Influence of CO2-pressure and moisture content of concrete on the pore structure of concrete during carbonation

碳化过程中CO2压力和混凝土含水率对混凝土孔结构的影响

• 批准号: 221646279
• 负责人: Professor Dr.-Ing. Christoph Gehlen
• 金额: --
• 依托单位: Centrum Baustoffe und Materialprüfung (cbm)
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2012
• 资助国家: 德国
• 起止时间: 2011-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/221646279?language=en
• 关键词: Influence; CO2; pressure; moisture; content

The resistance of concrete to carbonation is a decisive parameter with respect to damage to reinforced concrete caused by carbonation-induced reinforcement corrosion. Owing to higher atmospheric CO2 concentrations, the carbonation rate of concrete has, in theory, increased by an estimated 14% since 1960. At present, the carbonation resistance of concrete is determined in rapid tests with high CO2 concentrations which, according to current expert opinion, certainly do not correctly represent field conditions. The experimental results of the first funding period show that CO2 concentrations ≥ 4 vol.% lead to pronounced changes in pore structure and phase composition which were not observed after three years’ natural carbonation. On the other hand, the application of CO2 pressure changed the concrete microstructure far less while efficiently accelerating the carbonation process. In addition, moisture content profiles determined with 1H NMR showed that the formation of water during accelerated carbonation significantly affects the carbonation process. Thus water formation hinders carbonation less in specimens with lower portlandite contents and therefore binding capacities (e.g. CEM III).Whereas the penetration of CO2 by diffusion has been extensively studied, there is a need for research on permeation and its effect on the carbonation reactions. The use of CO2 gas pressure to assess of the carbonation behaviour of cementitious building materials requires investigation and deeper understanding of the mechanisms. To achieve this, cement type and w/c ratio, CO2 pressure and concentration as well as the humidity conditions are systematically varied. Precise conditions for the experiments are obtained by automatically controlling relative humidity and CO2 concentration. Thin mortar disks, mortar and concrete cylinders are stored in various CO2/N2 gas mixtures while varying gas pressure (0 to 10 bar) and its duration (a few hours up to 14 days) in a cyclic manner. Changes in the distribution of water in the surface region (1H-NMR), the formation and dissolution of mineral phases (XRD, TGA, Al-/Si-NMR) and pore structure (MIP) are observed. The ends of mortar and concrete cylinders with different moisture contents are exposed to CO2 and spatial distributions of water, phases as well as the depth of carbonation and surface air permeability determined as a function of time. Thermodynamic model calculations are used to help interpret the results. Finally, recommendations are made for an accelerated test for a realistic determination of carbonation resistance.

混凝土的抗碳化能力是碳化引起的钢筋腐蚀对钢筋混凝土造成损害的决定性参数。由于大气中二氧化碳浓度较高，理论上，自 1960 年以来，混凝土的碳化率估计增加了 14%。目前，混凝土的抗碳化性是通过高二氧化碳浓度的快速测试来确定的，根据目前的专家意见，这肯定不能正确代表现场条件。第一个资助期的实验结果表明，CO2浓度≥4vol.%会导致孔隙结构和相组成发生明显变化，而三年自然碳酸化后未观察到这种变化。另一方面，施加二氧化碳压力对混凝土微观结构的改变要小得多，同时有效地加速了碳化过程。此外，1H NMR 测定的水分含量曲线表明，加速碳酸化过程中水的形成显着影响碳酸化过程。因此，在硅钙石含量较低且结合能力较低的样品（例如 CEM III）中，水的形成对碳酸化的阻碍较小。虽然 CO2 通过扩散的渗透已被广泛研究，但仍需要研究渗透及其对碳酸化反应的影响。使用二氧化碳气压来评估水泥建筑材料的碳化行为需要对其机制进行研究和更深入的了解。为了实现这一目标，系统地改变水泥类型和水灰比、二氧化碳压力和浓度以及湿度条件。通过自动控制相对湿度和CO2浓度获得精确的实验条件。薄砂浆盘、砂浆和混凝土圆柱体储存在各种 CO2/N2 气体混合物中，同时以循环方式改变气压（0 至 10 bar）及其持续时间（几小时至 14 天）。观察到表面区域水分布的变化（1H-NMR）、矿物相的形成和溶解（XRD、TGA、Al-/Si-NMR）和孔隙结构（MIP）。具有不同含水量的砂浆和混凝土圆柱体的端部暴露于二氧化碳中，水的空间分布、相以及碳化深度和表面透气性随时间而确定。热力学模型计算用于帮助解释结果。最后，提出了加速测试的建议，以实际测定抗碳化性。