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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1. Introduction The production of Ordinary Portland Cement (OPC) significantly contributes to greenhouse gas emissions, as it releases nearly one ton of carbon dioxide (CO 2 ) for every ton of cement manufactured, making it a major human made source of CO 2 . Alkali-activated materials (AAMs) are considered an alternative to ordinary Portland cement (OPC) because they offer lower carbon emissions, superior strength, and resilience in harsh conditions (Shi et al., 2011). AAMs are synthesized by chemically activating aluminosilicate sources using alkaline solutions. They are commonly produced from industrial by-products, which help utilize waste and reduce environmental damage (Provis 2018). Developing AAMs starts with dissolving the raw materials in a highly alkaline solution. This involves a series of chemical reactions, forming a solid binder that often matches or surpasses the performance of regular cement (Duxson et al., 2007). Depending on the precursor, the resulting binder may form either a geopolymer gel (N-A-S-H) in low calcium systems or a C-A-S-H gel in high-calcium systems (Provis and Bernal, 2014). Fig.1 shows SEM images that revealed C-A-S-H gels formed in slag regions (Ca/Si ≈ 1.85) and N-A-S-H gels with Si/Al > 2 formed around fly ash particles (Vázquez-Rodríguez et al., 2023), Fig. 1. Interest in AAMs has grown quickly, with researchers exploring better mix designs, deeper insights into the chemical processes involved, and the long-term behavior of these materials (Juenger et al., 2011). Still, ongoing challenges persist, including preventing surface salt deposits (efflorescence), minimizing shrinkage, and establishing consistent production standards (Oladele et al., 2023). As the demand for greener construction practices rises, AAMs are likely to become a key solution in lowering the environmental issues related to the cement industry. Among the most widely used precursors for alkali-activated materials are industrial by-products such as blast furnace slag, fly ash, and red mud, which not only enhance the sustainability of AAMs but also help mitigate the environmental burden of industrial waste accumulation (B. Sun et al., 2022). Blast furnace slag, a by-product of iron and steel production, is rich in calcium and silica, forming high-strength C-A-S-H gels upon alkali activation (Shi et al., 2003). Fly ash, a residue from coal combustion, is predominantly aluminosilicate-based, making it suitable for producing low-carbon geopolymer binders (Hardjito et al. 2004). Red mud, a highly alkaline waste product from alumina refining, presents significant disposal challenges from the Bayer process. However, it exhibits potential as a supplementary precursor due to its high iron and aluminum content (Occhicone et al., 2021). Utilizing these wastes in AAMs reduces landfill demands and lowers the energy consumption and CO 2 emissions associated with traditional cement production (Turner and Collins 2013). Construction materials can significantly impact the environment by interacting with groundwater, surface waterways, marine waters, and soil, potentially releasing harmful compounds through leaching processes (Nativio et al., 2024). Therefore, a comprehensive review of the leaching potential of alkali-activated compounds is essential to identify risks and benefits, ensure safe and optimal material design, support environmental regulations, and promote wider use of AAMs in infrastructure and green buildings by assessing the environmental risks of waste or by-product-based AAMs, which is crucial for a circular economy. 2. Leaching Mechanism in Alkali-activated Materials 2.1. Primary Leaching Process There are several different ways that leaching takes place in AAMs. One of the most important processes is ion exchange, which involves replacing alkali metal ions like sodium (Na⁺) and potassium (K⁺) in the binder matrix with hydrogen ions (H⁺) from acidic surroundings. This process has been studied in the context of pore solution chemistry and chemical durability (Botti et al., 2022). Another key mechanism is diffusion-controlled release, in which ions slowly migrate across the material's linked pore networks (Zhang et al., 2022). The matrix dissolution, where the binder phases of the AAM break down, becomes significant in more severe chemical conditions (Sun 2024). Furthermore, surface wash-off might happen, which is defined by the quick release of ions that are loosely bound as soon as they come into contact with water (Sun et al., 2022).