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

混凝土抗碳化性能是影响钢筋碳化腐蚀损伤的决定性因素。由于大气中二氧化碳浓度的升高，理论上，自1960年以来，混凝土的碳化率估计增加了14%。目前，混凝土抗碳化性能的测定是在高CO2浓度的快速试验中进行的，根据目前的专家意见，这肯定不能正确地代表现场情况。第一个资助期的实验结果表明，CO2浓度≥4 vol.%会导致孔隙结构和相组成的明显变化，而这种变化在自然碳化3年后没有观察到。另一方面，施加CO2压力对混凝土微观结构的影响很小，但能有效地加速碳化过程。此外，1H NMR测定的含水率曲线表明，加速碳酸化过程中水的形成显著影响了碳酸化过程。因此，在硅酸盐含量较低的样品中，水的形成对碳酸化的阻碍较小，因此具有较低的结合能力（例如CEM III）。CO2的扩散渗透已经得到了广泛的研究，但对渗透及其对碳酸化反应的影响还需要进一步的研究。使用二氧化碳气体压力来评估胶凝建筑材料的碳化行为需要调查和更深入地了解其机制。为了实现这一目标，水泥类型和w/c比、二氧化碳压力和浓度以及湿度条件都是系统地变化的。通过对相对湿度和CO2浓度的自动控制，获得了精确的实验条件。薄砂浆盘、砂浆和混凝土圆柱体以循环方式储存在各种CO2/N2气体混合物中，同时改变气体压力（0至10 bar）和持续时间（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）和孔隙结构（MIP）的变化。将不同含水率的砂浆和混凝土圆柱体端部暴露于CO2中，并确定水、相、碳化深度和表面透气性的空间分布作为时间的函数。热力学模型计算被用来帮助解释结果。最后，对加速试验提出了建议，以便实际地确定抗碳化性。

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.