Sodium silicate activated slag-fly ash binders: Part III—Composition of soft gel and calorimetry

Sodium silicate activated, slag-fly ash binders are potential alternative binders to Portland cement. In this study, the early age properties of slag-fly ash binders namely, set time, and heats of reaction were investigated. Set time was investigated using a combination of two methods namely, the ASTM C403 penetration testing, and s-wave ultrasonic wave reflectometry (SUWR). The discrepancy in set time identified by these two methods suggested the presence of a soft gel which eventually hardened with time. The composition of this soft gel was analyzed by suspending the chemical reaction of the binder after the soft gel formed, but before it hardened. In order to analyze the composition of the soft gel, selective chemical extractions were performed on the binder. ²⁹ Si Magic Angle Spinning-Nuclear Magnetic Resonance (MAS-NMR), and FTIR spectroscopy were performed on binders and extraction residues. The soft gel contained a modified calcium silicate hydrate gel (C–N–S–H where N=Na), with a short mean chain length and no observable Al incorporation. Orthosilicate units were also found to be present in relatively high proportions when compared to hardened binders at later ages.