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In this study, the distributions of Ni+, Pb2+ and Cd2+ species during the co-composting of sewage sludge with fly ash (FA), steel slag (SS) and their mixtures were examined to evaluate how these additives influence the bioavailability of heavy metals. The results indicated that the optimum addition was FA-SS1.5 (or FA: SS = 1.5, w/w), which had the best passivation on Ni+, Pb2+ and Cd2+. Co-composting with FA or SS was effective to reduce Cd migration, while their mixtures considerably passivated Ni and Pb. Finally, 18d was the best time to obtain the best passivation effect on heavy metals. [ABSTRACT FROM AUTHOR]