This study investigated how Al ions affect slag structure. During vitrification, pure Al2O3, CaO, and SiO2 served as the encapsulation phases with the use of Al mol% as an operating parameter. All specimens with the same basicity (mass ratio of CaO to SiO2) of 2/3 were vitrified at 1400 degrees C and cooled by air cooling or water quenching. XRD was used to measure the volume fractions of crystalline and amorphous phases. In a non-Al environment, CaSiO3 was formed in air-cooled and water-quenched slags. With the addition of Al2O3, no crystalline phases were observed in water-quenched slags. With the increase of All mol% in specimens, the Al ions in air-cooled slags initially acted as an intermediate linking one tetrahedron chain to another and reducing the amount of crystalline phase, then behaved as a network former making the slags amorphous, and finally replaced Si ions in silicate frames to generate a large amount of CaAl2Si2O8. Air cooling improved the formation of crystallize structures with more leachable metal ions. A highly crystallized Al-framed structure is not suitable for encapsulating hazardous metals in vitrified slags. (C) 2009 Elsevier B.V. All rights reserved.