PSI - Issue 82

Samia M. Mohamed et al. / Procedia Structural Integrity 82 (2026) 213–219 S. M. Mohamed et al. / Structural Integrity Procedia 00 (2026) 000–000

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Fig. 3. Leaching patterns of different metals across a pH range that are classified into: a) neutral salts, b) cationic, c) Amphoteric, and d) Oxyanionic (Sun et al., 2019) 2.2.3. Red Mud (RM)-Based AAMs Red mud (RM)-based AAMs exhibit complex leaching behaviors influenced by pH, activator chemistry, and RM composition. As a highly alkaline byproduct of alumina production (pH 10–13), RM is increasingly used in AAMs but often blended with FA, GGBS, or MK to reduce leaching risks (Kumar et al., 2023). pH plays a critical role in leaching mechanisms. Ye et al. (2016) found that oxyanions like chromate and vanadate are most mobile in highly alkaline conditions (pH > 11), especially in RM-FA blends. Li et al. (2021) confirmed that vanadium, present in RM at 500–2000 mg/kg, leaches as VO₄³⁻ at high pH but can precipitate as Ca₃(VO₄)₂ at pH 8–10. Similarly, Cr(VI) shows increased mobility at pH ~12 due to weak retention in the geopolymer matrix (Tian et al., 2024). Sun et al. (2022) examined the leaching behavior of elements in five different mixes. The first mix (RMG-1) contains a high Ca content due to the presence of GGBS, which leads to high Ca leaching. Si showed higher leaching in the RMG-2 mix because it contains 50% RM-1, which has a higher silica content compared to RM-2, along with 50% GGBS. In contrast, RMG-4 includes 50% RM-2 and 50% GGBS, and since RM-2 has higher lead levels, this explains the increased Pb leaching. Vanadium (V) exhibited elevated leaching in RMG-5, which is composed of 50% RM-2 and 50% MK, because it contains less Ca than the other mixes, resulting in less precipitation and a more sustained release of vanadium (Sun et al., 2022), Fig. 4. Activator type significantly affects leaching. Sodium silicate improves heavy metal immobilization by forming Fe-O-Si bonds, resulting in a denser matrix (Ye et al., 2017). In addition, incorporating FA or GGBS improves both the mechanical properties and the leaching resistance of RM-based AAMs. Bobirică et al. (2020) found that a geopolymer composed of bottom ash, 10% red mud, and waste glass achieved compressive strengths of 7.3–14.5 MPa while maintaining low leaching levels. However, increasing RM to 30% decreased strength and elevated leaching rates (Bobirică et al., 2020). RM alone has limited immobilization capacity due to its high sodium content and weak gel structure. Hyeok-Jung et al. (2018) reported sodium leaching of over 22 mg/L in mixes with 30% RM, compared to 0.81 mg/L in slag-only mixes. RM–FA blends form stable aluminosilicate gels that trap contaminants. Zhao et al. (2022) showed RM-based binders could immobilize over 96% of Cu, Pb, and As under alkaline conditions. RM-based geopolymers exhibit different leaching behaviors for heavy metals such as Cu, Zn, Cd, Pb, Fe, and Cr. For example, it is reported that blending RM with FA and activating with sodium silicate significantly reduced heavy metal leaching compared to the raw materials, keeping concentrations within both the EU Landfill Directive and US EPA limits. Similarly, combinations with RHA showed comparable improvements in heavy metal immobilization, with most elements, especially lead, cadmium, and copper, being effectively bound within the aluminosilicate matrix (Nguyen et al., 2018). !