Stabilization of Acidic Uranium Tailings Residue Using a Cement–Fly Ash–Zero-Valent Iron Composite: Mechanical Performance and Long-Term Leaching Behavior

Acid-leached uranium tailings residues present a long-term management challenge because uranium mobility, sulfur-driven acidification, and poor structural stability must be controlled simultaneously. This study developed a multifunctional stabilization/solidification strategy combining Portland cement, fly ash (FA), and zero-valent iron (Fe0) for acidic uranium tailings residue generated after sulfuric-acid extraction. The residue contained approximately 0.011% U and high Si–Al and sulfur contents, and showed a marginal acid–base balance, with an acid-neutralization capacity of 83.3 and maximum potential acidity of 75.3. Fine particles exhibited higher porosity and greater uranium release. Cement-to-residue ratio, water-to-cement ratio, curing age, FA replacement, and Fe0 addition were systematically evaluated. A cement-to-residue ratio of 3:7 and water-to-cement ratio of 0.6 yielded a 28 d unconfined compressive strength of 28.7 MPa. Incorporation of 15–25% FA with 1% Fe0 maintained strength at approximately 30 MPa, while five freeze–thaw cycles caused only a 2.79% strength loss. Direct Fe0 treatment reduced cumulative uranium release by approximately 78%. Cement solidification maintained uranium concentrations below 10 μg L−1 during 90 d leaching under acidic, neutral, and alkaline conditions, corresponding to >99% suppression relative to untreated residue, while cement + Fe0 further reduced uranium release to <1 μg L−1. Sulfate release was reduced to less than one-third of that from untreated residue. Overall, cement, FA, and Fe0 provided complementary structural, matrix-modifying, and uranium-reactive functions, enabling simultaneous control of mechanical integrity and contaminant mobility.