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

• Applicant: Professor Dr.-Ing. Christoph Gehlen
• Subject Area: Construction Material Sciences, Chemistry, Building Physics
• Term: from 2012 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 221646279

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.

• DFG Programme: Research Grants
• Co-Investigator: Dr. Robin Edward Beddoe, Ph.D.