# The Use of Construction Waste to Remediate a Thermally Active Spoil Heap
## 3 Results and Discussion
### Sequential Extraction Analysis
#### 3.5.1 Aluminium  
Based on the study of acidity, it was proven that compared to burnt tailings (pH/H\({}_{2}\)O = 8.4, pH/CaCl\({}_{2}\) = 8.9 and H\({}_{a}\) = 1.4 mmol kg\({}^{-1}\)), unburned tailings show acidic properties (pH/H\({}_{2}\)O = 3.7, pH/CaCl\({}_{2}\) = 3.6 a H\({}_{a}\) = 205 mmol kg\({}^{-1}\)). In general, acidification helps to release aluminium from the parent rock minerals, so this potentially toxic aluminium can enter the soil solution, where it can become significantly phytotoxic. This problem could arise if unburnt tailings were covered with soil as part of reclamation. A high concentration of aluminium causes physiological problems to the root system, which can limit the growth of invasive plants in primary succession. In addition to the pH value, the ratio of Ca, Mg, and K cations to Al is also important. At a low ratio, aluminium settles at the exchange sites of cell membranes and thus prevents the active transport of ions. These processes, which cause the death of roots, poor intake of nutrients and overall weakening of the plant, are already present at a pH lower than 5.0 [58; 59; 60; 61]. The results obtained by BCR analysis for aluminium are shown in Table 7, while the total aluminium content determined by XRF analysis in unburnt tailings was 111,600 mg Al kg\({}^{-1}\) of dry matter. Aluminium toxicity is a potential factor limiting the growth of plants in acidic soils worldwide. [PERSON] reported that the concentration of aluminium in soil ranges from 1 to 30% (10,000 to 300,000 mg Al kg\({}^{-1}\)) [62]. During the first step of BCR analysis, the amount of released exchangeable metals and metals bound to carbonates can be assessed. The fraction bound to carbonates (soluble in water and acid) is very sensitive to changes in the pH value. The release of metals occurs at a pH value \(<\) 5.0, which means that the release of metals in the environment already takes place under weakly acidic conditions. This fraction is therefore the most dangerous for the environment. Carbonates and partly also oxides and hydroxides are dissolved in it [26; 63; 64; 65]. The results of the BCR analysis clearly show that the increasing addition of CDW leads to stabilization of aluminium ions. Considering the acidic reaction of unburnt tailings (pH = 3.7), it can be assumed that aluminium present in the tailings will be adsorbed on the surface by weak electrostatic interactions, and will therefore be easily released into the environment by ion exchange. Based on the second step of the BCR analysis (reducible fraction), it can be assessed whether Al present in the studied material is firmly bound to Fe/Mn oxides and hydroxides, which are very good metal sorbents. They can be released if the environment changes to an anoxic state (e.g., due to the activity of microorganisms). In strongly oxidizing conditions, this material degrades and the release of hazardous metal can occur [63; 64; 65; 66; 67; 68]. The positive effect of CDW addition on the properties of unburnt tailings is visible in this step as well. Based on the results, it has been found that both in CDW and in unburnt tailings, aluminium will most likely be firmly bound to Fe oxides and hydroxides, considering its higher content in the sample (250,000 mg Fe kg\({}^{-1}\)). Much stronger bonds already exist in the fraction bound to organic matter (including living organisms) and sulphides (oxidizable fraction). Considering the nature of the environment, Al is more likely to be bound to sulphides. The release of these metals can then occur in contact with oxygenated water [63; 66; 69]. Metals from this phase can be released in the environment, for example, during resuspension, which can be caused, for example, by spoil heap removal, but also during contact with richly oxygenated water. The results of the BCR analysis have revealed that aluminium from the tailings will be released from both the reducible and from the oxidizable fractions, where it will be primarily bound to sulphides.