# A Comprehensive Review on Mine Tailings as a Raw Material in the Alkali Activation Process
## 4 Pre-Treatment Methods
### Heat Treatment  
Conducting a preliminary heat treatment prior to alkali activation has displayed the potential to augment the reactivity of mine tailings [133]. The heat treatment of mine tailings is a process aimed at modifying their chemical and physical properties by subjecting them to high temperatures to induce changes in their mineralogy and reactivity, improving their suitability for various applications. One specific effect of this treatment is the removal or alteration of hydroxyl groups, which are oxygen and hydrogen atom pairs (-OH) found in minerals [134]. The specific temperature range used in heat treatments can vary depending on the desired outcomes and the composition of the tailings. The high temperatures applied during heat treatment can lead to the removal of hydroxyl groups from minerals containing hydroxyl groups, such as clays and certain silicates present in the tailings, and transform them into more stable phases with altered properties for the formation of a 3D structure [135]. Moreover, elevating the temperature can facilitate the dissolution of aluminum and silicon units from the original material, heightening its reactivity in alkaline conditions. When mine tailings are subjected to high temperatures during heat treatment, the intense thermal energy causes the minerals to change, leading to the generation of amorphous phases. The higher amorphous content means that a larger portion of the material is now in a highly reactive state, crucial for subsequent processing steps, like alkali activation [136]. A pioneering investigation used a thermal pre-treatment before alkali activation to enhance the reactivity of tungsten mine tailings, which were notably abundant in quartz and muscovite. The study revealed a substantial thermal resistance in quartz, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mine Tailings** & **Procedure Parameters** & **Mineralogy** & **Mineralogical Changes** & **Ref.** \\ \hline Vanadium tailings & 60 min grinding; 10:1 weight ratio of grinding ball to powder & Quartz, feldspar, diopside, mullite, and hematite & Decreased amount of crystalline quartz; higher mechanical strength of alkali-activated material obtained with higher & [127] \\ Copper tailings & 1–4 h grinding; 10:1 weight ratio of grinding ball to powder & Quartz, dolomite, chlorite, and albite & Decreased content of albite (lower crystalline silica content) & [130] \\ \hline Antigorite waste & 600 s grinding; 6:1 weight ratio of grinding ball to powder & Mg and Si minerals & Cementitious properties obtained in milled mine tailings & [131] \\ \hline Phyllite waste & 9–15 min grinding & Clay, quartz, and albite & Reduction in chamosite and muscovite; decreased albite content & [132] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of studies regarding mechanically activated mine tailings. whereas a portion of the muscovite (approximately 12%) remained unchanged even after exposure to temperatures exceeding 950 \({}^{\circ}\)C [137]. Subsequent research indicated that boron mine tailings, which included Ca and Mg minerals like colemanite and hydroboracite, transformed into an amorphous state at 600 \({}^{\circ}\)C [138]. Illite, a mineral with properties akin to those of muscovite, underwent decomposition at 800 \({}^{\circ}\)C, while albite decomposed only at temperatures above 900 \({}^{\circ}\)C [139; 140]. Another study delved into the utilization of heat-treated phosphate mine tailings as a raw material in alkali activation in conjunction with fly ash and metakaolin. During alkali activation, dolomite and calcite underwent a reduction process, whereas other phases, like quartz, fluorapatite, and multite, remained unaffected [141]. In the case of kaolinite tailings, calcination at 750 \({}^{\circ}\)C emerged as the most efficient approach to enhance their reactivity. This is attributed to the probable transformation of kaolinite into metakaolin during decomposition [142].