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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. Degradation can take the form of spalling, mineral dissolution and ultimately, cracking. These mechanisms increase concrete permeability, which can lead to two significant issues for decommissioning operations. First, cracking of the concrete increases water ingress, accelerating rebar corrosion and reducing the structural integrity of the buildings. This increases the complexity of decommissioning operations, which frequently involve the use of heavy plant, particularly for waste retrieval operations. Second, cracking of concrete waste containment structures can lead to contamination of the subsurface, increasing the complexity of environmental remediation operations. Microbially-induced calcite precipitation (MICP) may provide a low-cost, durable method for the reduction of permeability in aged or damaged concrete infrastructure. The method relies upon the ureolytic capacities of the bacterial strain Sporosarcina pasteurii to metabolise urea in the presence of calcium, resulting in precipitation of calcium carbonate. Bacteria are injected into the fractured concrete, alongside urea and calcium in solution, and layers of calcium carbonate are precipitated. In the research presented here, we treat fractured concrete cores in the laboratory, reducing the hydraulic conductivity by 2-3 orders of magnitude in concrete samples collected from UK Civil Nuclear sites. We then utilise X-CT imaging to quantify and visualise the calcite deposited within the fracture network. Our research indicates this treatment protocol can significantly reduce concrete permeability as well as improving structural integrity, thus increasing the longevity of degraded concrete nuclear assets.

Bacterial biomineralization provides a greener solution to a diverse range of civil engineering applications, reducing the carbon footprint of construction. Most successful to-date is microbially induced carbonate precipitation (MICP). MICP is being developed for a diverse range of applications including plugging fractures, stabilising soils and repairing building surfaces. In MICP, bacteria are injected within the soil, followed by injection of a fluid containing urea and calcium chloride. The bacteria hydrolyse the urea, which generates bicarbonate ion, increases pH and precipitates calcite. Advantages of MICP include: better penetration due to the low viscosity of the fluids, the ability to maintain free drainage in treated materials and the thousand-year durability of calcite. Despite the growing interest in MICP for geotechnical applications, MICP still remains largely confined to the laboratory with only a very small number of large-scale experiments having been completed and one commercial project. Use is constrained by (1) the ability to grow bacteria at a commercial scale and (2) the need to ensure homogeneous soil strengthening at a large-scale. In this research, conducted in collaboration with BAM Nuttall, we present the results of large-scale test for MICP treated sand. We treat a 1 m diameter cylinder of loose sand using multiple cycles of MICP. Unconfined compressive strength tests and triaxial tests of cores taken from the treated sand result in strengths of several MPa. Our results show that bacterial biomineralization could be a viable, low carbon alternative to cement and concrete for a range of earth infrastructure applications.

The concept of using bacteria to control the precipitation of calcium carbonate for engineering purposes, such as increasing soil strength and decreasing permeability, is well established in lab-scale experiments. What is not so clear is how to transition from these experiments to a practical field-scale ground improvement tool. One difficulty is that soil properties are highly site specific and also vary spatially within a site (e.g. porosity, permeability, particle shape and size distribution, mineralogical composition). Meanwhile, microbially induced carbonate precipitation relies on complex interactions between pore structure, fluid flow pathways, and injection strategies which all influence where the injected bacteria will attach, where CaCO3 will precipitate, and which evolve over time as CaCO3 is progressively precipitated and the pore structure is altered. To unpick these processes and optimise MICP treatment, we use light microscopy in microfluidic devices offering high time-resolution observations of bacterial attachment and CaCO3 crystal nucleation and growth in 2D systems. This is followed by X-ray μCT of sand packed columns offering more complex and realistic flow conditions in which we observe the evolving pore structure and relate this to changes in the flow fields through reactive-transport modelling with the software OpenFOAM. Results show that the processes of bacterial attachment and crystal growth are complex and highly dependent on micro-scale conditions, however feedback mechanisms, repeated treatment cycles, and operator controlled parameters such as flow velocity can act to minimise these local variations across a range of soil types.

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.

Microbially induced carbonate precipitation is a promising technique for ground improvement. In order for MICP to progress from a lab-scale process to a commercially viable alternative ground improvement option, a combination of field-trials and field-scale modelling of the process is required. We present the results of a field-scale model in which differing injection strategies are evaluated and find that longer treatment times make more efficient use of reagents, but may come with higher operational costs, and that utilising multiple strains of bacteria with different reaction kinetics and transport properties may improve efficiency.

Microbially induced CaCO3 precipitation (MICP) has shown great potential to reduce permeability in intact rocks as a means to seal fluid pathways in subsurface ground, for example to secure waste storage repositories. However, much less is known about how to apply MICP to seal fractured rock. Furthermore, there is limited information on the hydraulic and mechanical properties of MICP filled fractures, which are essential criteria to assess seal performance. Here, MICP injection strategies were tested on sandstone cores, aimed at obtaining a homogeneous porosity fill that reduced permeability by 3 orders of magnitude. The injection strategy resulting in the most homogenous calcite distribution was then applied to fractured granite cores, to yield transmissivity reduction of up to 4 orders of magnitude. Microscale analysis of these sealed granite cores using X‐ray computed tomography and electron microscopy showed that > 67% of the fracture aperture was filled with calcite, with crystals growing from both fracture planes, and bridging the fracture aperture in several places. Shear strength tests performed on these cores showed that the peak shear strength correlated well with the percentage of the fracture area where calcite bridged the aperture. Notably, brittle failure occurred within the MICP grout, showing that the calcite crystals were strongly attached to the granite surface. If MICP fracture sealing strategies can be designed such that the majority of CaCO3 crystals bridge across the fracture aperture, then MICP has the potential to provide significant mechanical stability to the rock mass as well as forming a hydraulic barrier.