1 INTRODUCTION Nowadays, there are growing pressures to use waste materials within the construction sector. This use of wastes in civil engineering requires an evaluation of both the environmental and technical suitability of the waste. From the environmental point of view the main concern is the leaching out of hazardous substances, such as heavy metals, to soil and water supplies while from the technical point of view issues like changes in cement hydration, durability analysis and economic evaluation should be addressed. Extensive research has been performed in to the use of solid residues from municipal solid waste incineration (MSWI) as cement or aggregate replacement in building materials (Al-Rawas et al., 2005, Aubert et al., 2006, Collivignarelli & Sorlini, 2002, Huang & Chu, 2003, Juric et al., 2006, Lin et al., 2003, Reijnders, 2007, Wainwright, 2000). Municipal solid waste (MSW) generation in Europe increased at an annual rate of 2% from 1995 to 2003 and, in 2003, 17.3% of these wastes were incinerated (Eurostat, 2005). Solid residues from MSWI have different chemical compositions and can be broadly classified as bottom ash and fly ash. Bottom ash is recovered from the base of the combustion chamber and fly ash from the heat recovery and air pollution control systems. Incineration of a tonne of municipal solid waste generated, on aver- age, 200 to 300 kg of bottom ash and 25 to 50 kg of fly ash (Chandler A. J., 1997). Owing to stringent air emission policies, fly ash which contains high contents of soluble salts, such as calcium carbonate, sodium and potassium chlorides and sulphates, heavy metals and re- sidual amounts of organic composites are classified as hazardous waste (Ferreira et al., 2005, Wan et al., 2006). In order to upgrade MSWI fly ash to a usable product it is essential to use preliminary treatments that minimize the prejudicial effects of some of these compounds. The composition of MSWI fly ash varies from plant to plant and it is not possible to establish a uni- versal treatment for them (Derie, 1996). At present, suggested treatments can be divided into Chemical characterization and leaching of treated fly ash from a MSWI plant I. M. Martins & A. M. Esteves Laboratório Nacional de Engenharia Civil, Lisboa, Portugal J. P. Forth University of Leeds, Leeds, United Kingdom ABSTRACT: With the aim of reducing landfill deposition the incorporation of industrial waste into the cement matrix of building materials is becoming common practice. An assess- ment of the elements and compounds present in the waste that have a detrimental effect on ce- ment hydration or on the environment, such as lead, zinc, chromium and sulphates, should al- ways be performed. Fly ashes from the heat recovery and air pollution control systems of MSWI plants are classi- fied as hazardous wastes owing to the concentration of potentially hazardous heavy metals as well as their high salt content. Waste pre-treatment represents a possible approach to prepare these residues for incorporation into building materials as it will reduce their pollution potential. This paper presents the results of the chemical characterization of two fly ash mixtures, col- lected at different locations from a Portuguese MSWI plant, before and after a selected pre- treatment, and an evaluation of their leaching behaviour.