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Author(s): Bianca J. Reeksting, Kevin Paine and Susanne Gebhard
Paper category: Proceedings
Book title: Proceedings of the Final Conference of RILEM 253-MCI Microorganisms-Cementitious Materials Interactions Volume II
Editor(s): Alexandra Bertron and Henk Jonkers
ISBN: 978-2-35158-207-7 (Set)
ISBN: 978-2-35158-210-7 (Volume 2)
e-ISBN: 978-2-35158-206-0
Publisher: RILEM Publications SARL
Publication year: 2018
Pages: 587-588
Total Pages: 308
Language : English

Abstract: The development of more sustainable, resilient infrastructure is needed to minimise the considerable environmental and economic impact currently associated with maintenance and repair of concrete. In particular, micro-cracks cause increased permeability of concrete, shortening the life-span and accelerating signs of deterioration. Although there is a broad range of water repellents and consolidants available for protection and remediation of cracks, microorganisms provide an environmentally friendly alternative that also addresses the shortcomings of more conventional treatments. Bacteria are capable of precipitating calcium carbonate within cracks, thus reducing the permeability of the concrete. However, there are limited studies on the effects that environmental factors common in concrete, such as low temperatures and high-salt conditions, may have on the ability of bacteria to precipitate calcium carbonate. Indeed, the choice of bacteria will be specific to the intended application, and there is thus a need to isolate and characterise new bacteria that demonstrate robust calcite precipitation under a variety of environmental conditions. In this study, new spore-forming, aerobic bacteria from environmental samples were isolated and screened to determine whether they were capable of microbial-induced carbonate precipitation. Environmental samples were collected from soils and cave scrapings. Pre-screening of isolates was based on their ability to grow in alkaline conditions on solid media. Over 70 initial isolates were obtained by preliminary screening and selection at a pH of 9. The selection of pH as an initial screening method was due to the requirement that bacteria survive and grow in the alkaline environment found in concrete (pH 9-14). A calcium source was included in the selection media in order to assess calcite precipitation by isolates. Calcite precipitation was found to be a common phenomenon amongst isolates, with the majority (> 90%) of isolates capable of calcite precipitation at high pH. Growth of these isolates was then assessed at high pH ranging from 9.5 to 11.5 on solid media. Thirty isolates were selected based on their alkaline tolerance on solid media, and optimum pH range was determined in liquid media. These 30 isolates were also subjected to 16S ribosomal DNA sequencing in order to assign a preliminary identity. Phylogenetic analysis indicated that the majority of isolates were Bacillus, with several clustering with well described species such as B. licheniformis, B. muralis, and B.cereus. Urease activity was determined using urea broth, and isolates were designated as ureolytic if a positive colour change reaction, from yellow to pink, was observed within 5 days. On solid calcium-containing media, ureolytic isolates were able to rapidly change the pH of their growth environment and facilitate precipitation of calcium over a large area, whilst non-ureolytic isolates showed a more localised precipitation on the bacterial colony without a significant change in pH. Isolates were further characterized by assessing their spore-forming capability using phase-contrast microscopy, with all isolates tested thus far forming spores in a standard sporulation medium. The impact of high salinity and temperature on bacterial growth and calcite precipitation capabilities were also assessed. High salt and low temperature were generally found to limit bacterial growth. The results obtained from this work will provide us with the information necessary to determine which isolate/s will be suitable to include in concrete. This will ensure robust calcite precipitation by bacteria that can survive and precipitate calcite in a wide range of adverse conditions, making them promising for application in self-healing building materials.

Online publication : 2018
Publication type : full_text
Public price (Euros) : 0.00

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