Calcite biomineralisation for the repair of damaged concrete

UK Civil Nuclear sites contain significant volumes of concrete infrastructure, including both external and internal structures. As a consequence, different concretes are exposed to differing environmental conditions, resulting in variable mechanisms and rates of concrete degradation. For example, external structures may be exposed to salt water and freeze-thaw cycles, while internal structures may be exposed to high temperatures and/or high levels of radiation. Key to minimising the degradation of concrete structures is the reduction of concrete permeability. High permeability permits the ingress of damaging chemical compounds such as sulfate, and permits carbonation which may damage steel-bearing reinforcements. Consequently, techniques to reduce permeability will improve durability of the concrete. Specifically, very low permeability concrete is highly desirable for radiation shielding structures, which must be impermeable to radioactively contaminated air and liquids. Microbially-induced calcite precipitation (MICP) may provide a low-cost, low-carbon method for the reduction of permeability in aged or damaged concrete infrastructure. The method used in this study relies upon the ureolytic capacities of the bacterial strain Sporosarcina pasteurii. We treat fractured concrete cores in the laboratory and show that our newly developed concrete treatment protocol successfully reduces hydraulic conductivity by at least 2 orders of magnitude in concrete samples collected from UK Civil Nuclear sites. We utilise XCT imaging to quantify and visualise the calcite deposited within the fracture network present in the concrete samples. Our research indicates this treatment protocol can significantly reduce concrete permeability and thus could be deployed to increase the longevity of degraded concrete nuclear assets.