The treatment of fly ashes from waste to energy (WtE) power plants is necessary due to the increasing cost of landfilling. Their use as a supplementary cementitious material (SCM) is a promising alternative. However, a treatment process is required to remove the chlorides, sulphates and heavy metals such as zinc and lead. A two-step process is proposed in which water washing removes the chlorides from an initial content of 16 to 1 wt%. The fly ash and water are mixed at an s/l ratio of 1:5 and heated to 60 °C for 1 h. The filtered material is then dried, combined with 18 wt% of sand (silica source), and heated to 1300 °C in a top submerged lance (TSL) reactor. Sulphur is removed through a reaction with silica. The heavy metals are then removed by reaction with the remaining chlorides and by means of the atmosphere generated by excess fuel addition in a reductive step in the same reactor. The final product has a composition similar to blast furnace slag and can be utilised as an SCM.