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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 to study the chemical composition of a cement-based hydration system.The Ca L3;-edge NEXAFS spectra from the examination of 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), the primary hydration product.O K-edge NEXAFS is also very useful to distinguish the chemical components in hydrated C3S.Based on the Ca L3,2-edge spectra and chemical component mapping,we concluded that CaCl2 prefers to coexists with unhdyrated 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 MAS NMR results, showing that the presence of CaCl2 in hydrated C3S generates a significant increase in the Q2/Q1 ratios.