Recycling mining wastes such as gangue for cemented rockfill is a key approach for sustainable development while ensuring the safety of underground structures in the mining industry. However, achieving sufficient strength for this low-cost material remains a fundamental challenge. This study explores an approach to strengthen cemented rockfill by using well-graded gangue, and by partially replacing Portland cement with fly ash and a premixed low-alkalinity activator. The effects of activator dosage and curing time on mechanical properties, composition and microstructure of cemented rockfill were investigated through uniaxial compression, XRD, FTIR, TG/DTG, MIP and SEM-EDS analyses. Results show overall improvements in the performance of cemented rockfill despite its low cement content, and the use of a low-cost low-alkaline activator. The 28-d compressive strength improved by over 30 % to reach 8 MPa with only 7.7 % wt. Portland cement and incorporating more than 60 % wt. of waste gangue in its mix composition. The main source of strength is calcium silicate hydrate and calcium aluminate silicate hydrate, the latter playing an increasing contribution with longer curing due to fly ash activation. Addition of premixed activator to fly ash enhanced the formation of Ca6Si6O17(OH)2, and CaO∙Al2O3∙xSiO2∙yH2O. Activator causes densification of the microstructure and this occurs predominantly on pores smaller than 1 μm. The optimum mix having the highest performance-cost ratio contains between 10 % and 15 % activator. This work reveals an effective and low-cost method of premixing fly ash with a low-alkalinity activator for recycling mining wastes and reducing cement.