Solidification of blast furnace water-cooled slag (BFWS) has been carried out using a hydrothermal processing method, in which the BFWS could be solidified in an autoclave under saturated steam pressure (1.56 MPa) at 473 K for 12 h by the addition of quartz. Experimental results showed that the strength development of the solidified specimen was significantly affected by the curing time and quartz particle size. With increasing curing time, the amount of tobermorite increased, and the formed tobermorite, in turn, enhanced the tensile strength. The particle size of the quartz added exerted a significant influence on the strength development, and the optimal quartz content for the strength development appeared to depend on the quartz particle size. The results also suggested utilization of finer quartz had the advantage of reducing the addition of quartz.