# A Comprehensive Review on Mine Tailings as a Raw Material in the Alkali Activation Process
## 3 Alkali Activation
A higher alkalinity promotes more robust reactions and leads to the formation of a denser and stronger structure, and higher concentration can accelerate the activation process, potentially leading to faster setting times and higher early strength development [87]. The most common types of alkaline reagents used as activators in alkali activation processes are hydroxides (e.g., sodium hydroxide and potassium hydroxide) and silicates (e.g., sodium silicate). Sodium hydroxide (NaOH) is a strong alkaline compound, is highly soluble in water, and provides a significant source of hydroxide ions (OH\({}^{-}\)) for the activation process. Potassium hydroxide (KOH) shares properties similar to NaOH, and like NaOH, KOH provides hydroxide ions to drive activation reactions. Sodium silicate (Na\({}_{2}\)SiO\({}_{4}\)) is an alkaline compound derived from the reaction of sodium carbonate and silica sand, providing both alkalinity and a source of reactive silicon, making it a versatile activator for alkali-activated materials [88]. Alkali-activated materials (AAMs) can immobilize harmful components like heavy metals due to their unique chemical and mechanical properties. AAMs have a robust chemical structure formed through polycondensation reactions between silicon (Si) and aluminum (Al) in the aluminosilicate raw materials. This structure is resistant to chemical breakdown, providing a stable matrix for encapsulating heavy metals [89]. The reactive sites within the AAMs' 3D structure can undergo ion exchange reactions. Heavy metal ions can bind to these sites, displacing other cations, and some heavy metals can be adsorbed onto the surfaces of AAM particles. Moreover, during the activation process, chemical reactions can lead to the formation of low-solubility phases. These phases can incorporate heavy metal ions, effectively immobilizing them within the material [90]. AAMs have good mechanical properties, so they can physically encapsulate heavy metal particles or ions within their structure, preventing their release into the surrounding environment. Finally, the chemical and physical properties of AAMs can reduce the reachability of heavy metals, which means that even under conditions in which an AAM is exposed to water, the release of heavy metals is minimized [91]. When different co-binder materials are introduced, the immobilization mechanism becomes more complex because each co-binder material may have its own chemical properties, which can lead to various interactions with heavy metals. When multiple co-binders are used together, their synergistic effects can enhance the immobilization process; the presence of multiple co-binders introduces heterogeneity to the mixture, making it more challenging to predict and analyze the immobilization process [92]. Co-binders can have different particle sizes and shapes, which affect their accessibility to heavy metals and the overall immobilization efficiency. Furthermore, the pH and redox conditions of the environment can influence how co-binders interact with heavy metals. These conditions can vary depending on factors such as local geology, water chemistry, and microbial activity [93; 94; 95]. Finally, alkali activation technology is a promising alternative to traditional Portland cement, but its practical application faces the following challenges: (1) The properties of alkali-activated materials are highly dependent on the source and composition of raw materials, such as fly ash, slag, and other industrial by-products, leading to inconsistent performance. (2) Design mixes for alkali-activated materials are challenging due to the lack of standardized guidelines and the need for the precise control of the activator concentration, water content, and curing conditions. (3) Alkali-activated materials can have excellent levels of durability, but there are concerns related to alkali-aggregate reactions, sulfate attack, and carbonation. More research is needed to fully understand and mitigate these issues.