Restoration of hydraulic properties in recycled cement pastes via thermal treatment

The primary objective of this study is to investigate the regeneration of hydraulic properties in hydrated cement pastes through thermal treatment between 400 °C and 800 °C, with a particular focus on the phase transformations and hydration mechanisms of the regenerated binders. This research is part of a broader effort to develop sustainable strategies for concrete recycling within a circular economy framework. Crushed cement paste, previously hydrated for 28 days, was thermally treated and characterized using ²⁹Si nuclear magnetic resonance (NMR) spectroscopy and X-ray diffraction (XRD) combined with Rietveld refinement.Results show that treatment at 600 °C leads to the formation of two C₂S polymorphs (α'_H-C₂S and β-C₂S), confirmed by both XRD and ²⁹Si-NMR, associated with the complete decomposition of C–S–H phases. Binder hydration obtained between 600°C and 800°C required more water than ordinary Portland cement, likely due to the high reactivity of the free lime formed during thermal treatment. After 28 days of rehydration, a significant proportion of the belite formed is consumed. These findings demonstrate that heat-treated recycled cement pastes can behave as low-temperature belitic binders, offering a promising low-carbon alternative to conventional clinker production.