Corrosion mechanism of reinforcement in LC3 cement pastes under coupled carbonation and chloride attack

Limestone Calcined Clay Cement (LC3) has good chloride (Cl−) binding capacity and is reported as a qualified binder for concrete manufacture with desalinated sea sand. In this study, corrosion mechanism of reinforcement in LC3 under coupled Cl− and carbonation attack was studied. Electrochemical measurement, optical microscopy and X-ray photoelectron spectroscopy were applied to assess safety of embedded reinforcement; OH− and Cl− profiles in LC3 paste were monitored to evaluate progress of carbonation degree. Results showed that LC3 pastes were carbonated with a pronounced speed, which was due to the decrease in calcium hydroxide content during its hydration stage. Due to decomposition of Friedel's salt and release of Cl−, obvious increase and redistribution of free Cl− were detected and progressed with carbonation duration. Depassivation of mild steel were observed at early stage of carbonation when CO2 had not reached its location, thus it was resulted from carbonation-induced Cl− increase rather than alkalinity decrease under the applied exposure. In summary, LC3 performed a good protection on embedded reinforcement under impact of the premixed Cl−, but this protection diminished quickly at a carbonation scenario.