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This paper addresses the hydrothermal reactivity of blast furnace slag and fly ash with lime, respectively. The test conditions were CaO-to-SiO2 ratio (C/S), autoclaving temperature (140oC and 180oC) and time (20 to 60 hours). The study was carried out in terms of the hydrothermal reactivity between SiO2 contained in each hydraulic material and (pure) lime, and the compressive strength of autoclaved specimens. Porosity measurement and the XRD analysis were also made in order to ascertain the hydraulicity of the siliceous materials. Compressive strength of the specimens was interpreted in terms of porosity and the reactivity of CaO and SiO2. And the XRD analysis showed the C/S change of the hydrates in the course of autoclaving process. SiO2 in the blast furnace slag was more reactive with CaO than that in the fly ash, and consequently the blast furnace slag specimens resulted in much higher compressive strength. A maximum compressive strength of 807 kg/cm2 was obtained for the blast furnace slag at the autoclaving condition of 180oC and 40 hours, while only 397 kg/cm2 was maximally to achieve with fly ash.