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. 2. Relative release for a) chlorine, fluorine, potassium, sodium, and b) aluminium, calcium, and silicon (Gijbels et al., 2019)

2.2.2. Fly Ash (FA)-Based AAMs FA-based AAMs show leaching behavior that varies according to pH, activator type, and material composition. FA is regularly used as the main binder in these investigations, frequently in conjunction with secondary components such as metakaolin, incinerator fly ash, or blast furnace slag. Koplík et al. (2016) showed that heavy metals like barium, lead, and copper were effectively immobilized in FA GGBS matrices through the formation of insoluble salts in amorphous phases, aided by the high-pH environment created during alkaline activation. On the other hand, Sun et al. (2019) classified leaching behaviors in FA-metakaolin mortars into three types: cationic (increased leaching in acidic conditions), amphoteric (leaching in both acidic and alkaline conditions), and oxyanionic (dual peaks at low pH, indicating weak retention in alkaline environments). Chloride and sulfate leach independently of pH, suggesting they exist in soluble forms (see Fig. 3). Chloride and sulfate leach independently of pH, suggesting they are in easily soluble forms. Consistent with previous research, Xu et al., 2019 confirmed that leaching depends on pH, FA content, and metal type (Xu et al., 2019). Škvára et al. (2009) found that low-calcium brown coal FA effectively immobilized zinc, copper, cadmium, and trivalent chromium due to strong bonding and gel encapsulation. However, arsenic and hexavalent chromium were poorly immobilized due to weak ionic interactions. Srinivasamurthy et al. (2023) studied long-term leaching (90 days) in binders with varying FA-slag ratios and Na₂O contents. They found Na⁺ leaching was reduced by slag addition and lower Na₂O content. Leaching of Si⁴⁺, Al³⁺, and Ca²⁺ indicated gel degradation, especially in mixes with high Na₂O or low slag content. Several studies confirmed that slag addition reduces leaching, Ivan Diaz-Loya et al. (2012) and Yliniemi et al. (2015) found that Ba, Pb, and Zn were strongly immobilized after alkali activation of FA. In contrast, Yliniemi et al. (2015) observed that As, Cu, Mo, Sb, and V showed increased leaching post-activation, despite their low mobility in raw FA. Based on the findings of Lin et al., 2020, the activator type was identified as the most influential factor for heavy metal leaching (Lin et al., 2020). For instance, Na 2 SiO 3 was more effective for stabilizing Pb and Zn, while DI water better reduced As, Hg, and Se. Using wood fly ash (WBA) with coal fly ash (CFA), toxic metals were effectively immobilized, especially with DI water, low water-to-ash ratios, and sieving. Nguyen et al. (2018) found that geopolymerization using sodium silicate significantly reduced heavy metal leaching in FA-red mud binders.