Mechanochemically Carbonated Recycled Cementitious Fines as a Reactive Carbonate–Silica Precursor for LC3: Hydration Synchronization and Strength Development

In this study, cement-rich recycled cementitious fines (RCF) were mechanically carbonated to produce carbonated recycled cementitious fines (CRCF), which were used to replace conventional limestone in LC3. CRCF acts as a reactive carbonate–silica-bearing component rather than an inert filler, leading to enhanced strength and reduced carbon emissions. CLC3 achieved significant strength improvements, with 1 day and 28 day compressive strengths exceeding those of LC3 by 89.2% and 12.9%, respectively. The early age enhancement is attributed to improved synchronization of aluminate–carbonate reactions, supported by reactive carbonate participation and silica-rich amorphous phases. These phases facilitate hydration through nucleation and promote the formation of carboaluminate phases such as monocarboaluminate (Mc). Pore structure analysis indicates a refinement of pore size distribution, characterized by a shift toward finer pores. The results demonstrate a mechanistic proof-of-concept for a cement-rich recycled precursor. The applicability within recycled concrete fines, where aggregates may also be present, requires further investigation.