## 3 Results and Discussion
### Results of the Characterization of Raw and Sintered Samples  
#### 3.2.1 Results of the Chemical Composition of the Waste Sludge and Raw Samples  
The results of chemical composition regarding heavy metal concentrations are presented in Table 1. According to _\"Standard Sl. Glasnik 56/2010\"_ as well as EN 12457/EN 16192: 2011, which applies in the Republic of Serbia, in the investigated waste sludge sample, the content of certain heavy metal ions exceeded the maximum values allowed for the classification of waste as nonhazardous; therefore, it can be classified as hazardous waste. However, based on guidelines prescribed by the _Rulebook on restrictions and prohibitions on the production, placing on the market and use of chemicals_ as well as by the _Rulebook on the categories, testing and classification of waste_, restrictions on the utilization of raw waste materials as an additive for industrial products in the energy industry have not been defined yet on the basis of the concentration of heavy metals. The waste sludge chemical composition in terms of the oxide content is shown in Table 2. The obtained oxide proportion confirms that the waste sludge material was of aluminosilicate composition with a non-negligible admixture of iron oxides and a rather large share of volatile organic matter. Moreover, alkali oxides are present in a great proportion, especially sodium oxides. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Concentrations, mg/kg, in Waste Sludge**} \\ \hline
**Parameter** & **LP Sludge** & **PE Sludge** & **LP+PE Sludge (1:1)** & **Ref. Values** \\ \hline Hg & \textless{}0.15 & \textless{}0.15 & \textless{}0.15 & 0.2 */2 ** \\ Sb & 0.85 & 85.5 & 43.175 & 0.7 */5 ** \\ Se & \textless{}0.2 & 2.5 & \textless{}1.35 & 0.5 */7 ** \\ Cu & 20.5 & 2050 & 1035.25 & 50 */100 ** \\ Zn & 37 & 175.5 & 106.25 & 50 */200 ** \\ Ni & 8 & 186 & 97 & 10 */40 ** \\ Cd & 2.5 & 6 & 4.25 & 1 */5 ** \\ Pb & 10 & 37 & 23.5 & 10 */50 ** \\ As & 2.5 & \textless{}0.5 & \textless{}1.5 & 2 */25 ** \\ Cr & 8 & 22 & 15 & 10 */70 ** \\ Mn & 600 & 4855 & 2727.5 & - \\ Tl & \textless{}0.05 & 2.5 & \textless{}1.275 & - \\ Fe & 18,750 & 13,250 & 16,000 & - \\ Co & 3.5 & 2330 & 1166.75 & - \\ \hline \hline \end{tabular} According to “Standard Sl. Glasnik 56/2010”: * _values refer to the disposal of nonreactive hazardous waste in landfills for nonhazardous waste in fields that are not used for disposal of biodegradable waste. ** values refer to disposal in landfills for hazardous waste._  
\end{table} 
Table 1: Chemical composition of the waste sludge—content of heavy metals. Figure 5: Sample GC\({}_{10}\) applied between ceramic bricks after thermal cycling, with larger waste particles on the surface indicating insufficient homogeneity. As can be observed from the results of the chemical composition determination of all the GC samples, given in Table 3, the content of Cu ions in the samples GC\({}_{15}\) and GC\({}_{20}\) exceeded the maximum concentration limits allowed for the classification of waste as nonhazardous according to the _Rulebook on categories, testing and classification of waste (\"Standard SI. Glasnik 56/2010\")_, as well as EN 12457/EN 16192: 2011, which applies in the Republic of Serbia to the disposal of nonreactive hazardous waste. However, the Cu content of GC15 was practically within acceptable limits if we consider the permissible error of measurements (\(\pm\)2.5%). Therefore, only the sample labeled as GC\({}_{20}\) did not meet the mentioned criteria for heavy metal content and is not ecologically acceptable to be used for commercial purposes. The maximum percentage of the clay-for-sludge replacement that can be achieved without affecting the stringent requirements concerning heavy metal content (in this case, Cu) in the composition is up to 15%. Therefore, the sample with the highest waste sludge content that satisfied the mentioned criteria and the most promising candidate for application as a ceramic adhesive or IT-SOFC sealant is GC\({}_{15}\).