# A Comprehensive Review on Mine Tailings as a Raw Material in the Alkali Activation Process
## 3 Alkali Activation
### Mine Tailings as a Filler Material in Alkali Activation
It was found that with a greater content of metakaolin, there was a greater amount of tetrahedral silicon (Si) coordination in the geopolymer gel, and the greater metakaolin content resulted in the final material having better mechanical properties [118]. Furthermore, gold mine tailings that were rich in dolomite, quartz, sulfur, and mullite were studied. The study highlighted the necessity of a suitable co-binder, such as blast furnace slag or metakaolin, for the alkali activation process and suggested that the gold mine tailings alone may not have been reactive enough to achieve the desired hardening. A matrix is suitable for construction material was generated by incorporating a higher co-binder content. However, employing a high concentration of NaOH (15 M) led to a notable reduction in the workability of the resulting paste, potentially exerting a substantial influence on the properties of the final material [119]. The use of alkali activation in paste backfill technology represents an intriguing avenue in sustainable construction practices. An inverse relationship is observed when employing alkali-activated blast furnace slag in conjunction with gold mine tailings containing quartz, albite, mica, sanidine, and microcline for the production of paste backfill material; as the compressive strength of the final matrix increases, the workability of the mixture decreases [120]. The superior strength exhibited by the matrix, compared to Portland-cement-based paste, under optimal curing conditions and with an ideal concentration of the alkali activator (a blend of NaOH and Na\({}_{2}\)SiO\({}_{3}\)), affirms the appropriateness of alkali-activated slag as a co-binder in paste backfill technology [121]. Alkali-activated fly ash can serve as a co-binder for sulfidic-rich mine waste rocks, enabling the production of paste backfill materials or construction materials. Utilizing sulfidic waste rock as the aggregate significantly reduces the acid generation potential of the waste in landfills, resulting in the production of environmentally acceptable materials. This stands as one of the foremost advantages of employing alkali activation in paste backfill technology, as opposed to ordinary Portland cement (OPC) [122]. Mine tailings, which may have a limited reactivity on their own, can contribute to the final material when used as an inert filler when suitable co-binder materials, like metakaolin or blast furnace slag, are chosen. Each type of mine tailing requires a meticulous study of mix design due to the influence of mineralogy, chemical composition, and physical properties on co-binder materials, alkali activators, and process conditions [123]. However, the accessibility of a co-binder is equally crucial, as the mining site may not always be in close proximity to the industry or manufacturing facility that produces a compatible co-binder. The summarized results of relevant studies are shown in Table 2. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Mine Tailings**} & **Procedure** & **Main** & \multirow{2}{*}{**Mineralogy**} & \multirow{2}{*}{**Final Properties**} & \multirow{2}{*}{**Ref.**} \\  & **Parameters** & **Elements** & & & \\ \hline Metal, coal, uranium, and potash & Liquid toxic waste & & & & \\  & mixed with a & & & & High immobilization of heavy metals; \\  & & geopolymer & - & & high mechanical performance \\  & tailings & precursor; & & & \\  & curing at 60 \({}^{\circ}\)C & & & & \\ \hline Kaltails & Fly ash as a & Si, Fe, Al, Ca, & & Maximum mine tailing content of & \\  & co-binder & Mg, Na, and & & 70 wt\% & \\  & & SO\({}_{3}\) & & stronger hardpan achieved with greater & \\ \hline Copper & Fly ash as a & & & & \\  & co-binder; & & & & Improved mechanical strength \\  & tailings & curing at 90 \({}^{\circ}\)C; & - & potassium & with higher \\  & pressure used in & & aluminum silicate, & co-binder content and elevated curing & \\  & preparation stage & & and gypsum & temperature & \\ \hline \hline \end{tabular}
\end{table}