Advanced characterization of chloride binding in OPC and LC³ pastes

Abstract

The incorporation of supplementary cementitious materials (SCMs) in cement pastes enables both decreasing CO2 emissions and increasing the concrete durability, notably with a better resistance against chloride ingress and thus retarding the chlorideinduced corrosion of steel rebars. Nevertheless, a better understanding of the chloride fixation in cement pastes is required to build an accurate model for the transport of chlorides in new types of cementitious systems with SCMs. The purpose of this study is to characterize the chloride binding capacity of Limestone Calcined Clay Cement (LC³) pastes compared to Ordinary Portland Cement (OPC) pastes. The binding of chlorides was investigated in hardened cement pastes equilibrated with 0.5M NaCl and 0.5M NaCl + 0.3M NaOH solutions whose chloride concentration is similar to that of sea water. The total chloride content was determined by acid dissolution of the paste after exposure, while chemically and physically bound chlorides were determined by X-Ray Diffraction (XRD) coupled with Rietveld analyses, Scanning Electron Microscopy with Electron Dispersive Spectroscopy (SEM-EDS) and water dissolution of the pastes. The results showed that the higher content of AFm phases in LC³-50 pastes at 28 days did not lead to a higher chloride binding capacity compared to the reference OPC pastes, for both investigated chlorine solutions. XRD and SEM-EDS provided coherent estimates of chemically bound chlorides into AFm phases for each system (i.e., different solid solutions of Hemicarboaluminate and Friedel’s salt). These chlorides fixed in AFm were found to be lower than the difference between total acid-soluble chlorides and watersoluble chlorides (by 0.4 to 2.3 mg of fixed chlorides per gram of paste, depending on the system), suggesting other mechanisms of chloride binding such as physical binding on the C-A-S-H. Overall, the findings in this study suggest that the amount of the different kinds of chlorides fixed in cement pastes can be evaluated with relatively straightforward methods such as XRD/Rietveld and acid-soluble chloride titration.