Soil heavy metal (HM) contamination is a widespread and serious environmental challenge. Even after treatment, contaminated soil may still pose a risk of secondary metal release under complex environmental conditions. In this study, a novel solidifying agent, a diatomaceous earth–fly ash-based geopolymer (D-FA), was synthesized from phosphoric acid (H₃PO₄), fly ash, and diatomaceous earth for the solidification/stabilization of contaminated lead-zinc red clay. Under coupled acid exposure and wet–dry cycling conditions, short-term effectiveness was evaluated using the toxicity characterization leaching procedure (TCLP), whereas the accelerated TCLP (ATCLP) was applied to analyze leaching kinetics and predict long-term performance. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD) were employed to characterize microstructural evolution and degradation mechanisms across multiple scales. A durability assessment framework based on cumulative leaching fraction (CFL) and effective diffusion coefficient (D_e) was established to predict the environmental safety period. The results showed that D-FA increased the strength of red clay by 6.59-fold and achieved fixation efficiencies of 99.99% for Pb²⁺ and 73.95% for Zn²⁺ within 28 days. Under long-term acid exposure and wet–dry cycling conditions, D-FA exhibited greater stability and durability in immobilizing Zn²⁺ than Pb²⁺. According to predictions based on Fick’s diffusion model, the environmental safety period reaches several decades for farmland at pH=5, exceeds 200 a for grassland and forest land, and may extend to several centuries for construction sites.