Reactivity contrast between hydrated and dehydrated cement pastes under alkali activation: Roles in gel evolution and compressive strength

This study evaluates the feasibility of recycling dehydrated cement paste (DCP) as a reactive calcium silicate source in fly ash-based alkali-activated materials (AAMs) and elucidates temperature-dependent phase evolution and gel chemistry. The performance of DCP was compared with that of hydrated cement paste and ordinary Portland cement under curing conditions of 25 °C and 60 °C. At 25 °C, the incorporation of DCP, containing residual C–S–H, promoted the early formation of C–(N)–S–H gel and improved initial strength via exothermic reactions. At 60 °C, the dissolution of both DCP and FA was significantly accelerated, leading to rapid phase evolution and substantially increased strength. Under these hydrothermal conditions, cancrinite was formed and subsequently transformed into C–(A)–S–H-type gel through ion-exchange and structural reorganization processes. These transformations were confirmed by X-ray diffraction, solid-state Si and Al nuclear magnetic resonance, and FE-SEM/EDS analyses. The results demonstrate that DCP acts not only as a reactive calcium source but also as a kinetic modifier that enhances both the reactivity and strength development of FA-based AAMs. This study provides new insight into the temperature-dependent behavior of recycled cementitious fines and demonstrates their potential as sustainable binder components. • DCP was recycled as a calcium silicate source using an alkali activation system. • Hydrothermal condition accelerated the phase evolution and produced a C–A–S–H gel. • N–(C)– A–S–H phase existing as cancrinite in the hydrothermal condition. • Portlandite and ettringite interfered with the phase evolution reaction. • C–S–H gel present in DCP facilitated the initial reaction period.