# The Use of Construction Waste to Remediate a Thermally Active Spoil Heap
## 3 Results and Discussion
### Acidification of Tailings
Active acidity (\(\mathrm{pH}/\mathrm{H_{2}O}\)), exchangeable acidity (\(\mathrm{pH}/\mathrm{CaCl_{2}}\)), and hydrolytic acidity \(\mathrm{H_{a}}\) were therefore monitored in all samples (Table 6). Active acidity (\(\mathrm{pH}/\mathrm{H_{2}O}\)) is determined by the activity of \(\mathrm{H_{3}O^{+}}\) cations in the solution, the sources of which are dissociated mineral and organic acids, acid salts, and acidoids [41]. It has a direct physiological significance, as it fundamentally affects the bio-chemical processes taking place in the soil, such as the availability and intake of nutrients especially by plants, but also the activity of microorganisms. In an acidic environment, for example, the growth of plant roots can slow down due to the toxic effects of aluminium. From the values listed in Table 6, it can be seen that by mixing tailings with \(\mathrm{pH}/\mathrm{H_{2}O}\) of 3.7 and CDW in a ratio of 9:1, the value of active acidity is significantly adjusted to a value of 6.1. Exchangeable acidity was evaluated in all samples as well. It is caused by adsorbed \(\mathrm{H^{+}}\) and \(\mathrm{Al^{3+}}\) ions (or Fe ions), which are exchanged for basic ions by a neutral salt solution (international standard 0.01M CaCl\({}_{2}\)). Even this potential part of protons is released into the solution and can thus affect soil processes. The results clearly show that protons will most likely be bound to the sorption complex in tailings and CDW, because the exchangeable acidity value was lower by 0.3 in both tailings and CDW than in \(\mathrm{pH}/\mathrm{H_{2}O}\). In this case, it can be stated that even a small addition of CDW has a positive effect on the properties of the tailings. At the same time, it is evident that with the increasing addition of CDW, the exchange of adsorbed cations no longer occurs. Based on the results of \(\mathrm{pH}/\mathrm{H_{2}O}\) and \(\mathrm{pH}/\mathrm{CaCl_{2}}\) and \(\mathrm{H_{a}}\), all metals will be dissolved in the solution. Due to the very low limestone content, it cannot be expected that the \(\mathrm{pH}\) will increase and the environment will turn alkaline. There will therefore be no deposition of metals due to the change in \(\mathrm{pH}\). Taking into account the fact that even extraction of the material using a neutral salt solution (\(\mathrm{CaCl_{2}}\)) fails to displace the more tightly bound \(\mathrm{H^{+}}\) ions in the sorption complex, the hydrolytic activity was therefore determined as well. It expresses the ability of the material to change the reaction of solutions of hydrolytically dissociable salts. Based on the obtained results, it is possible to confirm the hypothesis that a relatively large amount of \(\mathrm{H^{+}}\) bound in the sorption complex will occur in pure tailings. If the tailings from the surface of the body were used in an untreated state to recultivate the surrounding terrain, this could have a negative effect on the physical, chemical, and biological properties of the soil as a result. The influence of \(\mathrm{pH}\) is essential in the soil-forming process, because it affects weathering and substance flows, and the soil structure. It also has an effect on sorption, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample Description** & **pH/H\({}_{2}\)O** & **pH/CaCl\({}_{2}\)** &
\begin{tabular}{c} **H\({}_{a}\)** \\ **mmol kg\({}^{-1}\)** \\ \end{tabular} \\ \hline Tailings/CDW (ratio 9:1) & 6.1 & 6.5 & 54 \\ Tailings/CDW (ratio 8:2) & 6.3 & 6.7 & 34 \\ Tailings/CDW (ratio 7:3) & 6.5 & 6.9 & 21 \\ Tailings & 3.7 & 3.4 & 205 \\ Burning Tailings & 8.4 & 8.9 & 1.4 \\ CDW & 10.7 & 10.4 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Physical-chemical parameters of samples. and thus on the availability and mobility of both nutrients and risk elements. Fe, Mn, B, Cu, Zn, and Al become the most available in an acidic environment [42; 43]. If the pH value is too low, it is assumed that the potential plant growth will be inhibited by the phytotoxic effect of aluminium.