CaCl2-Accelerated Hydration of Tricalcium Silicate: A STXM Study Combined with29Si MAS NMR

Abstract

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 and29Si MAS NMR. STXM is demonstrated to be a powerful tool for studying the chemical composition of a cement-based hydration system. The Ca L3,2-edge NEXAFS spectra obtained by examining C3S hydration in the presence of CaCl2showed 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 L3,2-edge spectra and chemical component mapping, we concluded that CaCl2prefers to coexist with unhydrated C3S instead of C-S-H. In Si K-edge NEXAFS analysis, CaCl2increases the degree of silicate polymerization of C-S-H in agreement with the29Si CP/MAS NMR results, which show that the presence of CaCl2in hydrated C3S considerably accelerates the formation of middle groups (Q2) and branch sites (Q3) in the silicate chains of C-S-H gel at 1-day hydration.