Thermal and carbonation activation of recycled concrete powder: Mechanistic characterisation and performance in mortar

Recycled concrete powder (RCP), a fine byproduct of construction and demolition waste, remains underutilized due to low intrinsic reactivity and heterogeneous mineralogy. Preliminary R³ reactivity screening in this study confirmed that mechanical size reduction alone is insufficient to enhance reactivity, necessitating mineralogical activation. This study evaluates an integrated activation strategy combining thermal treatment (400–800 °C) and accelerated carbonation to modify RCP characteristics and improve its performance as a supplementary cementitious material. Thirty-eight mortar formulations incorporating 10–30 % RCP replacement were systematically evaluated via XRF, XRD, FTIR, TGA, FESEM, and compressive strength testing. Results identify 400 °C as the optimal thermal efficiency threshold for reactivating inert hydrate phases. Accelerated carbonation significantly intensified binder reactivity, with TGA quantifying a net CO₂ uptake between 8.65 % and 22.17 %. Notably, the 10 % RCP400 substitution variant demonstrated superior performance, exceeding control mortar strength by 8 % at 28 days. This improvement is attributed to microstructural densification, refined pore networks, and the pozzolanic consumption of portlandite. While excessive heating (800 °C) or high substitution levels caused dilution-driven strength declines, the synergistic thermal-carbonation pathway facilitates high-value valorisation of RCP in low-carbon construction. The integrated characterization provides a sharper understanding of microstructural transformations, advocating for RCP’s utilization to support circular economy goals. This research bridges the gap between laboratory-scale activation and structural application, offering a viable pathway for the construction industry to reduce its carbon footprint through enhanced recycled material integration.