CaCl2-Accelerated Hydration of Tricalcium Silicate: A STXM Study Combined with Si-29 MAS NMR

Abstract

The effect of calcium chloride (CaCl2) on tricalcium silicate (C3S) hydration was investigated by scanning transmission Xray microscopy (STXM) with Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra and Si-29 MAS NMR. STXM is demonstrated to be a powerful tool for studying the chemical composition of a cement-based hydration system. The CaL3,2-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 CaL3,2-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 Si-29 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.