# A Comprehensive Review on Mine Tailings as a Raw Material in the Alkali Activation Process
## 3 Alkali Activation
### Alkali-Activated Mine Tailings  
Mine tailings are a by-product of the mining industry, and their proper handling, disposal, and reuse are essential aspects of responsible mining practices to obtain various benefits, such as those shown in Figure 2. Testing the solubility of different elements in raw materials for alkali activation is crucial to understanding their reactivity. This procedure is a common method used to assess reactivity. After the reaction period, the solution is collected and analyzed using inductively coupled plasma spectrometry (ICP), which is a highly sensitive analytical method that can determine the concentrations of various elements in a sample [96]. These data help us understand which elements are readily available for reactions and incorporation into the final alkali-activated material. Elements that exhibit a high level of solubility under alkaline conditions are considered more reactive [97].  
Assessing the crystalline structure of a material through X-ray diffraction (XRD) and scanning electron microscopy (SEM) is another crucial method for estimating its reactivity. XRD provides information about the material's crystalline phases and their relative abundance, while SEM provides detailed images of the material's microstructure and surface morphology [99; 100]. Despite the abundance of silicon and aluminum in most mine tailings, their reactivity under alkaline conditions can be limited due to the highly crystalline structure of many mine tailings. The minerals in mine tailings often have well-defined crystalline structures. This means their atoms are arranged in a repeating pattern, which can resist dissolution and chemical reactions. Moreover, in highly crystalline materials, the surface atoms may be less chemically active compared to those in amorphous or poorly crystalline phases, reducing the likelihood of reactions with the alkaline solution [101]. The capacity of highly crystalline materials for ion exchange and chemical interactions with an activator may be limited and can hinder the dissolution of essential components needed for the activation process [102]. The poor reactivity of mine tailings is a significant challenge in their utilization for alkali-activated materials. Poorly reactive mine tailings may contribute less to the strength and durability of alkali-activated materials and can limit their usefulness as a primary component in construction. The slow reactivity of highly crystalline mine tailings can result in slower activation kinetics and require longer curing times or additional processing to achieve the desired properties [103]. Additional processing steps like mechanical activation or thermal treatment may be necessary to enhance  
Figure 2: Benefits of recycling and reusing mine tailings [98].  
the reactivity of mine tailings, adding complexity and cost to the recycling process. One way to enhance the reactivity of poorly reactive mine tailings is by combining them with a reactive co-binder, such as metakaolin or blast furnace slag, in an alkali activation process. This approach enhances the strengths of both materials to create a more effective and sustainable material [104]. However, the reactivity of mine tailings can be enhanced through various treatment methods aimed at modifying the physical and chemical properties of the tailings to make them more suitable for the alkali activation process. Finally, the decision to use mine tailings as a binder or inert filler should be made after carefully assessing their reactivity. This reactivity, in turn, is influenced by the mineralogical composition of the tailings and how they respond to various treatment processes.