# A Comprehensive Review on Mine Tailings as a Raw Material in the Alkali Activation Process
###### Abstract
Within paste backfill technology and the stabilization of mine tailings, the prevalent choice for a binding agent is Portland cement (including cement, lime, and other calcium-containing materials) [26; 27]. Nevertheless, the production of Portland cement results in significant CO\({}_{2}\) emissions, making it suboptimal for employment in paste backfilling [28]. Furthermore, in cases in which mine tailings possess the potential to generate acid, utilizing cement-based paste backfilling is not appropriate due to the adverse impact of the resulting acid on the formed matrix. On the other hand, the utilization of pozzolanic products in paste backfill technology is a notable approach that can enhance the properties and sustainability of the backfill material. Pozzolanic materials, such as blast furnace slag or fly ash, react with calcium hydroxide in the presence of water to form cementitious compounds, adding to the strength and durability of the mixture and offering enhanced acid resistance [29]. Since cement-based stabilization may not always be the optimal approach for mine tailings, an alternative strategy for repurposing mining waste is to utilize the tailings as a raw material in alkali activation to create an environmentally friendly construction material for subsequent use [30]. Alkali activation is a stabilization method that is particularly effective with raw materials rich in silicon and aluminum and relies on the reaction between alkaline solutions (often sodium or potassium hydroxide) and source materials containing silicon and aluminum. These elements are essential for the formation of the three-dimensional network of linked mineral compounds known as geopolymers. The raw material, which is rich in silicon and aluminum, is mixed with an alkaline solution, typically containing sodium hydroxide (NaOH) or potassium hydroxide (KOH), serving as an activator for the reaction and raising the pH of the mixture, making it highly alkaline [31]. The high-pH environment triggers the dissolution of silicon (Si) and aluminum (Al) components from the raw material. These elements are released as ions into the solution, undergo a complex series of chemical reactions, and combine with hydroxide ions from the alkaline solution to form new compounds, such as aluminosilicate gel, which is the basis of the three-dimensional network structure of the geopolymer [32]. In the newly formed aluminosilicate gel, the silicon and aluminum atoms are linked together through shared oxygen atoms, similar to how cement particles in traditional concrete are connected through calcium silicate hydrate (C-S-H) bonds [33]. Materials activated through alkali processes exhibit enhanced mechanical properties, a robust level of durability, and resistance to acids and thermal fluctuations [34]. Alkali-activated material shows the ability to immobilize hazardous components such as heavy metals within their structure, significantly contributing to environmental protection and the safe management of waste materials [35]. Mine tailings, especially those generated from the extraction of metal ores, have gained significant attention as potential precursors in alkali activation due to their abundant content of silicon and aluminum, which are essential elements for the formation of the geopolymeric structure through the alkali activation process [36; 37; 38]. One critical challenge associated with using mine tailings in alkali activation processes is the low reactivity of these materials, and it can vary depending on factors such as mineralogical composition, particle size distribution, pre-treatment, and chemical characteristics [39]. Mine tailings with poor reactivity can still be valuable as a filler material when paired with the appropriate co-binder. This approach allows for the utilization of tailings in construction applications while addressing their reactivity limitations. To deploy mine tailings as a reactive material in alkali activation, enhancing the reactivity of the minerals contained within them is essential [40].