Stabilization-solidification (s/s) method is widely applied to immobilize mining and industrial wastes. This technique uses cement chemical and physical properties to integrate trace elements (TE) during their hydration reactions. Prediction of cement phases and pollutants evolutions during weathering is needed to predict risk of pollution. TE can be integrated in cement phases like calcium silicate hydrate (CSH), Ca-Al hydroxy sulphate or Ca-zincate. The aim of this study is the determination of initial pollutant locations and TE-bearing phases in the cement matrix. A comparison of hydration reaction between synthetic sludge and mining waste is provided. Samples come from two sets of cement matrix, the first one is ordinary portland cement (OPC) only and the second one is a mix of OPC and fly ash (FA). Each set is subdivided as a function of the pollutant added that are either synthetic sludges (Cd, Cu, Pb, Zn) or tailings, (from a copper and zinc mine at Winston-Lake, Ontario, Canada). Initial TE-bearing phases were determined using physical characterization (XRD, SEM, microprobe). Samples with FA were depleted in portlandite. Trace elements are incorporated in the gel for synthetic sludge samples, whereas pollutants from tailings are localized in sulphide minerals and iron oxides.