The recent nuclear leak in Japan once again attracted people's attention to nuclear safety problems. Because of their poor thermal stability, those low-cost materials such as cement and asphalt cannot be used for the solidification of the radioactive wastes. In this work, the solidification behavior of 133Cs(+) by fly ash-based geopolymer was investigated. Leaching tests (carried out in deionized water, sulfuric acid and magnesium sulfate solutions) revealed that the geopolymer solidification had lower cumulative fraction leaching concentration (CFLC) of 133Cs(+) than that of cemented form. The thermal stability (high-temperature and freeze-thaw resistance) and acid-resistance of the geopolymer were also both better than that of cement. The geopolymer solidification block can acquire a compressive strength up to 30 MPa after 2 h calcination at 1000 degrees C. The morphology and mineral phases of the geopolymer and the geopolymer solidification block were characterized by SEM and XRD, and EDX analysis indicated that most of Cs associated with the amorphous geopolymer gel. These results gave encouragement for the idea that the fly ash-based geopolymer could be used as a low-cost and high-efficiency material for the immobilization of radioactive wastes. (C) 2013 Elsevier B.V. All rights reserved.