Activation regimes and mechanisms of heterogeneous recycled powders regulated by GGBFS in alkali-activated ternary binders

Construction and demolition waste generates large quantities of recycled powders (RPs), but their large-scale utilization remains limited by the pronounced heterogeneity of powders from different sources, especially recycled brick powder (RBP) and recycled concrete powder (RCP). In this study, the alkali-activation behaviors of RBP and RCP under three alkali-activators, namely Ca(OH)₂, NaOH, and Na₂SiO₃, were systematically investigated. Based on these results, ternary GGBFS-RBP-RCP systems were further developed to clarify the effects of the RBP/RCP ratio, total RP content, and activator type on mechanical performance and microstructural evolution. The results demonstrated 4 wt% Ca(OH)₂ resulted in the highest activity index for both RBP and RCP, whereas Na₂SiO₃ and NaOH were ineffective for the activation of RP because of excessive silica supply and surface passivation. In the ternary systems, the optimum RBP/RCP ratio depended strongly on the activation regime. A balanced RBP/RCP ratio of 50:50 was more favorable under Ca(OH)₂ and combined activation, whereas Na₂SiO₃ activation was more suitable for GGBFS-rich and RCP-rich mixtures. The Na₂SiO₃ system was highly sensitive to RP content and RBP proportion, showing substantial strength loss as both increased. This indicates that Na₂SiO₃ is more suitable for mixtures with a high GGBFS content and a high RCP proportion. In contrast, the combined Ca(OH)₂-Na₂SiO₃ activator significantly enhanced strength retention in systems with high RP contents, demonstrating better compatibility under conditions of low GGBFS content and high RCP proportion. This study provides a feasible solution for the targeted utilization of heterogeneous CDW.