THERMOCHEMICAL REGENERATION OF HYDRATED CEMENT PASTES: IN SITU MONITORING UNDER SYNCHROTRON RADIATION

Abstract

The recycling of construction and demolition waste into active supplementary cementitious materials requires a precise understanding of the thermal phase transitions within hydrated Portland cement. This study employs high-resolution static (ex situ, ESRF) and dynamic (in situ, SIRIUS) synchrotron X-ray diffraction, coupled with Rietveld refinement, to map the crystallographic evolution of cement paste during thermal treatment. A robust instrumental calibration using a TCHZ pseudo-Voigt profile ensured accurate microstructural quantification. The in situ dynamic analysis revealed a complex thermodynamic sequence: following the early thermal collapse of ettringite, a simultaneous dehydroxylation-carbonation anomaly occurred between 420 °C and 560 °C. Here, the decomposing portlandite yielded highly reactive nascent lime (CaO) that instantaneously carbonated into calcite, a process catalyzed by locally released water vapor. Above 650 °C, the decomposition of this secondary calcite flooded the matrix with free CaO, triggering a massive solid-state recombination with amorphous silica to regenerate active belite. Crucially, comparative analysis demonstrated that static ex situ calcination methods are severely compromised by rapid retrograde reactions (rehydration and recarbonation) during atmospheric cooling, underscoring the absolute necessity of in situ instrumentation to determine true high-temperature kinetic boundaries. These findings provide a definitive crystallographic framework for optimizing industrial heating protocols, demonstrating the thermochemical feasibility of the upcycling of hydrated cement pastes into sustainable and reactive clinker phases for the circular economy.