# A Comprehensive Review on Mine Tailings as a Raw Material in the Alkali Activation Process
## 3 Alkali Activation  
Alkali activation creates new construction materials from silicon- and aluminum-rich raw materials by mixing these raw materials with a highly alkaline solution. This solution, typically an alkali hydroxide or silicate, serves as an activator. It increases the pH of the mixture and facilitates the formation of a new 3D concrete-like structure. Various elements like iron (Fe), magnesium (Mg), and calcium (Ca) can also play crucial roles in the reaction. These elements contribute to the formation of calcium-(sodium) aluminosilicate hydrate (C(N)ASH), which is a complex amorphous gel-like structure with cementitious properties, similar to the products of cement hydration in ordinary Portland cement and Mg-Al-layered double hydroxide, which can contribute to the mechanical properties and stability of the resulting material. Alkali-activated mine tailing concrete and mortar show a promising level of durability against aggressive ions such as chlorides and sulfates. The durability of these materials can be influenced by the type of activator used, the mix design, and the curing conditions [80]. In terms of chloride resistance, the high alumina and silica content in mine tailings, when activated, can form a dense and chemically stable matrix, reducing the permeability of the concrete or mortar and limiting chloride ingress. The choice of activator (e.g., sodium hydroxide or sodium silicate) affects the pore structure and binding phases, producing more amorphous aluminosilicate gels that tend to enhance chloride resistance. Moreover, proper curing is crucial for developing a dense microstructure that improves chloride penetration resistance. Regarding sulfate resistance, alkali-activated materials often exhibit better levels of resistance than ordinary Portland cement (OPC) due to the absence of calcium hydroxide, which reacts with sulfates to form expansive compounds like ettringite. Meanwhile, incorporating supplementary materials, such as fly ash or slag, can further improve sulfate resistance by refining the pore structure and reducing permeability [81]. When the calcium content is low in alkali-activated materials (AAMs), they are called geopolymers. Geopolymers are a subset of AAMs with distinct characteristics and are defined as inorganic, amorphous to semi-crystalline aluminosilicate materials formed by the reaction of a source material rich in silicon and aluminum with an alkaline activator. Geopolymers, like other AAMs, form a three-dimensional structure through a process known as polycondensation. This results in a network of linked silicon and aluminum atoms [82]. The resulting 3D structure of geopolymers can vary in crystallinity. Some geopolymers may have a poorly crystalline structure, while others may exhibit more crystalline characteristics, depending on the specific raw materials and curing conditions used [83]. Geopolymers often possess excellent mechanical properties, high levels of resistance to chemical attack, and good levels of thermal stability. They also offer sustainability benefits similar to those of other AAMs, including the use of industrial by-products and waste materials, reduced carbon emissions, and lower energy consumption during production. These properties of alkali-activated materials (AAMs) position them as a more sustainable alternative to traditional construction materials like ordinary Portland cement (OPC) [84; 85]. The choice of raw materials in alkali activation depends on their reactivity and composition. Among the various aluminosilicate materials suitable for alkali activation, metakaolin and blast furnace slag have received an extensive amount of attention and research [86]. The properties of the alkaline activator play a crucial role in determining the final structure and properties of alkali-activated materials (AAMs).