A hydrothermal processing method has been used to solidify blast furnace water-cooled slag (BFWS), in which the BFWS could be solidified in an autoclave under saturated steam pressure (1.56 MPa) at 200 degrees C for 12 h by the additions of quartz or coal fly ash. The experimental results showed that the addition of the quartz or fly ash was favorable to the formation of tobermorite, and the tobermorite formation in turn exerted a signifiant influence on tensile strength. The strength development depended on both tobermorite formation and the density of the tobermorite formed. The excessive addition of quartz appeared to cause strength deterioration due to the fact that the residual quartz affected the formation of tobermorite in the solidified specimens. Fly ash could be used as an additive for the hydrothermal solidification of BFWS, which may offer both energy saving and cost reduction.