Influence of Carbonation on the Chloride Ingress Analyzed via Optical Sensors

Comprehensive understanding of the processes of chloride ingress and carbonation, particularly their combined effects, is imperative to find prevention strategies. In this study, 3 mortar samples (CEMI+ground granulated blast furnace slag) were exposed to three alternating 14 day cycles in a 3% NaCl solution and a carbonation chamber (3 vol% CO₂, 65% RH). After each cycle one sample was taken out for analysis. The water- and the acid-soluble chloride content was determined in powdered samples using a novel optical chloride sensor and via titration (reference method). The pH determination and assessment of the carbonation progress was done via optical pH imaging together with phenolphthalein coloration, as reference method. The results revealed a relatively constant carbonation rate ranging between 0.6 and 0.7 mm/√d. The pH imaging visually depicted the carbonation progress with depth and provided pH profiles in the pH range of 7.0–12. The chloride content decreased in the carbonated zone and the region with the highest chloride concentration shifted inwards. The results show a good correlation of the new optical sensors with the reference methods. Notably, optical pH imaging provides more insights into the pH variations over time compared to the phenolphthalein method. This investigation contributes towards advancing our comprehension of corrosion processes, particularly in the context of combined attacks involving carbonation and chloride ingress. The robust correlation observed between optical sensors and established methods highlights the potential of optical sensing technologies in enhancing our ability to monitor and understand corrosion phenomena in concrete structures.