# Influence of Unburned Carbon on Environmental-Technical Behaviour of Coal Fly Ash Fired Clay Bricks
## 3 Results and Discussion
### Microstructural Characterisation of Fired Clay Bricks
#### 3.4.2 SEM  
Figure 8a,b shows the microstructure observed by FESEM on the polished surfaces of FAA/clay and FAB/clay bricks, respectively. The main microstructural effect was the increase in porosity as the content of coal fly ash in the brick body increased. The increase in porosity was mainly associated with the presence of dolomite and unburned carbon in the mineralogical composition of the fly ashes (Figure 3). However, the incorporation of FAA and FAB led to different impacts on the porosity. For further discussion, Figure 9 presents details of the polished surface microstructure of samples prepared exclusively from clay B (100% Clay B) and those prepared with 50% fly ash incorporation (50% FAA/Clay B and 50% FAB/Clay B). The microstructure of the 100% clay B sample was characterised by the presence of open porosity consisting of interconnected and irregular cavities with a thickness of fewer than 5 \(\upmu\)m. This porosity is the result of the dehydroxylation of the clay during firing, which causes a volume reduction. The increase in porosity observed when fly ash was incorporated into the brick body is  
Figure 6: Concentration of contaminants of fly ash/clay bricks regarding the inert waste landfill limits (**a**) and the non-hazardous waste landfill limits (**b**) according to Decision 2003/33/EC. probably due to the combined action of two factors, namely the larger particle size of fly ash compared to clay, and the dolomite and unburned carbon content in fly ash. Thus, due to its larger average size, the inclusion of fly ash in the brick paste will result in green-shaped specimens with different porosity. According to [PERSON] et al. (2016), at equal pressing variables, a higher content of larger particles results in compacted specimens of higher bulk density (lower porosity), which, however, contain larger pores. As the pore size in the fired bodies depends more on the pore size in the green body than on the pore volume, the inclusion of fly ash in the brick paste will result in less densified fired bodies, and to achieve maximum densification of the product, a higher firing temperature would be required. On the other hand, fly ash contains dolomite and unburned carbon, which during the firing process will decompose with the consequent release of gases, causing the size of the open porosity to increase. FAA presents a low dolomite and unburned carbon content, as can be inferred from its LOI value (Table 2), which was even lower than that of clay B. Therefore, the major effect induced by the incorporation of FAA in the body of the brick was a slight increase in porosity associated with its larger particle size (Figure 9). In contrast, FAB contained much higher dolomite and unburned carbon than the clays, so that during firing, it will generate a high increase in porosity due to the release of the gases generated during its decomposition. This increase in porosity, which is evident in Figure 9, is associated with a significant increase in the water absorption of the brick specimens manufactured with this fly ash (Figure 5). Figure 7: XRD patterns of fired clay bricks prepared from clay B. (**a**) XRD patterns of FAA/clay; (**b**) XRD patterns of FAB/clay. The incorporation of fly ash into the brick composition affects not only the porosity of the fired pieces but also their crystalline composition and the distribution of crystalline phases. Figure 10 presents the FESEM images of polished and etched fracture surfaces of samples of 100% clay B and 50% FAB/clay B compositions, together with the mapping of the elements Al, Si, K, Ti, Fe, and Ca. The different crystalline phases that comprise the mineralogical composition of the fired bricks were clearly distinguished by the colour differences caused by the varying concentrations of these ions in each of the crystalline phases. The 100% clay B sample shows the conventional phase distribution in traditional fired clay materials composed mainly of quartz grains embedded in a ceramic matrix enriched in aluminium oxide.