<p>Incineration fly ash (IFA) presents serious environmental concerns, particularly due to the leaching of toxic heavy metals, which poses threats to soil and groundwater. Therefore, developing cementitious materials that can effectively immobilize these heavy metals while maintaining mechanical strength is essential. This study aims to determine the optimal IFA content that maximizes environmental safety without compromising structural performance. The findings indicated that IFA could lower the heat of hydration and extend the setting time. A proper dosage of IFA was found to enhance the density of repair materials, leading to improved mechanical strength. Specifically, a repair material incorporating 6% IFA exhibited fewer less harmful and multi-harmful pores compared to the control sample, which contributed to superior mechanical properties. After 28 days, the material with 6% IFA demonstrated a 24% increase in strength relative to the material without IFA. Thermogravimetric analysis of hydration indicates that with the increase in IFA content, IFA-12 has 74.53% less bound water compared to IFA-0, suggesting that IFA inhibits cement hydration. Additionally, cement effectively immobilized the heavy metals in IFA through mechanisms such as ion exchange, physical encapsulation, and precipitation, thereby minimizing the risk of heavy metal leaching. When using acid as the leachate to test the leaching concentrations of heavy metals in the cement mortar prepared with IFA, the immobilization levels for Cr, Ni, Cu, Zn, Cd, and Pb in the cement ranged from 94.60% to 99.99%, demonstrating effective immobilization of heavy metals in the IFA.</p>