# Use of Mineral Waste for the Production of Artificial Aggregates
## 3 Slags from Iron and Steel Production
### Phase and Chemical Composition of Slags  
Slags are products formed by exposure to high temperatures, which determine their mineral (phase) composition (Table 3). Slags deposited on heaps are subject to hypergenic processes, which can lead to the formation of new minerals. The most common phase components found in slags include the following:  
* glass, which is the predominant constituent. It is formed by the rapid cooling of the melt. Subject to hypergenic processes, the glass may devitrify. Other components such as metallic aggregates and mineral phase crystals may be present in the glass;
* metallic aggregates are solidified droplets and aggregates of metallic iron or iron alloys with other metals, which have not separated from the gangue during the metallurgical process and have not migrated into the molten pig iron or steel alloy;
* oxide phases, which are mainly represented by various forms of iron oxides, such as wustite FeO, hematite Fe\({}_{2}\)O\({}_{3}\), magnetite Fe\({}_{3}\)O\({}_{4}\), as well as ilmenite Fe, TiO\({}_{3}\), chromite FeCr\({}_{2}\)O\({}_{4}\), periclase MgO, and calcium ferrites. Slags also contain native lime CaO, which can hydrate in contact with water, leading to an increase in the pH of the environment (a strongly alkaline environment is created). The hydration of lime results in the formation of Ca hydroxides, which causes an increase in the volume of the material;
* silicate phases crystallize from the melt and the rapid cooling rate of the melt means that they usually represent individual crystals dispersed in the glass. Of the silicates found in slags, the most commonly described are dicalcium silicates (lamrite 2 CaO-SiO\({}_{2}\)), melilites (akermanite and gehlenite), and pyroxenes represented by augite, clinoenstatite, and others. The mineral composition of slags often includes mullite, which is a transformation product of kaolinite admixed in carbonate rocks used as a flux. Also found is alite, a silicate phase with the formula 3 CaO-SiO\({}_{2}\), which does not occur in nature, but is characteristic of clinkers. Among the least abundant phases reported in steelmaking and blast furnace slags are olivines (mainly fayalite Fe\({}_{2}\)[SiO\({}_{4}\)]) and monticellites. Occasionally, quartz is also identified as an impurity in the feedstock, which may have been transformed into cristobalite or tridymite by exposure to high temperatures [24; 26; 27; 28; 29; 30; 31]. A richer mineral composition is observed in slags deposited on heaps that have been subjected to hypergenic factors for a long time, compared to the currently produced slags. The mineral inventory is enriched with secondary phases, mainly represented by gypsum, calcite, aragonite, goethite, and brucite [24; 27; 29; 32]. \begin{table}
\begin{tabular}{l l l} \hline \hline
**Group** & **Phase** & **Chemical Formula** \\ \hline \multirow{6}{*}{oxide phases} & wüstite & FeO \\  & hematite & \(\alpha\)–Fe\({}_{2}\)O\({}_{3}\) \\  & magnetite & Fe\({}_{3}\)O\({}_{4}\) \\  & ilmenite & FeTiO\({}_{5}\) \\  & chromite & FeCr\({}_{2}\)O\({}_{4}\) \\  & periclase & MgO \\  & lime & CaO \\ \hline \multirow{6}{*}{silicate phases} & glass & - \\  & larnitte & 2 CaO-SiO\({}_{2}\) \\  & akermanite & Ca\({}_{2}\)Mg[Si\({}_{2}\)O\({}_{7}\)] \\  & gehlenite & Ca\({}_{2}\)Al[(SiAl)\({}_{2}\)O\({}_{7}\)] \\  & pyroxenes & Mg\({}_{2}\)Si\({}_{2}\)O\({}_{6}\)-Fe\({}^{2+}\)\({}_{2}\) Si\({}_{2}\)O\({}_{6}\) \\  & mullite & Al\({}_{6}\)Si\({}_{2}\)O\({}_{13}\) \\  & olivines (fayalite) & Fe\({}_{2}\)[SiO\({}_{4}\)] \\  & monticellite & CaMg[SiO\({}_{4}\)] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Main phase components of slags from iron and steel production [23; 24; 27; 28; 26; 18]. The main chemical component of slags from the iron and steel industry is CaO, the content of which can reach more than 50 wt.%. The SiO\({}_{2}\) content in blast furnace slags is higher (up to 45 wt.%), while in steelmaking slags it usually does not exceed 15 wt.%.