X-ray spectromicroscopic study of interactions between NaCl and calcium silicate hydrates

X-ray diffraction (XRD), scanning transmission X-ray microscopy (STXM) and X-ray absorption near edge structure (XANES) spectra were used to investigate the interactions between sodium chloride (NaCl) and calcium silicate hydrate (C-S-H), the primary binding phase of hydrated Portland cement. XRD analysis indicates the interlayer expansion of C-S-H with a high Ca/Si ratio. Contrast maps of STXM images collected from C-S-Hs with different Ca/Si ratios reacted with NaCl solution reveal that sodium interacts with C-S-H with a low Ca/Si ratio, while chloride is intercalated into the interlayer of C-S-H with a high Ca/Si ratio. Silicon K-edge XANES spectra were analysed to understand the variation in the structure of silicate chains. This revealed that the interaction with the sodium cations depolymerises the silicate chains of C-S-H with a low Ca/Si ratio. The mechanism involved for the selection of adsorbed ions is probably due to the residual charge between the positive charge of interlayer calcium ions and the negative charge of the end group of silicate chains in C-S-H.