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

Abstract

The effect of calcium chloride (CaCl 2 ) on tricalcium silicate (C 3 S) hydration was investigated by scanning transmission X‐ray microscopy (STXM) with Near Edge X‐ray Absorption Fine Structure (NEXAFS) spectra and 29 Si 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 C 3 S hydration in the presence of CaCl 2 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 C 3 S. Based on the Ca L 3,2 ‐edge spectra and chemical component mapping, we concluded that CaCl 2 prefers to coexist with unhydrated C 3 S instead of C‐S‐H. In Si K‐edge NEXAFS analysis, CaCl 2 increases the degree of silicate polymerization of C‐S‐H in agreement with the 29 Si CP/MAS NMR results, which show that the presence of CaCl 2 in hydrated C 3 S 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.