Effects of Different Pretreatment Methods for Recycled Fine Aggregates on the Properties of Geopolymer Mortar Incorporating Recycled Powder

Although low-carbon geopolymers incorporating construction and demolition waste (CDW) offer a promising circular economy pathway, the synergistic mechanisms between pretreated recycled fine aggregates (RFA) and geopolymer binders have not been systematically elucidated. This study investigated the comprehensive performance of geopolymer mortar containing recycled powder (RP) incorporating RFA; 50% Fly ash, 25% slag, and 25% RP were incorporated as precursor for the production of geopolymer binders, while the replacement ratios (0%, 20%, 40%, 60%, 80%, 100%) and the pretreatment methods (carbonation and prewetting) of RFA were taken as experimental parameters. The effect of these parameters on the fluidity, setting time, water absorption, compressive strength, and microstructure of recycled geopolymer mortar (RGM) and recycled cement mortar (RCM) was analyzed. Results showed that as the RFA replacement ratio increases, the measured properties generally decline. However, pretreating the RFA, particularly through carbonation, effectively mitigates these drawbacks. The use of 60% carbonated RFA enhanced the compressive strength of RGM by 12% compared to untreated RFA at equivalent replacement ratio. A comparative evaluation of the performance variations between RGM and RCM revealed that geopolymer mortar exhibited lower fluidity, faster setting time, and higher compressive strength. The microstructure analysis by SEM showed that the geopolymerization reaction between adherent cement paste in RFA and geopolymer binders significantly enhanced the microstructural compactness compared to RCM. Furthermore, carbonation and prewetting treatments can mitigate cracks and pores in the mortar. The results demonstrate that RGM prepared with carbonated RFA offer an estimated 76% reduction in net CO2 emission and 14.3% reduction in total cost relative to conventional cement mortar. This study established a framework that compares the mechanisms of RFA pretreatment and equip engineers with validated pretreatment strategies for upcycling CDW into construction materials.