The effects of replacing calcined clay with sewage sludge ash (SSA) treated under room temperature and high temperature ranging from 500 °C to 900 °C in limestone calcined clay cement (LC3) have been investigated in this paper. The optimal calcination temperature for SSA was found to be 800 °C based on the results of strength and microstructure observations. The main inorganic components of sludge ash are Fe₂O₃, SiO₂, Al₂O₃, and CaO, which are very similar to the components of calcined clay in LC3, but with a very high content of Fe₂O₃ (55–61%) and P₂O₅ (9–10%). With different levels of the replacement of calcined clay with calcined SSA, setting time, compressive strength, XRD, TG/DSC, and SEM analyses of the modified LC3 pastes were conducted to identify the chemical compositions, physical properties, hydration products, microstructure, and the heavy metal contaminants within the pastes, which were compared to the results for normal LC3 paste. The incorporation of SSA significantly altered the morphologies of Ca(OH)₂ and CaCO₃, as well as modified the microstructure of the LC3 paste. In comparison to the pure OPC group, the LC3 pastes containing SSA exhibited a reduced Ca(OH)₂ content and an increased CaCO₃ content. Furthermore, the modified LC3 pastes with calcined SSA effectively facilitated the immobilization of heavy metal ions in SSA. The findings indicate the potential viability of utilizing calcined SSA as a replacement for calcined clay in LC3.