5 pages.-- Final full-text version of the paper available at: http://dx.doi.org/10.1016/j.jhazmat.2008.04.086.The resistance of class C fly ash belite cement (FABC-2-W)to concentrated sodium sulphate salts associated with low level wastes (LLW) and medium level wastes (MLW) is discussed. This study was carried out according to the Koch and Steinegger methodology by testing the flexural strength of mortars immersed in simulated radioactive liquid waste rich in sulphate (48 000 ppm) and demineralised water (used as a reference), at 20ºC and 40ºC over a period of 180 days. The reaction mechanisms of sulphate ion with the mortar was carried out through a microstructure study, which included the use of Scanning electron microscopy (SEM), porosity and pore-size distribution and X-ray diffraction (XRD). The results showed that the FABC mortar was stable against simulated sulphate radioactive liquid waste (SSRLW) attack at the two chosen temperatures. The enhancement of mechanical properties was a result of the formation of non-expansive ettringite inside the pores and an alkaline activation of the hydraulic activity of cement promoted by the ingress of sulphate. Accordingly, the microstructure was strongly refined.Funding for the present research was provided by the Minister of Science and Technology under Project n◦ MAT 2002-04023-CO3. The authors wish to thank the Thermal Station of Cercs (Catalonia) for the fly ash supplied.Peer reviewed