# Case Study on Secondary Building Materials
## 3 Results
### Environmental Compatibility of Incleration Bottom Ash (IBA)  
Two quality levels for IBA are defined in the Secondary Building Materials Decree [21], \"HMVA-1\" and \"HMVA-2\", see Table 2. The classification is based on column percolation or batch tests at an L/S of 2 L/kg. IBA is usually extracted via a wet discharge system behind the incineration chamber. Then, ferrous and non-ferrous metal are separated with magnets and eddy current separators, respectively. Subsequently, the ash is stored for 6 weeks or more for the completion of aging reactions [19; 20]. The aging reactions (uptake of CO2 by CaO yielding CaCO3, formation of hydraulic phases (CSH phases), corrosion of elemental metals) do not influence the electrical conductivity and the concentrations of chloride and sulfate in the standard leaching tests, but the pH of the leachates. Initially, with fresh IBA, high pH values of around 12.5 are measured. With progressing aging, the pH values decrease to values around below 11. Most heavy metal compounds exhibit a strong dependence on the solubility in the pH. If the solubility is high at high pH values due to the formation of metal hydroxo complexes (Me(OH)4\({}^{2-}\) with Me as two-valent ion), the pH then decreases to a minimum around pH 6-8 [29]. This trend was observed in the long-term investigation of the leaching behavior of IBA, see Figure 6.  
Figure 5: Distribution pattern of PAHs in column test eluates at L/S 2 L/kg of soil-like materials. The concentrations of vanadium and antimony depend on the concentration of Ca\({}^{2+}\) ions. At high Ca concentrations, soluble calcium vanadates and antimonates are formed. The aging processes lead to lower Ca\({}^{2+}\) concentrations due to the formation of stable CaCO\({}_{3}\). This behavior is described in detail elsewhere [30; 31]. In the complete survey, no exceedance of the leaching limit values, according to the quality level for \"HMVA-2\", was observed. The values for \"HMVA-1\" were not reached simply due to the stringent values for chloride, sulfate, and electrical conductivity. These values could possibly only be reached with an advanced washing process. As previously stated, CaCO\({}_{3}\) is formed during the aging process of IBA. CO\({}_{2}\) can be transported into an ash storage pile via atmospheric CO\({}_{2}\) or by CO\({}_{2}\) dissolved in rainwater. The carbonation of IBA has already been studied elsewhere [32; 33; 34]. In our studies, similar results were obtained, see Figure 7. The initial carbonate concentration was around 5%, increasing to maximum values of 9%. The experimental data were fitted using the solver module in MS Excel with the function y(t) = y(0) + K\({}_{\rm F}\) t\({}^{1/n}\), with y being the carbonate concentration, K\({}_{\rm F}\) as arbitrary constant, and t the storage time in days. Best fit was achieved with K\({}_{\rm F}\) = 0.36 and 1/n = 0.49. So, within 90 days, 0.32 mol CO\({}_{2}\) (13.9 g) could be sequestered per kg IBA. This is in the same order of magnitude as for recycled concrete aggregates, where values between 2.8 and 9.5 g CO\({}_{2}\) per kg material were found [9]. \begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **HMVA-1** & **HMVA-2** \\ \hline pH & 7–13 & 7–13 \\ Electr. conductivity (uS cm\({}^{-1}\)) & 2000 & 12,500 \\ Chloride (mg L\({}^{-1}\)) & 160 & 5000 \\ Sulfate (mg L\({}^{-1}\)) & 820 & 3000 \\ Antimony (\(\mu\)g L\({}^{-1}\)) & 10 & 60 \\ Chromium (\(\mu\)g L\({}^{-1}\)) & 150 & 460 \\ Copper (\(\mu\)g L\({}^{-1}\)) & 110 & 1000 \\ Molybdenum (\(\mu\)g L\({}^{-1}\)) & 55 & 400 \\ Vanadium (\(\mu\)g L\({}^{-1}\)) & 55 & 150 \\ \hline \end{tabular}
\end{table} 
Table 2: Limit values for leaching c\({}_{\rm i}\) at L/S = 2 L/kg for the utilization of IBA. The release E\({}_{\rm i}\) can be calculated according to E\({}_{\rm i}\) = c\({}_{\rm i}\)\(\times\) L/S. Figure 6: pH values in leaching tests according to DIN 19529 as a function of the storage time of IBA (**left**) whereby the differently colored points represent two sampling sites and two L/S ratios (2 and 10 L/kg).