Bio-mineralization process of CaCO₃ induced by bacteria isolated from Egypt for sustainable bio-concrete

Microbial-induced calcium carbonate precipitation (MICP) has garnered significant attention for its construction and geotechnical engineering applications. In this study, 24 bacterial isolates were obtained from various edges of Wadi El-Natron Lake, Egypt, and subsequently assessed for their capacity for calcium carbonate (CaCO₃) precipitation. Among these isolates, strain D16 exhibited the highest CaCO₃ precipitation, yielding 0.404 g/100 mL, alongside robust bacterial growth and a final pH of 9.09. Morphological and biochemical characterization revealed that the isolate was rod-shaped, Gram-positive, Catalase-positive, Urease-positive, and Spore-forming. The optimal growth conditions for the isolate included a pH of 8, with ideal Ca²⁺ and urea concentrations of 25 mM and 20 g/L, respectively, at an incubation temperature of 30 °C over seven days. Molecular identification confirmed the isolate as Bacillus tropicus strain D16, which has been recorded in GenBank under the accession number PQ817131. The precipitated CaCO₃ was quantified and characterized using scanning electron microscopy (SEM) equipped with energy-dispersive X-ray (EDX) analysis, Fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), and the N₂ desorption/adsorption isotherm (BET) method. The effect of calcium carbonate nanoparticles (CaCO₃-NPs, denoted as NC) on the properties of cement paste was investigated. Four composite pastes were prepared with varying dosages of CaCO₃-NPs: NC0.0, NC0.5, NC1.0, and NC1.5. These pastes were subjected to a series of tests, including compressive strength, bulk density, total porosity, and chemically combined water content, over a hydration period of up to 90 days. The results demonstrated that the addition of NC enhanced the compressive strength of the cement paste up to an optimal dosage content of 0.5%, beyond which the strength decreased due to nanoparticle agglomeration. These findings were further corroborated by X-ray Diffraction (XRD), Differential Thermal Thermogravimetric Analysis (DTG/TGA), and Scanning Electron Microscopy (SEM), which provided microstructural and phase composition insights. Overall, the results indicate that the inclusion of an optimal dosage of CaCO₃-NPs can significantly improve the performance of cement composite pastes.