Slurry widely existes in river dredging, cast-in-place piles, pipe-jacking construction and other projects, whose treatment is a long-term problem in the engineering field. In this study, solid waste geopolymer which was synthesized by coal gangue, blast furnace slag and fly ash was adopted to solidify the slurry with a moisture content of 60% (mass fraction). The mechanical properties, durability and microscopic mechanism of the solidified soil were investigated. The feasibility of the solidified soil as the backfill material was also estimated. The results show that strength of the solidified soil increases with the content of solid waste and alkaline activator. When 20% (mass fraction) solid waste is added, the cohesion c, the internal friction angle φ and strength of the solidified soil were 60 kPa, 19°, and 400 kPa after 7 d solidification. The solidified soil meets the properties requirements of the backfill soil. In addition, the geopolymer-solidified soil displayes better freeze-thaw resistance than the cement-solidified soil with the same content of cement. The gel products of geopolymer-solidified soil are mainly calcium silicate hydrate and gismondine, which can bind soil particles and fill the pore structure, then strengthenes the solidified soil performance.