Multiscale structural, optical, thermal, and mechanical characterization of microbial-induced calcium carbonate precipitation in bio-concrete

Calcium carbonate precipitation by microbes (MICP) is a promising, bio-mediated method for improving the durability and self-healing capacity of cementitious materials. The paper under study is a systematic investigation of the effect of Bacillus subtilis on the structural, optical, thermal, and mechanical properties of bio-modified concrete. The analysis using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) indicates enhanced calcite formation, increased crystallinity, and decreased vibrational disorder at 4% bacterial content in the concrete. Thermal analysis indicates greater stability for carbonates, with high calcium carbonate decomposition activity observed at approximately 850°C. Optical studies indicate clear modifications to defect states in the cement matrix. Photoluminescence (PL) spectra of both conventional and bio-modified concrete exhibit a shift between broad emission bands in the violet-blue (378–440 nm) to blue-shifted UV-violet emission (354–374 nm) in conventional and bio-modified concrete, respectively, with a shorter Stokes shift and a chromaticity shift toward the UV in CIE 1931 color space. These optical signatures are attributed to microstructural refinement and carbonate enrichment resulting from microbial mineralization. Mechanical testing also supports the positive effects of MICP; compressive and split tensile strengths increase by approximately 30% in the best modified mixtures. Comparison of structural, optical, thermal, and mechanical measurements shows that coordinated multiscale adjustments in the cementitious matrix are supported by microbial activity. The results reveal that optical spectroscopy is a complementary, non-destructive method for assessing microbial mineralization and indicate that MICP-based bioconcrete could be a long-lasting, sustainable construction material for environmentally friendly advanced infrastructure projects.