# Influence of Unburned Carbon on Environmental-Technical Behaviour of Coal Fly Ash Fired Clay Bricks
## 3 Results and Discussion
### Estimation of Gas Emissions during Firing Process  
The content of Cl, F, S, C, and N for clay B and fly ash A (low LOI) and B (high LOI), as well as their corresponding acid gas emissions HCI, HF, SO\({}_{2}\), and CO\({}_{2}\) after the firing stage at the temperature of 1058 \({}^{\circ}\)C simulating the industrial processes and estimated by  
Figure 11: Microstructure FESEM images taken at different regions of 100% clay B. (**a**) The FESEM images of different regions observed in the microstructure of 100% clay B brick; (**b**) The high potassium ox-ide content area in ceramic matrix; (**c**) FESEM observations at high magnification. Figure 10: FESEM images and mapping of elements of samples 100% clay B and 50% FAB/clay B.  
mass balance (Equation (1)) are summarized in Table 4. The content of N was below the detection limit of the equipment used. Chlorine emissions are related to its content in the samples. The chlorine in the clay comes from micas, halite, and organic matter, while in the ash, it mainly forms chlorinated compounds associated with its organic matter. The significant increase in chlorine emissions in clay compared to fly ash, despite having the same initial content, may be due to the presence of chlorine as sodium chloride (halite), which is very unstable during the firing process. While the difference in emissions between the two types of ashes is directly related to the unburned content, the higher the LOI, the higher the decomposition of chlorinated compounds. Fluorine emissions also depend on the initial concentration of this element in the samples. The difference observed between the clay and the ashes may be due to the decomposition of phyllosilicates present in the clay while the difference between both ashes may be due to the formation of thermally stable compounds (CaF\({}_{2}\)) in samples with low carbonate content [58]. Sulphur emissions depend on the mineralogy of the samples. Due to the oxidation and decomposition of pyrite and organic matter during the firing process, high values of sulphur emissions are obtained for ashes with a high content of unburned carbon [59]. Finally, in the case of CO\({}_{2}\) emissions, a direct relationship can be observed between the initial carbon content in the raw materials and the level of emissions. Gas emission for the mixtures of FAA/clay and FAB/clay fired at the temperature of 1058 \({}^{\circ}\)C simulating the industrial processes and estimated by mass balance (Equation (1)) are summarized in Figure 12. Emission limit values for HF, HCl, and SO\({}_{2}\) proposed by [PERSON] et al. (2011) [58], aimed to include them in the Andalusian-Spain legislation in response to the indications of the IPPC, have also been considered. From a study conducted with 40 ceramic factories in Andalusia, Spain by means, median, and the 90\({}^{\text{th}}\) percentile values of 180 samples, three ranges of emission values were established: \"acceptable\", \"recommended intervention\", and \"compulsory intervention\". The introduction of fly ash in the mixtures reduces the emissions of HCl due to low chlorine emissions from fly ash. Incorporating up to 90% of FAA and FAB, a reduction in emissions of HCl was achieved compared with the brick of 100% clay. The percentage of reduction obtained was 61% for FAA and 25% for FAB. The reduction in emissions by introducing FAA was higher due to the lower value of emission with respect to FAB. As indicated above, it is probably due to a greater decomposition of chlorinated compounds associated with organic matter, that is, the content of unburned matter. The introduction of fly ash in the mixtures of FAA/clay and FAB/clay reduced the emissions of HF due to the low contents of F that fly ash present. The reduction in HF emissions was higher for fly ash due to the lower contents of F before the firing process compared to clay.