Comprehensive understanding long-term heavy metals release characteristics and environmental impacts of...

China's aggressive air pollution control policies have led to the widespread generation of ultra-alkaline fly ash (UAFAs). Compared with conventional fly ash, the risk characteristics, release mechanisms, and control strategies of UAFAs during its reuse in construction materials-such as roadbeds-remain fundamentally different yet poorly understood. This study systematically evaluates the lifecycle groundwater contamination risks of UAFAs-recycled road materials through accelerated carbonation experiments coupled with reactive transport modeling and Monte Carlo uncertainty analysis. Initial field simulations revealed temporarily stabilized acid-base equilibrium in UAFAs roadbeds, producing low heavy metal leaching (0.038-0.23 mg/L) within acceptable risk thresholds. However, progressive carbonation disrupted this metastable state, triggering dramatic heavy metal remobilization (5.6-1073 times) that exceeded China's Groundwater Category III standards by 1.6-12 times. Particularly, UAFAs from grate furnaces with "semi-dry deacidification + activated carbon + baghouse" systems exhibited extreme contamination potential, leaching Zn, Cd and Pb at 2.26-38.75 times higher concentrations than other treatment configurations. Regional risk simulations identified 5-fold spatial variability in heavy metal exposure concentrations across shallow aquifers, driven by hydrogeochemical heterogeneity. These findings demonstrate that conventional leaching tests neglecting carbonation dynamics significantly underestimate the lifecycle contamination risks of UAFAs applications by 47-89 %. We propose an urgent methodological reform integrating time-dependent carbonation models into risk assessment frameworks and recommend strict controls on UAFAs sources and regionalized management strategies for road construction applications.