Enhancing recycled concrete aggregates via cement-free recycled concrete powder slurry: The impact of aggregate age and residual portlandite

This study investigates the efficacy of Recycled Concrete Powder (RCP) as a cement-free slurry treatment for enhancing the physical properties of Recycled Concrete Aggregates (RCA). To elucidate the critical influence of aggregate origin and chemical condition, two distinct RCA types were examined: an industrially produced old aggregate (RB) and a laboratory-produced young aggregate (YNG). The experimental program evaluated standalone RCP slurries, calcium hydroxide pre-treatments, and combined methods through water absorption, open porosity, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and calcimeter testing. Results demonstrate that treatment efficiency is fundamentally governed by the residual portlandite content within the adhered mortar. The young YNG aggregates exhibited significant reductions in water absorption (up to 14.2%) and pronounced microstructural densification, driven by active pozzolanic reactions with intrinsic portlandite. In contrast, the old RB aggregates showed limited improvement (maximum 4.2%) due to carbonation-induced calcium depletion, which restricted chemical interactions and confined modifications to physical pore-blocking mechanisms near the surface. These findings indicate that while RCP slurry offers a promising low-carbon strategy for aggregate valorization, its performance is intrinsically dependent on the service history and residual reactivity of the parent concrete.