Municipal Solid Waste Incineration Bottom Ashes (MSWI BA) have been submitted to a thermal treatment leading to their complete vitrification. The composition and the pozzolanic activity of the vitrified, grounded bottom ashes is evaluated, as far as the mechanical and microstructural properties of mortars containing up to 30 wt. % of this fraction (on cement weight), as a replacement of the hydraulic binder. At sufficiently long curing times, modified composites exhibit higher mechanical strengths and lower porosity than those of the unmodified ones. The same mortars show lower permeation to chlorides and sulphates and the tendency to depress the expansion generated by alkali-silica reactions (ASR). 1359-5997 © 2004 RILEM. All rights reserved.