One of the main objectives of this work is to present an effective alternative for the final destination of sludge from urban waste water treatment plants by its use as a component of mortar or concrete. A binding and stabilizing matrix of sludge-cement and sludge-cement-coal fly-ash was investigated and the effects of various percentages of waste and binder, on the behavior of sludge in the system are presented. Assessment of the environmental quality of the final product and the consequent guarantee of its use in the building industry demand that it meets a number of requisites, one of which is that the effluents extracted by water action should be contamination-free, or at least that the concentration of contaminants should be below certain pre-set limits. For this a number of leaching tests must be carried out, such as the Netherlands Leaching Test .