Landfill leachate concentrate (LLC) is a high-salinity organic wastewater produced in the process of membrane separation and has become an urgent problem because of its complex composition and high biological toxicity. Solidification/stabilization (S/S) technology using geopolymers provides a feasible and economical zero-discharge LLC treatment method. This study developed one-part fly ash-based geopolymers using LLC as the only solvent. The results showed that the solid alkali activator significantly increased the LLC treatment capacity with a high liquid−solid ratio of 0.55. The high-salinity LLC significantly promoted the substitution of Al for Si and improved the compressive strength of the geopolymers. Three leaching experiments showed that the prepared geopolymers had excellent stability, high S/S rates for organic matter (92.9%) and NH3-N (91.4%), and a high contaminant removal rate to reduce acute biological toxicity. Microscopy tests showed that sodium aluminosilicate hydrate (N-A-S-H) gel was the main component of the fly ash-based geopolymers after the addition of LLC. Cl− attached to the surface of the geopolymer gels through exchange with OH−, while SO42− was bound within the geopolymer structure in the form of Na2SO4 crystals. There was no evidence that organic matter and NH3-N were chemically combined with the geopolymer material, rather, they were physically encapsulated within the geopolymer structure. This study proposes a technical strategy for LLC S/S using fly ash-based geopolymer. The geopolymer can be used as a new building material, which provides a new perspective for the treatment of LLC and the utilization of fly ash resources. Display omitted •Landfill leachate concentrate (LLC) can replace water to prepare one-part geopolymer.•High-salinity concentration of LLC accelerates the substitution of Al for Si.•Fly ash-based geopolymers can effectively immobilize organic matter and NH3-N.•The leaching toxicity of the immobilized LLC decreases from 72.5% to 13.8%.•Geopolymers save 20% in costs and reduce 2/3 of CO2 emissions compared to cement.