Microbially induced calcite precipitation by a novel alkaliphilic Bacillus albus strain for sustainable self-healing bio-mortar with enhanced mechanical performance and durability

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Abstract Micro-crack formation is a major factor limiting the durability of concrete structures. This study investigates a sustainable self-healing approach based on microbially induced calcium carbonate precipitation (MICP). Fifty bacterial isolates were obtained from alkaline soils in Wadi El-Natrun, Egypt, and screened for their ability to precipitate calcium carbonate. Isolate code W39 exhibited the highest activity, producing 0.453 g/100 mL of CaCO₃. Molecular identification by 16 S rRNA sequencing confirmed the strain as Bacillus albus (Accession number: PQ288981). Optimal precipitation occurred at pH 8, with 25 mM CaCl₂ and 20 g/L urea, after seven days of incubation at 30 °C. Instrumental analyses, including scanning electron microscopy (SEM) coupled with an energy-dispersive X-ray (EDX) analyzer, high-resolution transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), and the N₂ desorption/adsorption isotherm (BET) method, verified that the biogenic CaCO₃ consisted of nanoscale, high-purity calcite. The self-healing potential was evaluated by embedding viable Bacillus albus cells in cement mortar at optical densities (OD₆₀₀) of 0.5, 1.0, and 1.5, and curing in urea-enriched media with varying CaCl₂ concentrations (25–100 mM). Bacterial incorporation significantly enhanced mechanical performance over a 90-day curing period. The optimal dosage depended on Ca²⁺ availability: OD₆₀₀ 1.5 provided the greatest improvement at 25 mM CaCl₂, whereas OD₆₀₀ 0.5 was more effective at 50–100 mM. Microstructural analyses, comprising x-ray diffraction (XRD), differential thermal/thermogravimetric analysis (DTG/TGA), and scanning electron microscopy (SEM), confirmed the formation of additional C–S–H and calcite, resulting in reduced porosity and a denser matrix. The bio-mortar also demonstrated increased durability, including resistance to exposure to MgSO₄ and MgCl₂, as well as thermal stability up to 1000 °C. These findings demonstrate that the indigenous Bacillus albus strain is a promising agent for durable, self-healing bio-concrete. Overall, these results show that microbial incorporation improves the pore system, encourages the formation of more hydrates and CaCO 3 , and creates a more resilient mortar that can withstand extreme thermal exposure.

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