Reinforcement corrosion in carbonated fly ash concrete

Abstract

The cement industry contributes about 8% of the anthropogenic CO2 emissions, partly due to large consumption of cement and the emissions associated with the production (for Portland cement (PC) approximately 800 kg/t). One approach to lower the CO2 footprint is to substitute part of the Portland clinker by supplementary cementitious materials (SMCs), e.g. fly ash blends (PCFA). Blended cements may present advantages compared to PC. However, a main drawback is a lower carbonation resistance. This disadvantage is known and taken into account in the standards. The objective of the PhD study was to improve the understanding of carbonation-induced corrosion in reinforced concrete structures containing fly ash blends. For the tested materials and exposure conditions, the carbonation depth measured using thymolphthalein compared to carbonation depth determined using optical microscopy, calcium hydroxide to calcium carbonate profiles determined using thermogravimetric analysis, and free alkali metals (Na, K) in the pore solution determined using cold water extraction. Based on this, the pH indicator was used in the present study for detection of carbonation. Thymolphthalein solution sprayed on a freshly split surface allows to determine the spatial variation of the carbonation depth. Based on investigations of the microstructure and solid phases, carbonation up to 5% CO2 appears representative for natural carbonation. An increased degree of carbonation and an increased content of sulphate in the pore solution were observed in samples carbonated at 100% CO2. However, the impact of the exposure condition was limited compared to the changes upon carbonation (from non-carbonated to carbonated condition). Corrosion initiation is reported in the literature before the carbonation front compares to the concrete cover. We found that corrosion did not start until the concrete (mortar)-steel interface was carbonated as identified using pH indicator thymolphthalein. A relationship between the carbonated fraction of the mortar-steel interface (as identified using thymolphthalein) and the open circuit potential was observed. Carbonation causes a decrease in the hydroxyl concentration, variations into the carbon content, a reduction in the carbonate-to-bicarbonate ratio, and a release of detrimental ions such as sulphates and chlorides in the pore solution. This strongly increases the probability of corrosion compared to non-carbonated pore solution. Microcell corrosion rates of reinforcement embedded in carbonated concrete were determined at different moisture conditions. The moisture content was found to have a large impact on the microcell corrosion current. High microcell current densities were observed for wet samples, whereas low microcell current densities were found over drying. The microcell current density was slightly higher in carbonated PCFA compared to PC. When reinforcement embedded in wet carbonated concrete was electrically connected to reinforcement in wet non-carbonated concrete, a macrocell corrosion rate of the same magnitude as the microcell corrosion rate was measured. The relative contribution of the partial processes in macrocell corrosion (cathodic polarization of passive steel, difference in potential between active and passive steel, and anodic polarization of active steel) depended on the cathode-to-anode ratio of the macrocouple. Overall PCFA presented lower carbonation resistance and the embedded reinforcement corroded slightly faster than in PC. However, it appears that the corrosion rate in carbonated PC and PCFA concrete is only of importance at high moisture content. Reinforcement in carbonated PCFA concretes kept constantly at 90% RH corroded at low rate. In summary: The total (microcell and macrocell) current density should be used for service life prediction of reinforced concrete structures exposed to CO2 and periods of wetness (i.e. XC4) The possible use of PCFA concrete exposed to CO2 and periods of wetness (i.e. XC4) should be carefully considered