## 3 Results and Discussion
### Effect of the BA Natural Weathering  
We determined the effect of natural weathering on the different BA fractions using analyses of the chemical and mineralogical compositions, and leachate and eluate parameters. The chemical compositions of the different untreated BA fractions and those after 3 and 6 months of natural weathering are shown in Table 2 and Figure 6 in terms of major and minor elements (provided in the oxide form) by the percentage of weight and trace elements in milligrams per kilogram. Additionally, we weathered the <40.0 mm fraction under laboratory conditions (constant precipitation amount and temperature) for comparison. The main part of BA (more than 65% by weight) consisted of the following element oxides: SiO\({}_{2}\), CaO, Al\({}_{2}\)O\({}_{3}\), and Fe\({}_{2}\)O\({}_{3}\). We noted that all element (except Fe\({}_{2}\)O\({}_{3}\) and TiO\({}_{2}\)) concentrations decreased during natural weathering. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Fraction**} & \multicolumn{8}{c}{**Element**} \\ \cline{2-9}  & **SiO\({}_{2}\)** & **CaO** & **Fe\({}_{2}\)O\({}_{3}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **K\({}_{3}\)O** & **TiO\({}_{2}\)** & **P\({}_{2}\)O\({}_{3}\)** & **SO\({}_{2}\)** \\ \hline \multicolumn{9}{c}{**5.6 mm**} \\ \hline Untreated & 27.700 \(\pm\) 0.189 & 26.655 \(\pm\) 0.161 & 6.506 \(\pm\) 0.046 & 5.117 \(\pm\) 0.142 & 1.093 \(\pm\) 0.010 & 1.375 \(\pm\) 0.037 & 2.007 \(\pm\) 0.036 & 6.818 \(\pm\) 0.048 \\ \hline
3 months & 27.424 \(\pm\) 0.517 & 26.834 \(\pm\) 0.424 & 6.222 \(\pm\) 0.141 & 5.454 \(\pm\) 0.409 & 1.029 \(\pm\) 0.028 & 1.348 \(\pm\) 0.125 & 1.740 \(\pm\) 0.100 & 5.967 \(\pm\) 0.120 \\ \hline
6 months & 26.376 \(\pm\) 0.517 & 26.104 \(\pm\) 0.433 & 6.293 \(\pm\) 0.129 & 5.407 \(\pm\) 0.413 & 0.910 \(\pm\) 0.026 & 1.315 \(\pm\) 0.117 & 1.844 \(\pm\) 0.101 & 4.368 \(\pm\) 0.098 \\ \hline \multicolumn{9}{c}{**5.6-11.2 mm**} \\ \hline Untreated & 42.670 \(\pm\) 0.257 & 20.223 \(\pm\) 0.119 & 6.108 \(\pm\) 0.041 & 5.587 \(\pm\) 0.135 & 1.014 \(\pm\) 0.009 & 0.992 \(\pm\) 0.032 & 1.231 \(\pm\) 0.033 & 3.555 \(\pm\) 0.031 \\ \hline
3 months & 34.441 \(\pm\) 0.603 & 23.054 \(\pm\) 0.375 & 6.388 \(\pm\) 0.124 & 5.483 \(\pm\) 0.413 & 0.906 \(\pm\) 0.026 & 1.128 \(\pm\) 0.111 & 1.534 \(\pm\) 0.098 & 3.528 \(\pm\) 0.085 \\ \hline
6 months & 32.526 \(\pm\) 0.592 & 22.634 \(\pm\) 0.387 & 6.372 \(\pm\) 0.126 & 5.121 \(\pm\) 0.397 & 0.836 \(\pm\) 0.025 & 1.184 \(\pm\) 0.114 & 1.731 \(\pm\) 0.102 & 3.321 \(\pm\) 0.079 \\ \hline \end{tabular}
\end{table} 
Table 2: Bulk chemical composition of untreated BA and BA after 3 and 6 months of weathering. Figure 6: Concentrations of heavy metals in the different fractions of untreated bottom ash eluate. The concentrations of metals, including the main elements, in all the fractions of BA, decreased during natural weathering (Figure 7). We found the largest decreases in Mn, Cu, and Sr in the <40.0 mm fraction for both weathering types (natural and laboratory conditions). The Mn concentration was lower by 678 \(\pm\) 159 and 725 \(\pm\) 162 mg kg\({}^{-1}\), Cu by 1096 \(\pm\) 125 and 1088 \(\pm\) 122 mg kg\({}^{-1}\), and Sr by 346 \(\pm\) 125 mg kg\({}^{-1}\) for natural and laboratory conditions, respectively. The decreases in Zn concentration in both <40.0 mm fractions were similar: 1121 \(\pm\) 112 mg kg\({}^{-1}\) (natural weathering) and 1119 \(\pm\) 110 mg kg\({}^{-1}\) (laboratory ageing). Decreases in the Pb concentrations of these fractions were relatively low: From 104 \(\pm\) 12 to 142 \(\pm\) 15 mg kg\({}^{-1}\) after 6 months of weathering. The highest decrease in Zn concentration (2359 \(\pm\) 140 mg kg\({}^{-1}\)) was found in the 5.6-11.2 mm fraction after 6 months of natural weathering. We concluded that heavy metal concentrations in laboratory-aged BA decreased more rapidly than in naturally weathered BA during the first 3 months.