# Influence of Unburned Carbon on Environmental-Technical Behaviour of Coal Fly Ash Fired Clay Bricks
## 4 Conclusions  
The influence of the amount of unburned carbon present in coal fly ash on the properties of fly ash-fired clay bricks was studied through the dosage of increased amounts of fly ashes with different unburned carbon content: FAA (low LOI) and FAB (high LOI).  
The main microstructural effect was the increase in porosity as the content of coal fly ash (FAB) in the brick body increased. The porosity was associated with the release of the gases generated during the decomposition of dolomite and the content of unburned carbon of the fly ash. This increase in porosity corresponded with a significant increase in the water absorption and therefore, a bulk density and flexural strength reduction. Although unburned carbon plays an important role on the final properties, no detriment was observed in the properties of ceramic bricks containing up to 30% FAA and percentages of 20% FAB.  
The leaching behaviour of critical contaminants associated with the presence of fly ash showed an immobilisation of Cr and Se in fired clay bricks while Mo reduced its mobility to values close to the inert landfill limit values. No influence of porosity on Mo leaching was observed, but a higher retention, in samples with FAB, due to the adsorption of Mo on the surface of amorphous iron oxides. HF and HCl emissions were below the emissions of the 100% clay bricks, so introducing fly ash would reduce the emissions of these pollutants in the firing process. The decomposition of chlorinated compounds associated with the unburned material was the main difference in the emission reduction values between both ashes. SO\({}_{2}\) emissions of FAB/clay mixtures showed high levels due to the presence of the combination of different sulphur associated with unburned carbon, mainly.  
On the other hand, the level of CO\({}_{2}\) emissions should be estimated based on the total annual production of the company. Therefore, unburned carbon separation option from fly ash would be advisable to obtain acceptable emission values in the CFB process.  
Conceptualisation, A.A. and E.C.; Methodology, J.D. and M.R.; Formal analysis, and investigation, J.D., E.C., M.R., T.L. and A.A.; Resources, A.A. and E.C.; Data curation, J.D. and T.L.; Writing--original draft preparation, J.D. and M.R.; Writing--review and editing, A.A. and E.C.; Project administration and funding acquisition, A.A. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the project \"Valorisation of fly ash in ceramic products and fly ash based products\" funded by Solvay Quimica SL.  
Not applicable.  
This work was presented at the workshop \"Engineering and circular economy: the road to sustainability\" funded as a part of the ECO-MET-AL Project (PID2019-109520 RB-I00), \"Can industrial and mining metalliferous wastes produce green lightweight aggregates? Applying the Circular Economy\" funded by the Spanish Ministry of Science, Innovation, and Universities and ERDF funds, framed in the \"Grants for \"R&D&I Projects\" in the framework of the State Programmes for the Generation of Knowledge and Scientific and Technological Strengthening of the R&D&I System and R&D&I oriented to the Challenges of Society, Call 2019\".  
The authors declare no conflict of interest.