Under-calcination thermal reactivation of hydrated cement pastes and construction and demolition waste: A comparison of different commercial products

This study explores the properties of supplementary cementitious materials (SCMs) obtained from the under-calcining thermal reactivation of construction and demolition waste fines (CDW) and of pure hydrated cement pastes (HCPs) derived from different commercial cements (CEM I, CEM II, CEM III, and CEM IV, classified according to EN 197-1). Samples were thermally treated at 350 °C (low-energy activation) and 600 °C (high-reactivity optimization) to increase pozzolanic and/or hydraulic behavior without inducing CO₂ release from calcination. Phase transformations were analyzed by X-ray Powder Diffraction with Rietveld refinement, thermal behavior by TGA/DTA, and morphological evolution by SEM, while reactivity and hydration kinetics were assessed using isothermal calorimetry. Results indicate pronounced differences in thermal response among the HCPs and CDW, governed by their specific phase assemblages and initial SCM content (limestone, slag, and pozzolans). At the selected under-calcining high-reactivity temperature (600 °C), γ-C₂S formation is favored. Thermal reactivation of slag-rich cement (CEM III) yielded the most reactive circular SCM, combining both high hydraulic (260 J/g - EN 196-11) and pozzolanic (329 J/g - ASTM C1897-20) performances with reduced carbon impacts.