## 3 Results and Discussion
### Effect of the BA Natural Weathering
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mineral** & **Chemical** & **Untreated** & \multicolumn{2}{c}{**Laboratory Ageing**} \\ \cline{3-5}  & **Formula** & **Sample** & **3 Months** & **6 Months** \\ \hline \hline \multicolumn{5}{c}{**Carbonates**} \\ Calcite & CaCO\({}_{3}\) & XXXX & XXXX & XXXX \\  & & **Oxide** & & \\ Quartz & SiO\({}_{2}\) & XXXX & XXXX & XXXX \\ Hematite & Fe\({}_{2}\)O\({}_{3}\) & X & X & X \\ Magnetite & Fe + 2 Fe + 3O\({}_{4}\) & XX & XX & XX \\  & & **Silicates** & & \\ Diopside & CaMgSi\({}_{2}\)O\({}_{6}\) & X & XX & XX \\ Gehlenite & Ca\({}_{2}\)Al\({}_{2}\)SiO\({}_{7}\) & XX & XX & XX \\ Akermanite & Ca\({}_{2}\)Mg(Si\({}_{2}\)O\({}_{7}\)) & X & X & X \\ Microcline & K(AlSi\({}_{3}\))O\({}_{8}\) & X & X & X \\  & & **Phosphates** & & \\ Hydroxylapatite & Ca\({}_{5}\)(PO\({}_{4}\))\({}_{3}\)(OH) & X & X & XX \\  & & **Sulfates** & & \\ Anhydrite & Ca(SO\({}_{4}\)) & X & X & X \\ \hline \hline \end{tabular} Note: XXX—very high amount (>20%); XXX—high amount (10–20%); XX—medium amount (5–10%); X—small amount (trace) (<5%).  
\end{table} 
Table 3: Mineral compositions of untreated BA and BA after 3- and 6-month laboratory aging. The mineralogical changes in bottom ash during the stabilization process can be explained by the following formula:  
\[\text{Ca(OH)}_{2}+\text{CO}_{2}\rightarrow\text{CaCO}_{3}+\text{H}_{2}\text{O} \tag{1}\]  
One of the major processes for the stabilization of BA is carbonization (Formula (1)), which occurs in two steps: First, carbon dioxide is absorbed into the water in the pores of BA. Then, calcium carbonate is formed by calcium hydroxide (Ca(OH)\({}_{2}\)) reacting with dissolved CO\({}_{2}\) (water is necessary for the reaction) [39; 40]. Later, the formation of calcium silicate hydrates is possible (Formula (2)):  
\[\text{Ca(OH)}_{2}+\text{SiO}_{2}\rightarrow\text{CaH}_{2}\text{SiO}_{4} \tag{2}\]  
The analogous formation of calcium aluminate hydrates occurs. Other scientists [26; 27] have shown that calcium silicates and calcium aluminate hydrates have a high sorption capacity for heavy metals. Along with the carbonization reaction, a sulfate formation reaction plays an important role in the stabilization of BA. During the sulfate reaction, calcium sulfate dissolves in pore water and is transported. Subsequently, an aqueous sulfate phase such as gypsum is formed. Due to the increased reaction of sulfates with aluminum compounds, during the formation of the calcium aluminate phase, ettrinige forms in the pore water (Formula (3)):  
\[\text{Ca}_{3}\text{Al}_{2}\text{O}_{6}+3\text{CaSO}_{4}\cdot 2\text{H}_{2} \text{O}+26\text{H}_{2}\text{O}\rightarrow\text{Ca}_{6}\text{Al}_{2}(\text{ SO}_{4})_{3}(\text{OH})_{12}\cdot 26\text{H}_{2}\text{O} \tag{3}\]  
The reactions described above are illustrated by a graph depicting the dependence of pH on the calcium oxide concentration in the eluates (Figure 8). Six months after the beginning of the stabilization process, all pH and Ca concentration values decreased. The graph shows that Ca(OH)\({}_{2}\) in the \(<\)5.6 m and \(<\)40.0 mm fractions was hydrolyzed to CaCO\({}_{3}\) during the stabilization process. The concentrations of copper (Cu), lead (Pb), and zinc (Zn) in the BA eluates and filtrate during natural weathering and laboratory aging are shown in Figures 8-10. We compared the metal concentrations with the limit values (in an eluate) that would enable BA to be used in civil engineering in Lithuania [41]. Figure 8: Changes in calcium solubility and pH during 6 months of natural weathering of BA eluates as a function of particle size. As with the other elements, the concentration of copper in the eluates decreased with increasing stabilization time (Figure 9). In the <40.0 mm fraction, the Cu concentration in the eluate exceeded the leaching limit value (150 \(\mu\)g L\({}^{-1}\)), with values of 257.12 \(\pm\) 5.14 and 178.60 \(\pm\) 3.57 \(\mu\)g L\({}^{-1}\).