Hydrothermal treatment is useful in the recycling of slag and glass discharged from metallurgical or ash melting processes, because it enables valuable porous ceramic materials to be fabricated. Under hydrothermal conditions, raw materials dissolve in a high pressurized water vapor or aqueous solution to form an intermediate hydrate phase containing water. Our previous studies revealed that the hydrothermal reaction of sodium borosilicate glass forms a hydrate glass phase with abundant water, and the glass containing water expands to form a porous glass when annealed, because of water vaporization in the hydrate glass phase. The blast-furnace (BF) slag forms needle-shaped hydrate crystals such as tobermorite (5CaO center dot 6SiO(2)center dot 5H(2)O) between the initial slag particles. In this study, we have fabricated functional porous glass with hydrate crystals by the hydrothermal treatment of a mixture of BF slag and sodium borosilicate glass particles. The glass/slag mixture can contain water in the hydrate glass phase after hydrothermal treatment, and porous glass was obtained by annealing the glass/slag containing water. Needle-shaped hydrate crystals were formed by the hydrothermal reaction of borosilicate glass and BF slag components with highly pressurized water, and have been observed in the porous glass microstructure. Furthermore, we have investigated the effect of slag/glass mass ratio and raw material size on water content and the formation of hydrate crystals in the resulting material after hydrothermal treatment.