Impact of meta-stable chloride hydrate phases on concrete durability at near-freezing temperatures

混凝土路面的冬季养护实践在过去十年中发生了重大变化。基于传统岩盐（NaCl）的除冰/防冰策略已经扩展到包括基于氯化镁、氯化钙和乙酸钾的替代化学品，仅举几例。这些新化学品和新应用方法对混凝土基础设施耐久性的影响尚未完全了解。该研究计划的目标是了解混凝土降解机制，该机制归因于在接近0°C的温度下发生的二次氯化物相的膨胀沉淀。在接近水的冰点的温度下，复杂的氯氧化钙（Ca-氧）水合物相的形成已被证明会导致混凝土的膨胀和损坏，但这些相在室温和湿度下的不稳定性阻碍了进一步的研究。该研究计划结合了实验室方法和暴露于氯离子溶液和冻融（F-T）环境的混凝土的探索性真实世界采样。在实验室部分，将在混凝土混合物基质上进行仔细控制的恒定浸泡和蒸发前沿暴露实验，并结合定期取样，通过质量和长度变化测量、强度测试、扫描电子显微镜能量色散X射线光谱法（SEM/EDS）、X射线微荧光（µXRF）、X射线计算机断层扫描（CT）和X射线衍射（XRD）技术。此外，钙氧水合物将被合成和表征，以确定其晶体学和热力学性质。为配合实验室研究，将确定一系列现场，并对混凝土进行取样和回收，以保持现场温度和湿度条件，以便进行XRD分析，特别是保存钙氧水合物相。收集的信息将用于帮助将这些次生相的形成整合到现有的模型中，以预测氯化物渗透和混凝土降解。

Winter maintenance practices for concrete pavements have undergone major changes within the last ten years. De-icing/anti-icing strategies based on traditional rock salt (NaCl) have expanded to include alternative chemicals based on magnesium chloride, calcium chloride, and potassium acetate, to name a few. The impact of these new chemicals and new application methodologies on the durability of concrete infrastructure is not fully understood. The goal of the research program is to understand concrete degradation mechanisms attributed to the expansive precipitation of secondary chloride phases that occur at temperatures near 0°C. At temperatures near the freezing point of water the formation of complex calcium oxychloride (Ca-oxy) hydrate phases has been shown to cause expansion and damage in concrete, but the instability of these phases at room temperature and humidity has hampered further study. This research program incorporates both an experimental laboratory approach, and the exploratory real-world sampling of concrete exposed to chloride ion solutions and freeze-thaw (F-T) environments. In the laboratory portion, carefully controlled constant-immersion and evaporative-front exposure experiments will be conducted on a matrix of concrete mixtures, and coupled with periodic sampling to assess the development of secondary minerals and damage through a combination of mass and length change measurements, strength testing, combined scanning electron microscopy energy dispersive x-ray spectrometry (SEM/EDS), x-ray micro-fluorescence (µXRF), x-ray computed tomography (CT), and x-ray diffraction (XRD) techniques. In addition, Ca-oxy hydrate will be synthesized and characterized to determine its crystallographic and thermodynamic properties. To compliment the laboratory study, a series of field sites will be identified and the concrete sampled and recovered in such a manner as to maintain field temperature and humidity conditions for XRD analysis, particularly for the preservation of Ca-oxy hydrate phases. The information collected will be used to help integrate the formation of these secondary phases into existing models for the prediction of chloride penetration and concrete degradation.