Thermal stability as well as sulphate resistance of alkali activated slag (AAS) pastes prepared from Ground Granulated Blast Furnace Slag (GGBFS) and 5% fly ash (FA)are examined. AAS pastes are accelerated using alkaline solution of sodium hydroxide with concentration 1, 1.5 and 2 M and sodium silicate with fixed concentration (0.28 mol/Kg).AAS pastes exposed to 5% MgSO₄ solution and to high temperature (200, 400, 600 and 800 ^oC). The compressive strength, bulk density, total porosity, chemically combined water content and phase composition are determined in this study. Concentration of 2M NaOH with 5% FA promotes the best properties of AAS pastes that exhibit the highest resistance to firing up to 400^oC by achieving the greatest compressive strength. However, AAS pastes activated by 2M NaOHgive a high sulphate resistance.