Hydration kinetics and phase evolution in thermo-mechanically activated recycled cementitious composites

This study investigates the hydration kinetics, setting behavior and compressive strength development of recycled cementitious composites. Ordinary Portland cement (OPC) was partially replaced with recycled cement (RC) powders at varying levels, maintaining a consistent water-to-cement ratio across all mixes. Isothermal calorimetry revealed distinct hydration kinetics: slow and subdued heat evolution for raw RC, accelerated early heat release for ball milled RC (BMRC) due to high surface area and for thermally treated ball milled RC (HBMRC), heat flow closely parallels that of pure OPC. Vicat setting times correlated well with calorimetric data, with HBMRC showing high correlation with isothermal data. Compressive strength development exhibited strong linear correlations with cumulative hydration heat, especially beyond 24 h, underscoring the efficacy of calorimetry to predict strength gains. Thermogravimetric analysis and X-ray diffraction confirmed suppressed portlandite accumulation, transformation of carbonate phases, and formation of metastable alite/belite polymorphs. Scanning Electron Microscope with Energy Dispersive X-ray Spectroscopy showed enhanced nucleation density and densified C-S-H/C-A-S-H gels.