# Influence of Unburned Carbon on Environmental-Technical Behaviour of Coal Fly Ash Fired Clay Bricks
## 1 Introduction
For this reason, in practice, only 30% of the total ash produced in the world is re-used [27]. These limitations in the recovery of ashes in the manufacture of concrete reinforce the need for a more rigorous study of its recovery in other industrial processes, specifically in the ceramic industry. Many studies have investigated the properties of clay-based bricks containing CFA to evaluate the technical performance of FCBs [9; 17; 28; 29; 30; 31; 32; 33]. Specifically, the study of the percentage of unburned in CFA on the final properties of FCBs, only [PERSON] et al. [34] considered it as an input parameter in an artificial neural network model. On the other hand, there is no harmonisation to determine the environmental behaviour of ceramic products incorporating waste; the most widely used leaching tests to evaluate them at the end of their useful life, as construction and demolition waste (C&DW), whether deposited in landfills or used as secondary raw material, are the compliance tests: TCLP (Toxicity Characteristic Leaching Procedure) [35; 36; 32] and the Equilibrium Leaching Test EN 12457 [37; 38; 39; 40; 41; 42; 43]. Specifically, in the case of ceramic products manufactured with coal ash, only very few authors [29; 32; 43] have reported equilibrium leaching test data of heavy metal mobility with different proportions CFA. Only [PERSON] et al. (2019) [44] showed comparative results of batch and column leaching test of metal release, and the difference between bricks with/without CFA was minimal. Furthermore, from the point of view of the environmental performance of the process, firing is a critical stage due to emissions into the atmosphere [45]. For this reason, the ceramic industry is included in the pollutant release and transfer register (PRTR), establishing the best available technologies (MTDs) as well as the emission limit values of the process. Considering that the incorporation of waste can affect gaseous emissions, some authors have studied these effects on an industrial scale [37; 41; 42; 46; 47; 48] and also on a laboratory scale [49; 50; 51]. However, only very limited information has been obtained on the effects of the composition, mineral transformation, and firing parameters on these emissions. As highlighted in the review of [PERSON] et al. [52], it is essential for a technological transfer to the industrial practice of a correct evaluation of the technical performance of the FCBs incorporating waste as well as an assessment of the environmental behaviour through the leaching test of the final products and gas emissions during the firing process. In this way, there are no works that have performed an integral study of the incorporation of CFAs into FCBs. Therefore, this paper proposes the substitution of part of a virgin raw material such as clay for a residual material that currently does not have a recovery route due to its high concentration in unburned materials. In this research, the incorporation of two coal fly ashes with different content of unburned carbon, with additions varying from 0 to 100 wt% in FCBs was conducted. The technological properties, leaching behaviour, and microstructural characterisation of the final products as well as the estimation of the gas emissions during the firing process were performed with the aim to study the impact of influence of unburned carbon on CFA-FCBs from a technical and environmental point of view.