# Influence of Unburned Carbon on Environmental-Technical Behaviour of Coal Fly Ash Fired Clay Bricks
## 3 Results and Discussion
### Technological Properties of Fired Products  
The effect of unburned carbon of fly ash on the linear shrinkage is shown in Figure 5. FAB/clay ceramics present a higher value of linear shrinkage for percentages of fly ash greater than 50%. There was a difference over three units between FAA and FAB for both clays. For concentrations between 10-50% of fly ash, fired clay ceramics with FAB had a linear shrinkage close to the bricks with FAA. The difference between ashes was lower than 0.5 units and presented a linear shrinkage close to the bricks with 100% clay. The same tendency of increasing the linear shrinkage, due to the addition of waste, has been observed in other studies [53]. Figure 4: Thermogravimetric behaviour of (**a**) clay A and clay B; (**b**) FAA and FAB. Figure 3: XRD pattern of clay A (**a**), clay B (**b**), FAA (**c**), and FAB (**d**). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Leaching Concentrations (mg/kg)**} & \multicolumn{3}{c}{**Landfill Limits 1**} \\ \hline  & **Clay A** & **Clay B** & **FAA** & **FAB** & **Inert** & Non-Hazardous \\ \hline pH & 7.72 & 9.35 & 11.01 & 12.52 & - & - \\ As & 0.461 \(\pm\) 0.02 & 0.296 \(\pm\) 0.01 & 0.146 \(\pm\) 0.16 & 0.110 \(\pm\) 0.03 & 0.5 & 2 \\ Ba & 1.075 \(\pm\) 0.036 & 11.337 \(\pm\) 0.036 & 4.668 \(\pm\) 0.220 & 2.163 \(\pm\) 0.099 & 20 & 100 \\ Cd & 0.003 \(\pm\) 0.0001 & 0.013 \(\pm\) 0.0002 & 0.006 \(\pm\) 0.004 & 0.004 \(\pm\) 0.001 & 0.04 & 1 \\ Cr & 0.006 \(\pm\) 0.005 & 0.008 \(\pm\) 0.0002 & 1.375 \(\pm\) 0.13 & 1.998 \(\pm\) 0.128 & 0.5 & 10 \\ Cu & 0.106 \(\pm\) 0.003 & 0.095 \(\pm\) 0.001 & 0.046 \(\pm\) 0.004 & 0.059 \(\pm\) 0.005 & 2 & 50 \\ Hg & 0.002 \(\pm\) 0.0001 & 0.0013 \(\pm\) 0.0002 & 0.008 \(\pm\) 0.007 & 0.009 \(\pm\) 0.006 & 0.01 & 0.2 \\ Mo & 0.0183 \(\pm\) 0.0001 & 0.004 \(\pm\) 0.001 & 5.450 \(\pm\) 0.065 & 5.196 \(\pm\) 0.007 & 0.5 & 10 \\ Ni & 0.047 \(\pm\) 0.005 & 0.016 \(\pm\) 0.001 & 0.015 \(\pm\) 0.001 & 0.011 \(\pm\) 0.001 & 0.4 & 10 \\ Pb & 0.046 \(\pm\) 0.001 & 0.057 \(\pm\) 0.002 & 0.035 \(\pm\) 0.01 & 0.021 \(\pm\) 0.01 & 0.5 & 10 \\ Sb & 0.008 \(\pm\) 0.0002 & 0.008 \(\pm\) 0.0002 & 0.021 \(\pm\) 0.001 & 0.013 \(\pm\) 0.001 & 0.06 & 0.7 \\ Se & 0.021 \(\pm\) 0.007 & 0.087 \(\pm\) 0.0007 & 2.617 \(\pm\) 0.255 & 0.619 \(\pm\) 0.053 & 0.1 & 0.5 \\ Zn & 1.599 \(\pm\) 0.035 & 1.60 \(\pm\) 0.012 & 0.943 \(\pm\) 0.044 & 0.370 \(\pm\) 0.019 & 4 & 50 \\ \hline \end{tabular} \({}^{1}\) Limits for inert and non-hazardous wastes according to Landfill Decision 2003/33/EC.  
\end{table} 
Table 3: pH values and leaching of major elements from raw materials according to the compliance leaching test EN 12457-2. Figure 5: Main technological properties of fired clay bricks. Weight loss increases as the percentage of FAB increases due to the high carbon contents [31]. Samples were fired up to 1058 \({}^{\circ}\)C and at this range, FAB experiences a thermal decomposition of carbonates and unburned carbon [54]. This decomposition supposes a loss of weight and was confirmed with the thermogravimetric analysis (Figure 4). Fired clay bricks with FAA presented results of weight loss close to the bricks made with 100% clay. Percentages of FAA above 60% had slightly higher values due to the thermal decomposition of water and carbonates. The difference between fired bricks was 5 units for 10% of fly ash, while 15 and 20 units for 60% and 90% of FAB, respectively, so unburned carbon significantly influenced the weight loss of fired clay bricks. Fly ash is less dense than the clays used, thereby lowering the density with increasing the ash content in the bricks. The percentage of ash used considerably affects the densification. The presence of unburned carbon in the fly ash provides less dense bricks. Bricks with 10% of fly ash presented 100 units of difference between FAA and FAB and with 50% had 400 units of difference. With percentages up to 20% of fly ash, results of bulk density were very close between FAA and FAB, regardless of the clay used.