While simultaneous processing of heavy metal-rich waste and municipal sewage sludge (MSS) in cement kilns may be a useful measure for protecting the environment, simultaneous co-disposal of these two types of wastes has seldom been reported in detail. In this study, we examined how MSS influenced the fixation of heavy metals during clinkerization and determined the main controlling mechanisms through analyzing the polymorphism of tricalcium silicate (C₃S), mineral composition of MSS, and element distribution in the clinker. The results showed that MSS had negative effects on the fixation of heavy metals, with reductions of 12.9%, 8.7%, 3.2%, and 1.2% in the amounts of Cu, Ni, Cr, and Zn fixed, respectively. These changes were mainly attributed to the presence of trace elements – in particular phosphorus from the MSS, which caused the polymorphism of C₃S to change in the order: rhombohed ral→monoclinic→triclinic. As well as occurring as C₂S-C₃P, phosphorus also occurred in new phases of K₂NiP₂O₇, K₂Cu(PO₃)₄, and Cu₄O(PO₄)₂ in cement clinker. In general, trace elements from the MSS, especially phosphorus, decreased the solubility of heavy metals in cement clinker through changing the polymorphism of C₃S, and resulting in a decrease in the fixation ratios of heavy metals.