## 3 The Role of Mining Tailings in Advancing Sustainable Construction Practices
### Tailings-Based Geopolymer  
Geopolymers are synthesized from a complex three-dimensional network primarily composed of Si-O-Si and Al-O-Si bonds. These networks typically form through the reaction of aluminosilicate minerals (like fly ash or slag) with a concentrated alkali hydroxide or silicate salt solution. The geopolymerization process begins with the formation of Si(OH)\({}_{4}\) or Al(OH)\({}_{4}\) oligomers in the presence of an alkali solution enriched with Al and Si. This process progresses until the condensation of the metal-oxygen bonds occurs, ultimately forming the geopolymer structure. During alkali activation, it is also possible to form hydrated C-S-H or C-A-S-H cementing gels [182]. The geopolymerization process is influenced by numerous factors, including the type and concentration of the alkali activator, the curing conditions, and the mineralogical composition of the aluminosilicates used. Other variables that can impact the process include the SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio, the concentration of the alkali solution, the curing time and temperature, the liquid-to-solid ratio, and the pH [183]. These variables allow for the utilization of geopolymers in various applications, such as in cement and concrete, fire-resistant materials, and toxic waste containment [184].  
Given their silica/aluminosilicate-based mineral composition, mine tailings are considered potential candidates for the geopolymer industry [185]. However, unlike pozzolanic materials, like slag, which mainly consist of amorphous minerals (e.g., SiO\({}_{2}\)), mine tailings typically have a more crystalline mineral structure. This crystallinity is due to the fact that mine tailings are usually produced through wet chemical processes (like flotation), unlike other by-products formed at high temperatures (like slag). This makes mine tailings less chemically reactive in the geopolymerization process. For instance, silica (-SiO\({}_{2}\)), an inert mineral in mine tailings, reacts less with alkali solutions due to its ordered crystalline structure [186]. Therefore, pretreatment (such as thermal) or blending with other aluminosilicates might be necessary to enhance the reactivity of mine tailings in geopolymer applications. Nonetheless, the formation of a silica polymer (Si-O-Si derived from silica sol) around the tailings' particles can help stabilize and build the tailings-based geopolymer structure [172]. Recent applications of tailings-based geopolymers have included iron tailings/fly ash-based geopolymers used as binder materials due to their high thermal conductivities, low porosity, and microcracks, as well as enhanced setting times and ultimate compressive strength [181]. Phosphate tailings-based geopolymers also serve as binder materials due to their characteristics [187]. Slag/metakaolin/gold tailings-based geopolymers are used for lightweight aggregates thanks to their high strength characteristics [188]. Copper tailings-based geopolymers find application in road base construction materials [179] and bricks [189], with copper tailings/cement kiln dust-based geopolymers noted for their stability and enhanced ultimate compressive strength. Silica-rich vanadium tailings/fly ash-based geopolymers are employed in fire-resistant materials due to their stability over a wide range of temperatures [172].