Effects of Incorporating High-Volume Fly Ash into Tricalcium Silicate on the Degree of Silicate Polymerization and Aluminum Substitution for Silicon in Calcium Silicate Hydrate

This study assesses the quantitative effects of incorporating high-volume fly ash (HVFA) into tricalcium silicate (C₃S) paste on the hydration, degree of silicate polymerization, and Al substitution for Si in calcium silicate hydrate (C-S-H). Thermogravimetric analysis and isothermal conduction calorimetry showed that, although the induction period of C₃S hydration was significantly extended, the degree of hydration of C₃S after the deceleration period increased due to HVFA incorporation. Synchrotron-sourced soft X-ray spectromicroscopy further showed that most of the C₃S in the C₃S-HVFA paste was fully hydrated after 28 days of hydration, while that in the pure C₃S paste was not. The chemical shifts of the Si K edge peaks in the near-edge X-ray fine structure of C-S-H in the C₃S-HVFA paste directly indicate that Al substitutes for Si in C-S-H and that the additional silicate provided by the HVFA induces an enhanced degree of silicate polymerization. This new spectromicroscopic approach, supplemented with 27Al and 29Si magic-angle spinning nuclear magnetic resonance spectroscopy and transmission electron microscopy, turned out to be a powerful characterization tool for studying a local atomic binding structure of C-S-H in C₃S-HVFA system and presented results consistent with previous literature.