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

混凝土的抗碳化能力是一个决定性的参数，相对于碳化引起的钢筋腐蚀所造成的钢筋混凝土损坏。由于大气中二氧化碳浓度较高，混凝土的碳化率自1960年以来理论上估计增加了14%。目前，混凝土的抗碳化性是在高CO2浓度的快速测试中确定的，根据目前的专家意见，这当然不能正确地代表现场条件。第一个资助期的实验结果显示，二氧化碳浓度≥ 4体积%导致孔隙结构和相组成的显著变化，这在三年的自然碳酸化之后没有观察到。另一方面，CO2压力的应用改变混凝土微观结构少得多，同时有效地加速碳化过程。此外，用1H NMR测定的水分含量曲线表明，在加速碳酸化过程中水的形成显著影响碳酸化过程。因此，水的形成阻碍碳酸化与较低的氢氧化钙含量，因此结合能力（如CEM III）的标本较少。而通过扩散的CO2渗透已被广泛研究，有必要对渗透及其对碳酸化反应的影响进行研究。使用CO2气体压力来评估水泥基建筑材料的碳化行为需要调查和更深入地了解其机理。为了实现这一点，水泥类型和水/碳比，二氧化碳压力和浓度以及湿度条件系统地变化。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体压力（0至10 bar）及其持续时间（几小时至14天）。观察了水在表面区域的分布（1H-NMR）、矿物相的形成和溶解（XRD、TGA、Al-/Si-NMR）以及孔结构（MIP）的变化。具有不同含水量的砂浆和混凝土圆柱体的端部暴露于CO2和水的空间分布，相以及碳化深度和表面空气渗透率确定为时间的函数。热力学模型计算用于帮助解释结果。最后，提出了一个现实的测定抗碳化加速试验的建议。

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.