Electrochemical response of solidification Cu²⁺ contaminated soil influenced by red mud/fly ash ratio

The main purpose of this work was to study a new method for evaluating the solidification of contaminated soil based on electrochemical impedance spectroscopy (EIS). To explore how the EIS parameters were affected by the pore structure and mesostructure of the cured system, the physical and mechanical properties, leaching toxicity, microstructure, and EIS of the stabilized contaminated soil were tested after 7, 28, 60, and 90 days of curing. Based on the EIS results, a physical and equivalent circuit model of the stabilized contaminated soil's impedance response was established to reveal the mechanism of binder-heavy metal ion-soil interaction. The results showed that as the red mud (RM)-fly ash (FA) mass ratio and curing age increased, the strength and structural compactness of the solidified body also increased. The best curing effect was achieved with an RM-FA mass ratio of 7:3 after curing for 90 days. The equivalent circuit model of the solidified body obtained by EIS was R_s (Q₁ (R_ct1W) Q₂R_ct2). The pore solution resistance R_s, solid-liquid interface ion transfer resistance Rct 1, and unconfined compressive strength (UCS) q_u all showed an increasing trend with increasing RM-FA mass ratio and increasing curing time. Fitting the model demonstrated that both R_s and R_ct1 were closely correlated with the strength of the solidified bodies. These conclusions were further verified by scanning electron microscope (SEM) experiments. Overall, this work demonstrates that the strength characteristics of solidified bodies can be evaluated by EIS and reveals the microscopic mechanism of the solidification of Cu²⁺-contaminated soil.