Isolation of alkaliphilic calcifying bacteria and their feasibility for enhanced CaCO3 precipitation in bio-based cementitious composites.

Microbially induced calcite precipitation (MICP), secreted through biological metabolic activity, secured an imperative position in remedial measures within the construction industry subsequent to ecological, environmental and economical returns. However, this contemporary recurrent healing system is susceptible to microbial depletion in the highly alkaline cementitious environment. Therefore, researchers are probing for alkali resistant calcifying microbes. In the present study, alkaliphilic microbes were isolated from different soil sources and screened for probable CaCO3 precipitation. Non-ureolytic pathway (oxidation of organic carbon) was adopted for calcite precipitation to eliminate the production of toxic ammonia. For this purpose, calcium lactate Ca(C3 H5 O3 )2 and calcium acetate Ca(CH3 COO)2 were used as CaCO3 precipitation precursors. The quantification protocol for precipitated CaCO3 was established to select potent microbial species for implementation in the alkaline cementitious systems as more than 50% of isolates were able to precipitate CaCO3 . Results suggested 80% of potent calcifying strains isolated in this study, portrayed higher calcite precipitation at pH 10 when compared to pH 7. Ten superlative morphologically distinct isolates capable of CaCO3 production were identified by 16SrRNA sequencing. Sequenced microbes were identified as species of Bacillus, Arthrobacter, Planococcus, Chryseomicrobium and Corynebacterium. Further, microstructure of precipitated CaCO3 was inspected through scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermal gravimetric (TG) analysis. Then, the selected microbes were investigated in the cementitious mortar to rule out any detrimental effects on mechanical properties. These strains showed maximum of 36% increase in compressive strength and 96% increase in flexural strength. Bacillus, Arthrobacter, Corynebacterium and Planococcus genera have been reported as CaCO3 producers but isolated strains have not yet been investigated in conjunction with cementitious mortar. Moreover, species of Chryseomicrobium and Glutamicibacter were reported first time as calcifying strains.

Bio-mineralized self-healing recycled aggregate concrete for sustainable infrastructure.

Various carriers have been investigated by researchers to introduce bacteria inside the concrete however, factors such as local availability, cost and long-term protection of bacterial cells have barred the application of this contemporary technology in the construction industry. In the present study, bacteria were immobilized via recycled coarse aggregate (RCA) and virgin fine aggregate (FA) besides direct induction to preserve natural resources and emulate sustainability. The application of RCA in substitution of virgin coarse aggregate is dropping anthropogenic emissions, minimizing energy consumption and managing construction waste effectively. Vegetative cells of Bacillus subtilis bacterium were incorporated in RCA through vacuum impregnation to boost crack healing efficiency. Crack healing efficiency was studied by quantifying the crack healing widths and percentage of strength regained after pre-cracking at 3,7 and 28 days. Similarly, mechanical properties were gauged via compressive and split tensile strengths at specified intervals while healing precipitate was characterized using analytical means. Results of experimental work revealed that specimens having RCA and 50% virgin FA as bacteria immobilizers exhibited the most efficient crack healing remedy by healing crack widths up to 1.1 mm and recovering 85% of compressive strength. Specimens containing RCA exclusively displayed a maximum of 0.7 mm crack healing widths and 76% strength recovery while direct incorporation of bacteria lagged behind with 0.6 mm crack healing width having 69% strength recovery. Likewise, synergetic formulation and direct induction depicted increase in compressive strength of 4% and 6% respectively while exclusive RCA formulation decreased the compressive strength up to 3% Moreover, field-emission scanning electron microscopy (FE-SEM), thermo-gravimetric analysis (TGA), X-ray diffraction (XRD) and X-ray fluorescent (XRF) characterized the crack healing precipitate as bio-mineralized calcite crystals.