Abstract
Characterization of alkaliphilic Bacillus species for spore production and germination and calcite formation as a prelude to investigate their potential in microcrack remediation in concrete. Conditions, extent and timing of endospore production was determined by dark-field light microscopy; germination induction and kinetics were assessed by combining reduction in optical density with formation of refractile bodies by phase-contrast microscopy. Bacillus pseudofirmus was selected from several species as the most suitable isolate. Levels and timing of calcium carbonate precipitated invitro by B. pseudofirmus were evaluated by atomic absorption spectroscopy and structural identity confirmed as calcite and aragonite by Raman spectroscopy and FTIR. The isolate produced copious spores that germinated rapidly in the presence of germinants l-alanine, inosine and NaCl. Bacterial cells produced CaCO3 crystals in microcracks and the resulting occlusion markedly restricted water ingress. By virtue of rapid spore production and germination, calcium carbonate formation invitro and insitu, leading to sealing of microcracks, B. pseudofirmus shows clear potential for remediation of concrete on a commercial scale. Microbial sealing of microcracks should become a practicable and sustainable means of increasing concrete durability.
Highlights
Metabolic activities in microbes yield insoluble organic and inorganic materials, intra- or extra-cellularly (Lappin-Scott et al 1988)
Often for the first time in the context of bacteria used in concrete remediation, selection from three alkaline-tolerant Bacillus spp., evaluation of speed and extent of endospore formation, triggers for spore germination, ability of spores to survive in concrete, and the kinetics, amounts and structural confirmation of calcite production
Sporulation in B. pseudofirmus was detectable at 20h and apparently 100% of cells viewed by phase contrast microscopy contained endospores by 26 h, at which stage divisional growth had ceased (Fig. 1 a, b)
Summary
Metabolic activities in microbes yield insoluble organic and inorganic materials, intra- or extra-cellularly (Lappin-Scott et al 1988). The processes that lead to production by living organisms of inorganic material such as phosphorites, carbonates, silicates and iron and manganese oxides in the form of shells, skeletons and teeth are termed bio-mineralization (Beveridge et al 1983; Ghiorse 1984). Biocalcification involves precipitation of calcium carbonate polymorphs and occurs commonly in soil, fresh water and marine environments (Shirakawa et al 2011). Research into and potential application of bio-calcification has included restoration of limestone on historic monuments and ornamental stone (Rodriguez-Navarro et al 2003), soil stabilization and microbiologically enhanced crack repair (De Muynk et al 2010). The methods mimic what has been occurring naturally, since many carbonate rocks have been cemented over eons by microbeinduced calcium carbonate precipitation (Rodriguez-Navarro et al 2003)
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