The capability of the blast furnace slag to be vitrified was investigated by means of differential thermal analysis (DTA), scanning electron microscopy, and X-ray diffraction (XRD) in samples prepared by melting without the addition of any other ingredients. Differential thermal analysis revealed the presence of two major exothermic peaks. XRD indicates the presence of more than one crystalline phase: gehlenite Ca2Al2SiO7, BaAl2Si2O8, and pyroxene Ca(Mg,Al)(Si,Al)(2)O-6. The activation energy of crystallization was estimated on the basis of DTA carried out on particle size 312-500 mu m at different heating rates. Analysis of non-isothermal DTA data yielded values of 457.5 KJ/mol and 2.21 for the activation energy of crystallization and the Avrami exponent, respectively. A value for the activation energy corresponding to structural relaxation at temperatures around the glass transition has also been determined for this glass.