Characterisation of Friedel’s Salt in Cementitious Materials with Raman Spectroscopy

Friedel’s salt (C₃A∙CaCl₂∙H₁₀), a product formed between chloride ions (Cl⁻) and calcium aluminates (C₃A), is usually believed to be responsible for the chemical binding of chloride ions in hardened cement matrix. Therefore, characterising the condition of the Friedel’s salt formed in cementitious materials can improve the understanding of the interactions between the chloride and the aluminate phases in concrete and, thus, facilitate the prediction of service life of reinforced concrete structures because only those ‘free’ chloride can initiate the corrosion of reinforcing bars in concrete. Raman spectroscopy, a vibrational spectroscopy working on the principle of inelastic scattering, is an outstanding analytical tool which combines both qualitative and quantitative analysis capacities. However, up to now, there has been no research on the application of Raman spectroscopy for the characterisation of Friedel’s salt. In this study, a bench-mounted Raman spectrometer was employed to characterise the pure Friedel’s salt synthesized under controlled laboratory condition as well as the Friedel’s salt formed in a Portland cement paste after being exposed to a NaCl solution. Under Raman spectroscopy, various fingerprint bands of Friedel’s salt were successfully identified. The results obtained clearly indicate that Raman spectroscopy is capable of characterising Friedel’s salt in cementitious materials.