Performance and Life-Cycle Assessment of Fly Ash–Based Pelletized Aggregates Partially Replaced with Spent Catalyst Waste

This study investigates the development of sustainable geopolymer-based pelletized aggregates using fly ash (FA) and spent catalyst (SC) waste. Taguchi experimental design was employed to optimize key parameters influencing aggregate production, including Na 2 O content (6%–8%), water content (20%–22%), and SC proportion (20%–30%). Pelletized FA-SC aggregates were produced through disc pelletization and cured under ambient conditions. Their physical and mechanical properties—including aggregate impact value (AIV), aggregate crushing value (ACV), individual pellet strength (IPS), and water absorption (WA)—were evaluated along with microstructural and thermal analyses using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). Among the nine mixes investigated, the FASC-9 mix (8% Na 2 O , 22% water, and 20% SC) exhibited the best overall performance, showing improved mechanical strength and reduced water absorption. Microstructural results confirmed the formation of dense geopolymeric gel networks contributing to enhanced thermal stability up to 700°C. Life-cycle impact assessment further indicated lower environmental impacts, particularly reduced global warming potential, compared with conventional natural aggregates. The results demonstrate the feasibility of valorizing spent catalyst waste into sustainable geopolymer aggregates for construction applications.