Nanoscale insights: unraveling fungal-induced precipitation of CaCO3 polymorphs for self-healing concrete

Routine methods of concrete production contribute 9% to anthropogenic CO₂ emissions and demands 2-3% energy, along with 9% water consumption. Despite these environmental costs, concrete structures frequently undergo deterioration due to unavoidable physical, chemical, and biochemical stressors, resulting in cracks that permit gas diffusion, water, and pollutants penetration, ultimately compromising its integrity and internal steel reinforcement. Microbially induced CaCO₃ precipitation has emerged as a sustainable way of concrete protection and self-healing. However, the detailed mechanisms and formation of various CaCO₃ polymorphs remain inadequately explored.

In this ongoing study, samples were prepared by inoculating a growth medium, containing urea and nutrients, with different fungi under diverse growth conditions. High-resolution Scanning Transmission X-ray Microscopy (STXM) in the Ca 2p energy range (340−360 eV) were employed to investigate the fungal-induced formation and chemical speciation of CaCO₃ at the cellular base or interface between hypha and the surrounding ions. To discriminate potential absorption saturation effects, only spectra (NEXAFS) extracted from thin regions (≈ 30 nm) of the entire sample thickness were considered for spectral analysis. Furthermore, SEM with EDS was used to reveal morphology and elemental distribution, and composition in studied sample thin sections.

The preliminary results suggest that the samples spectra resembled those of pure calcite and aragonite, according to reference spectra. These are the most stable CaCO₃ biomaterials. Notably, the intensity of weak peaks preceding each main resonance peak of the Ca L3 and Ca L2 edges were relatively smaller for aragonite-dominated spots than in calcite-dominated spots. As revealed by the spectral analysis, some fungi showed the ability to form CaCO₃, predominantly in the form of either calcite or aragonite. Other fungal strains demonstrated a more heterogeneous precipitation behavior by forming both phases, albeit in distinct nano spots within the same sample. Furthermore, a few fungal species exhibited the ability to precipitate other crystalline Ca minerals, most likely CaPO₄, as shown by SEM/EDS analyses.

In conclusion, the results of this ongoing investigation provided not only valuable insight on distinctive fungal behaviors in the biomineralization process, but also revealed spatial nanoscale heterogeneity in CaCO₃ speciation under the same fungal conditions.