechanical strength (compression and flexural strength). Additionally, the printed walls exposed to weathering (60 days) exhibited shrinkage cracks that were subsequently sealed significantly by biomineralization. Taken together, the results reveal the feasibility of integrating bacterial self-repair systems into cementitious mixtures for 3D printing processes, constituting an effective strategy for improving the durability of printed structures.