To address the challenge of pile foundation waste slurry disposal, this study proposes a resource utilization approach based on alkali activation technology, enabling rapid solidification of the waste slurry into a flowable solidified soil suitable for engineering backfill. An orthogonal experimental design was employed to determine the optimal mix ratio of the solidifying agent, identified as cement: slag: fly ash = 0.5:0.35:0.15, with NaOH accounting for 6% of the total mass. Based on this, the mechanical and durability properties of the solidified soil were systematically investigated. Results indicate that within a certain range, both unconfined compressive strength and splitting tensile strength increase linearly with higher solidifying agent content and longer curing age, and a fitting relationship between the two was established. An increase in construction delay time weakens the mechanical performance, highlighting the need for appropriate time control during construction. In terms of durability, resistance to wet-dry cycles improves with greater solidifying agent content, though a saturation effect is observed. The water stability coefficient shows different growth trends with changes in solidifying agent content and immersion age. Freeze-thaw cycles significantly degrade the strength, and increasing the solidifying agent content does not effectively improve frost resistance. The findings provide theoretical and technical support for the engineering application of premixed flowable solidified soil.