Microstructure and Phase Characterization of Carbonation Accelerated Concretes Proposed for Deep Geologic Repositories using Simultaneous Synchrotron X-Ray Computed Microtomography (XCT) and Spatially Resolved Energy Dispersive X-Ray Diffraction (EDX

Long-term storage and eventual disposal of spent nuclear fuel (SNF) is a challenging materials problem, with deep geologic repositories (DGR) widely considered the best solution. The general design of DGRs is widely agreed upon, requiring an engineered barrier system (EBS) and disposal site-specific local geology to contain SNF on geologic timescales of 100,000 – 1M years, but specific details are less certain. An understanding of the stability of the materials used in the EBS in geologic conditions is crucial in a once-through nuclear cycle, such as in the United States, where SNF disposal needs are greatest. Cement and cementitious barriers are a key component in EBS systems and serve as a barrier between steel casques that contain the SNF and the local geologic environment its embedded in, creating separation of the waste from the biosphere by trapping radionucleotides that escape the primary containment particularly in the first 10,000 years of storage. There are two key issues in this field of study. First, projecting the stability of cementitious materials over geologic timescales by developing an accelerated aging model that is reliable. Second, characterization techniques of corrosion mechanism in concrete, particularly specific to DGR conditions such as high pressure, high temperature, temperature cycling, radiolysis, chloride ion attack, entrapped gasses (methane, carbon dioxide), and interfacial corrosion between the steel casque and cementitious barrier. This work details a preliminary approach addressing both concerns, an accelerated aging protocol of cements in carbonating conditions and simultaneous synchrotron methods to nondestructively characterize carbonation effects on cement phase and microstructure.<br/><br/>To develop aging techniques, representative cement cylinders were aged in 100% CO₂ atmosphere at room and elevated temperatures for varying durations of time. The pore structure and carbonation front progression were studied using x-ray computed microtomography (XCT) finding a clear delineation between the carbonated and uncarbonated regions. Additionally, spatially resolved energy dispersive x-ray diffraction (EDX-XRD) was used to identify phase changes to the cement’s alkaline portlandite buffer, a key component of the cement’s barrier capabilities to radionucleotides, as it was depleted with the dissolution of CO₂ into the cement's pore solution during the carbonation process. These phase changes showed clear signatures in the EDX-XRD and corroborated the carbonation depth found from the XCT. Both XCT and EDX-XRD were measured at Sector 7-BM at the Advanced Photon Source. The kinetics and near-field chemistry of concrete and cementitious materials has many unanswered questions as it’s difficult to model and quantitatively measure, especially under conditions expected in DGRs. This work has shown the ability to study concrete subjected to DGR conditions and measure one of the key aspects in its usefulness as a ESB material.