# Use of Mineral Waste for the Production of Artificial Aggregates
## 2 Coal Mining Wastes
### Chemical and Mineral Composition  
The mineral composition of unburned coal mining waste is dominated by quartz and clay minerals, mostly represented by kaolinite. Dispersed organic matter (coal) also makes a significant contribution, especially in claystones, shales, and siltstones. Minerals in the carbonate group are represented by siderite, dolomite, calcite, and ankerite, which likely constitute the matrix material in sandstones. Feldspars, muscovite, and biotite are also present. Sulphides are mainly represented by pyrite and marcasite, the content of which is particularly high in residues from coal flotation. Over time, sulphides can oxidize, leading to the acidification of the environment, and transform into sulphates [8; 11; 15; 17].  
The chemical composition of the waste is dominated by SiO\({}_{2}\), the content of which can be up to 50%. Al\({}_{2}\)O\({}_{3}\) and loss on ignition (LOI) follow in second place. Their content exceeds 20 wt.%. The Fe\({}_{2}\)O\({}_{3}\) content varies around 5 wt.%. K\({}_{2}\)O is usually present in an amount of around 2-3 wt.%. The CaO and MgO contents do not exceed 2 wt.%. The fraction of other constituents does not exceed 1% [3; 5; 8; 15; 18; 19].  
Waste heap fires result in a change in the mineral and chemical composition. Coal is burned out. Exposed to elevated temperatures, minerals of the carbonate group (siderite, calcite, dolomite) undergo thermal decomposition. Kaolinite is dehydroxylated and transformed into metakaolinite. Iron-containing minerals are oxidized and transformed into maghemite and hematite [8; 17]. As a result of these processes, the proportion of LOI decreases and the share of other ingredients increases (Table 1).  
If the temperature exceeds 1000 \({}^{\circ}\)C, the partial melting of the rock material occurs. Glass is formed in the waste, from which high-temperature minerals such as mullite, sillimanite, cordierite, and pyroxenes can crystallize. Under reducing conditions, magnetite and even native iron may crystallize (Figure 2) [5; 8; 13; 15; 17].  
\begin{table}
\begin{tabular}{l l l l l l l} \hline \hline \multirow{2}{*}{**Component**} & \multicolumn{2}{c}{**Unburned Coal Mining Waste**} & \multicolumn{2}{c}{**Thermally Altered Coal Mining Waste**} \\ \cline{2-7}  & **Min** & **Max** & **Average** & **Min** & **Max** & **Average** \\ \hline SiO\({}_{2}\) & 33.43 & 60.00 & 50.09 & 45.98 & 62.88 & 56.98 \\ Al\({}_{2}\)O\({}_{3}\) & 17.00 & 28.00 & 20.25 & 23.25 & 31.49 & 25.99 \\ Fe\({}_{2}\)O\({}_{3}\) & 1.50 & 5.54 & 4.81 & 3.48 & 16.44 & 8.05 \\ CaO & 0.03 & 2.70 & 2.09 & 0.11 & 3.21 & 0.83 \\ MgO & 0.30 & 3.76 & 1.95 & 0.40 & 1.93 & 1.17 \\ Na\({}_{2}\)O & 0.01 & 0.70 & 0.35 & 0.15 & 0.66 & 0.35 \\ K\({}_{2}\)O & 0.06 & 5.50 & 1.87 & 1.91 & 4.14 & 3.49 \\ SO\({}_{3}\) & 0.04 & 1.80 & 0.93 & 0.01 & 2.12 & 1.08 \\ LOI & 18.79 & 35.23 & 24.94 & 0.81 & 40.14 & 18.41 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of major chemical composition (in wt.%) of coal mining waste [3; 5; 13; 15].  
The mineral composition of thermally transformed waste differs significantly from the original one, which is revealed, among other ways, during the identification of phases by diffraction (XRD) (Figure 2 and Table 2).  
Transformations in mineral composition are accompanied by changes in chemical composition. Coal burnout increases the content of the other chemical constituents.