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

混凝土的抗碳化性能是碳化引起的钢筋锈蚀对钢筋混凝土损伤的决定性参数。由于大气中二氧化碳浓度较高，自1960年以来，混凝土的碳化速率理论上增加了约14%。目前，混凝土的抗碳化能力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，这肯定不能正确地反映现场条件。第一个注资期的实验结果表明，二氧化碳浓度≥4vol.%导致了孔结构和物相组成的显着变化，这是三年自然碳化后没有观察到的。另一方面，施加CO2压力对混凝土微观结构的影响很小，却有效地加速了碳化过程。此外，~1H核磁共振测定的水分分布表明，加速碳化过程中的水的形成对碳化过程有显著的影响。因此，在孔雀石含量较低的样品中，水的形成对碳化反应的阻碍较小，因此结合能力较低(例如CEM III)。由于二氧化碳通过扩散的渗透已被广泛研究，因此有必要研究渗透及其对碳化反应的影响。使用二氧化碳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，水泥类型和水灰比、二氧化碳压力和浓度以及湿度条件都会发生系统的变化。通过自动控制相对湿度和二氧化碳浓度，获得了实验的精确条件。薄砂浆圆盘、砂浆和混凝土圆筒储存在各种二氧化碳/氮气混合物中，同时以循环方式改变气体压力(0到10巴)及其持续时间(几个小时到14天)。观察了水在表面区的分布(1H-核磁共振)、矿物相的形成和溶解(X射线衍射、热重分析、Al-/Si-核磁共振)和孔结构(MIP)的变化。不同含水率的砂浆和混凝土圆筒的末端暴露在二氧化碳中，水的空间分布、相以及碳化深度和表面空气渗透率随时间变化而确定。热力学模型计算被用来帮助解释结果。最后，建议进行加速试验，以确定实际的抗碳化性能。

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.