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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2.2.

Leaching Characteristics of Different By-Product-Based AAMs

2.2.1. Ground Granulated Blast-Furnace Slag (GGBS)-Based AAMs The leaching behavior of AAMs based on GGBS varies with the type of activator, formulation, and curing conditions. Gijbels et al. (2019) found that sodium silicate in Sample 1 improved leaching resistance, especially for mobile ions like Na⁺, K⁺, and S. In contrast, Samples 2 and 3, activated with NaOH alone, showed higher leaching, with Sample 2 performing worst due to excess water weakening the gel structure, as shown in Fig. 2.

Fig. 1. Pastes activated with 4M of NaOH: a) Hydration gels of 60S40MFA L/S = 0.25, b) chemical reaction of FA with NaOH (Vázquez Rodríguez et al. 2023)

Sithole et al. (2021) reported minimal heavy metal leaching using 15 M NaOH, with leachate concentrations decreasing over time, indicating improved immobilization. In comparison, Chen et al. 2022 found that the geopolymer approach can be used in the solidification/stabilization of heavy elements. Mohamed & Al Khattab (2022) found good acid resistance and minimal mass loss when mortar specimens were submerged in 10% sulfuric acid for up to 90 days, with mixes made with 50% fly ash showing excellent resistance to acid. Similarly, Li et al. (2024) found that slag mortars with 20% fly ash enhanced stabilization, with ettringite formation keeping chromium, antimony, and lead below threshold levels. Salas et al. (2024) showed that although raw materials like rice husk ash (RHA), spent diatomaceous earth (SDE), and almond shell biomass ash (ABA) initially exceeded leaching limits, alkali activation reduced hazardous trace elements to below regulatory thresholds such as WAC and Dutch SQD. Likewise, Ling et al. 2024 showed that incorporating biomass bottom ash (BBA) reduced leaching of lead, chromium, molybdenum, and zinc, though higher BBA levels increased chloride and sulfate leaching. Liu et al. (2023) confirmed that activating bottom ash (BA) with magnesia and sodium aluminate significantly reduced leaching. Activator chemistry plays a key role in leaching and durability. NaOH-activated systems tend to leach more sodium due to excess alkali, as noted by Gong et al. (2019). In contrast, Sodium silicate-based systems form denser gels with lower porosity, reducing ion diffusion and improving leaching resistance (Bernal et al., 2014). Slag composition also matters; a higher CaO/SiO₂ ratio promotes calcium-alumino-silicate-hydrate (C-A-S-H) gel formation (Li et al., 2025); however, this may increase calcium leaching. GGBS contains trace elements like chromium, manganese, and vanadium, with Cr(VI) being particularly mobile and environmentally concerning (Zhang et al., 2024). High alkalinity (pH > 12) in AAMs affects metal mobility. Guo et al. (2017) showed that cationic metals like lead, cadmium, and zinc are immobilized through hydroxide precipitation. On the other hand, it is described how some oxyanions, such as chromate, arsenate, and selenate, become more mobile in such alkaline environments (Izquierdo et al., 2009). Over time, carbonation can lower pH and remobilize previously stabilized metals (Bernal et al., 2014). Curing and exposure conditions further shape durability. Heat curing at 60–80 °C accelerates polymerization and yields denser, less porous matrices with lower leaching potential (Najafi Kani et al., 2012). Conversely, ambient curing often results in an incomplete reaction, leaving behind more unreacted alkalis, which can contribute to elevated leaching levels, particularly of sodium and hydroxide ions (C. Liu et al., 2023). Environmental exposure also matters repeated wetting– drying cycles tend to accelerate leaching relative to continuous immersion, likely because drying induces microcracking and mechanical stresses that open transport paths(Li and Li, 2020). In marine conditions, seawater’s high chloride content increases Cl⁻ ingress and can elevate Na⁺ leaching, as reported by Jun et al. (2021).

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