CaCl₂-accelerated hydration of tricalcium silicate: A STXM study combined with ²⁹Si MAS NMR

The effect of calcium chloride (CaCl₂) on tricalcium silicate (C₃S) hydration was investigated by scanning transmission X-ray microscopy (STXM) with Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra and ²⁹Si MAS NMR. STXM is demonstrated to be a powerful tool for studying the chemical composition of a cement-based hydration system. The Ca L_3,2-edge NEXAFS spectra obtained by examining C₃S hydration in the presence of CaCl₂ showed that this accelerator does not change the coordination of calcium in the calcium silicate hydrate (C-S-H), which is the primary hydration product. O K-edge NEXAFS is also very useful in distinguishing the chemical components in hydrated C₃S. Based on the Ca L_3,2-edge spectra and chemical component mapping, we concluded that CaCl₂ prefers to coexist with unhydrated C₃S instead of C-S-H. In Si K-edge NEXAFS analysis, CaCl₂ increases the degree of silicate polymerization of C-S-H in agreement with the ²⁹Si CP/MAS NMR results, which show that the presence of CaCl₂ in hydrated C₃S considerably accelerates the formation of middle groups (Q 2) and branch sites (Q 3) in the silicate chains of C-S-H gel at 1-day hydration.