## 4 Discussion and Conclusions
Our results reveal that IBA surely complies with the limit set for HMVA-2, but the quality requirements for HMVA-1 were not reached, mainly due to the parameters chloride and sulfate. The solid contents of chloride and sulfate are too high (see Table 1) and alkaline, and alkaline earth chlorides and even calcium sulfate exhibit higher solubilities than the allowed limits. As shown in the present study, ettringite with a low solubility is not stable in the course of the aging process and is transformed into other, more soluble calcium sulfate phases. The use of IBA as aggregate in concrete was studied (see e.g., [67]), but it is not well-developed in practice due to the presence of alkali salts, fine Al particles which could evolve hydrogen, and other disturbing substances. The main areas of application are landscing (on landfill sites) and road construction, thus helping to save natural resources such as sand and gravel [47]. The geochemical modelling software Visual MINTEQ was able to predict the concentrations of relevant heavy metals in the leaching tests, as well as the precipitation of solids. Modelling complexation, as observed in the present study for copper, is also possible but \begin{table}
\begin{tabular}{l l l l l l l} \hline
**Parameter** & **Unit** & **HMVA-1** & **HMVA-2** & **DK 0** & **DK 1** & **DK II** \\ \hline pH value & - & 7–13 & 7–13 & 5.5–13 & 5.5–13 & 5.5–14 \\ Electrical conductivity & \(\upmu\)S/cm & 2000 & 12,500 & & & \\ DOC & mg/kg & & & 500 & 800 & 1000 \\ Cl & mg/kg & 320 & 10,000 & 800 & 15,000 & 25,000 \\ SO4 & mg/kg & 1640 & 6000 & 1000 & 20,000 & 20,000 \\ Sb & \(\upmu\)g/kg & 20 & 120 & 60 & 300 & 700 \\ Cr (total) & \(\upmu\)g/kg & 300 & 920 & 500 & 3000 & 10,000 \\ Cu & \(\upmu\)g/kg & 220 & 2000 & 2000 & 10,000 & 50,000 \\ Mo & \(\upmu\)g/kg & 110 & 800 & 500 & 3000 & 10,000 \\ V & \(\upmu\)g/kg & 110 & 300 & & & \\ As & \(\upmu\)g/kg & & & 500 & 2000 & 2000 \\ Pb & \(\upmu\)g/kg & & & 500 & 2000 & 10,000 \\ Cd & \(\upmu\)g/kg & & & 40 & 500 & 1000 \\ Ni & \(\upmu\)g/kg & & & 400 & 2000 & 10,000 \\ Hg & \(\upmu\)g/kg & & & 10 & 50 & 200 \\ Zn & \(\upmu\)g/kg & & & 4000 & 20,000 & 50,000 \\ Total dissolved matter & mg/kg & & & 4000 & 30,000 & 60,000 \\ TOC (solid content) & \% & & 1 & 1 & 3 \\ \hline \end{tabular}
\end{table} 
Table 2: Legal requirements for the use and landfilling of IBA. The limit values have been converted from concentrations to leached amounts because different leaching procedures are anticipated to check the requirements for use (HMVA-1 and-2: L/S = 2 L/kg) and landfilling (DK 0–II: L/S = 10 L/kg). was not examined within the present study, along with sorption processes that are relevant for the release of Sb. The leaching behavior of most heavy metals in IBA depends mainly on the pH of the leaching test. Antimonate (Sb-V) forms a stable mineral with Ca. Therefore, the mobilization of Sb depends on the concentration of Ca\({}^{2+}\) ions, which is initially low due to the existence of ettrinigite and later, with the advancement of aging in IBA, due to the formation of CaCO\({}_{3}\) (K\({}_{\text{L}}\) = 1 \(\times\) 10\({}^{-8}\) mol\({}^{2}\) L\({}^{-2}\)). Nevertheless, IBA fulfills the requirements for landfilling and use for a variety of applications (e.g., the installation methods allowed for HMVA-2) after storage. Even aged IBA remains a reactive material. Demanding requirements such as Germany's HMVA-1, with low release of sulfate, chloride, and Sb, are not achievable. The exclusion of the fine fraction 0-2 mm (or 0-4 mm) where chloride and sulfate are enriched might be helpful to reach this goal. However, that would affect approximately 50% of the mass of IBA, which then might be disposed of on landfills. The problem of increasing Sb release as a result of decreasing Ca concentrations would still remain in such a scenario, especially in the long-term view, when most Ca is transformed into CaCO\({}_{3}\). A different approach could involve further thermal treatment of IBA, e.g., vitrification of IBA [68; 69], which has, however, a high energy demand.