[Purposes] To address the environmental burden caused by the large-scale stockpiling of red mud and fly ash,a novel green grouting material was developed by leveraging the synergistic complementarity between the strong alkalinity of red mud and the activity of fly ash. [Methods] The effects of the water-solid ratio,red mud content,alkali activator content,and sodium silicate modulus on the performance and mechanical strength of the slurry were systematically investigated, and the microscopic mechanism of its strength formation was revealed by using scanning electron microscopy. [Findings] The results indicate that the water-solid ratio is the critical factor in governing the rheology of the slurry. The optimal threshold for red mud content is 30%, at which the slurry water separation rate drops to 3.5% with a high stone ratio.The alkali activator significantly accelerates polycondensation of the system, enhancing early mechanical strength of materials. The mechanical performance of materials peaks when the sodium silicate modulus is 1.2. Consequently,the optimal mix ratio is determined to be a water-solid ratio of 0.8, red mud content of 30%, alkali activator content of 10%,and a sodium silicate modulus of 1.2. Microscopic analysis confirms that the C-(A)-S-H/N-A-S-H composite gel network generated by alkali activation and the micro-aggregate filling effect of unreacted particles work synergistically.They are critical to material mechanical strength formation. [Conclusions] The research findings provide a new pathway for the resource utilization of red mud and fly ash.