Improved strength and durability of fly ash-amended concrete by microbial calcite precipitation

Fly ash acts as a partial replacement material for both Portland cement and fine aggregate. An innovative approach of microbial calcite precipitation in fly ash-amended concrete has been investigated. This is the first report to discuss the role of microbial calcite precipitation in enhancing the durability of fly ash-amended concrete. The present study investigated the effects of Bacillus megaterium ATCC 14581 on compressive strength, water absorption and water impermeability of fly ash-amended mortar and concrete. Mortar specimens were used for compressive strength and water absorption tests, while concrete specimens were used for water impermeability tests. At the fly ash concentrations of 10%, 20% and 40% in mortars, bacterial cell enhanced mortar compressive strength by 19%, 14% and 10%, respectively, compared to control specimens. Treated mortar cubes absorbed more than three times less water than control cubes as a result of microbial calcite deposition. Microbial deposition of a layer of calcite on the surface of the concrete specimens resulted in substantial decrease of water uptake and permeability compared to control specimens without bacteria. Microbial cells also prevented ingress of water effectively in different concentrations of fly ash-amended concrete. Scanning Electron Micrography (SEM) analyses evidenced the direct involvement of bacteria in calcite precipitation. The approach of the present study gives us dual environment friendly advantages. First, use of fly ash-a recovered resource reduces depletion of natural resources and also reduces the energy-intensive manufacturing of other concrete ingredients, leading to savings in both energy usage and emissions of greenhouse gases. And second, use of bacterial cells to improve strength and durability of fly ash-amended concrete further provides greener and economic options.