# The Use of Construction Waste to Remediate a Thermally Active Spoil Heap
## 3 Results and Discussion
### Sequential Extraction Analysis
#### 3.5.1 Aluminium
Aluminium is generally one of the problematic metals found in the environment. In tailings, it is stabilized in the form of feldspars, either K-feldspar (KAlSi\({}_{3}\)O\({}_{8}\)), Na-feldspar (NaAlSi\({}_{3}\)O\({}_{8}\)), or Ca-feldspar (CaAl\({}_{2}\)Si\({}_{2}\)O\({}_{8}\)). This explains the high content of aluminium in the sample (about 10%). It is a naturally occurring mineral in the Moravian-Silesian Region. This also supports the results in terms of Si content in the XRF analysis. Additional Si occurs in quartz, which is also abundant here. This fact was also confirmed by the XRF analysis. Aluminium is generally very mobile in acidic environments. Thus, by creating a mixture of tailings and CDW, an increase in pH value can be achieved due to the higher calcium content in CDW (confirmed by XRF analysis). The released calcium ions (Ca\({}^{2+}\)) present in the CDW will displace the aluminium ions (Al\({}^{3+}\)) in the tailings, which will result in partial reduction in its availability and will therefore be stabilized. This can be simplified as follows:  
\[\text{CDW--Ca}+\text{Al}^{3+}\leftrightarrow\text{CDW--Al}+\text{Ca}^{2+}\] \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample Description** & **Fraction I** & **Fraction II** & **Fraction III** \\ \hline Tailings/CDW (ratio 9:1) & 283 & 1026 & 401 \\ Tailings/CDW (ratio 8:2) & 166 & 1139 & 389 \\ Tailings/CDW (ratio 7:3) & 138 & 1397 & 368 \\ Tailings & 790 & 719 & 493 \\ CDW & 51 & 1757 & \textless{}0.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Results of BCR analysis for Al in mg kg\({}^{-1}\) of dry matter. Aluminium stabilisation can also take place by the formation of low-soluble aluminium compounds that are less toxic to plants. Their formation can be facilitated by the addition of various supplements, such as material rich in calcium or phosphate fertilisers. The stabilisation of aluminium using the calcium cations (Ca\({}^{2+}\)) present in CDW can also be mediated by their reaction with aluminium ions to form the less soluble compound Ca\({}_{3}\)Al\({}_{2}\)(OH)\({}_{12}\), which is less soluble in acidic environments. The equation describing this stabilization may be as follows:  
\[2\;\text{Al}^{3+}+3\;\text{Ca}^{2+}+12\;\text{OH}^{-}\rightarrow\text{Ca}_{3} \text{Al}_{2}\text{(OH)}_{12}\]  
The addition of phosphate-based fertilizers can not only reduce the availability of aluminium in the reclaimed subsoil, but can also promote the growth of vegetation in the reclaimed area. The reactions are complex and can involve various interactions, including the precipitation and ion exchange processes. For example, when phosphate fertilisers are added, aluminium may combine with phosphate ions to form the low-soluble AlPO\({}_{4}\). A simplified equation describing this stabilisation is as follows:  
\[\text{Al}^{3+}+\text{H}_{2}\text{PO}_{4}{}^{-}\rightarrow\text{AlPO}_{4}+2\; \text{H}^{+}\]  
However, when applying phosphate fertilisers, a careful site-specific assessment/evaluation of the problem must always be carried out to ensure that, for example, surface water is not affected by the possible eutrophication.