A novel bacterial strategy for self-healing mortars: the case of Bacillus thuringiensis

Abstract Self-healing mortars represent a promising innovation by enabling the autonomous sealing of cracks and microcracks, contributing to increased durability and extended service life of concrete structures. Among the different self-healing mechanisms reported in the literature, the use of Bacillus bacteria stands out due to their ability to precipitate calcium carbonate (CaCO₃) through biomineralization processes. Although Bacillus thuringiensis has previously been investigated in microbial self-healing systems, its behavior in cementitious matrices modified with highly reactive supplementary cementitious materials remains insufficiently understood. Therefore, this study investigates the development and performance of biological mortars incorporating Bacillus thuringiensis in combination with metakaolin and silica fume, aiming to assess the potential interaction between bacterial biomineralization and pozzolanic modification of the cementitious matrix. The methodology included the characterization of raw materials, the production of mortar specimens with different bacterial incorporation levels, and the use of pozzolanic additions, followed by the evaluation of physical, mechanical, and durability properties. The healing efficiency was assessed through digital microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDS). The results indicated that the composition containing 7.5 × 10 7 UFC/g of bacteria showed expressive CaCO₃ precipitation along cracks, promoting partial pore filling and enhanced microstructural densification, whereas the reference mortar exhibited no consolidation products. Quantitative analysis of crack closure further demonstrated the progressive development of self-healing, with the crack closure rate increasing from 72.22% after 21 days to 94.29% after 42 days for the 7.50 composition. Although complete crack closure was not achieved, the results demonstrate that bacterial activity significantly contributed to the sealing process and that the healing efficiency continued to increase with curing time. Overall, Bacillus thuringiensis represents a promising strategy for inducing self-healing in cementitious materials.