Ladle slag (LS) and urban incineration bottom ash (BA) are two wastes which are necessarily disposed of with few attention for re-use or recycling. Their chemical composition contains silica and alumina accompanied by lime and iron oxides. Mineralogically speaking a number of silicate and silicoaluminate phases are present combined with 20-30 vol% of amorphous matrix. The presence of an important amount of vitreous phase makes LS and BA particularly reactive in alkali media. The geopolymerisation process of 70 wt% of LS or of BA in a metakaolin matrix has been characterized in terms of morphology and crystalline phases evolution.