# Influence of Unburned Carbon on Environmental-Technical Behaviour of Coal Fly Ash Fired Clay Bricks
## 1 Introduction  
Development of innovative solutions of resource efficiency in the construction sector, both in products and in infrastructure works, may include huge amounts of waste and thus promotes the circular economy for a sustainable future and also contributes to achieving the long-term United Nations Sustainable Development Goals [1]. Clay minerals, mainly aluminosilicates, are a resource widely used in industry for their well-known properties such as plasticity, fire resistance, and their wide distribution, but from an environmental engineering point of view, the relevant properties are cation exchange capacity, structure, texture, and surface area. The use of clay minerals as potential sorbent material for organic and inorganic contaminants has increased in recent decades [2]. For example, a sustainable application is the capture of carbon through carbon dioxide adsorption by modified clays [3]. On the other hand, among the many industrial processes in which clay is used is the manufacture of fired clay bricks (FCBs), one of the most extended construction materials [4]. FCBs are frequently produced from non-renewable materials or cementitious materials at high firing temperatures, which implies environmental damage due to the extraction of raw material, GHG emissions, and air pollution globally [5]. The incorporation of industrial wastes in clay based-ceramics, as a clay replacement, contributes toward cleaner production practices, and has been addressed by many scientific works during the last decade [6]. Four groups of waste can mainly be determined according to their nature [7,8]: sludge from different industrial processes [9, 10, 11]; inorganic waste, mainly from mining and metallurgy waste processes [12,13]; organic wastefrom paper manufacturing plants and also biomass of different biological activities [14; 15; 16] and ashes, mainly from coal energy production, and municipal solid waste and biomass incineration [9; 17; 18; 19]. Among all of them, it is important to underline coal fly ash (CFA), which is generated in thermal power plants, typically constituting around 80% of the total ash, and approximately 30% of the mass of coal consumed [20]. It has been estimated that the world's 2300 coal-fired power station fleet creates approximately one billion tonnes of new fly ash each year [21]. Therefore, the incorporation of CFA in the production of FCBs can be an important application, bearing in mind the large quantities of raw materials needed for FCB production [22]. The valorisation of CFA contributes to the reduction in natural resources extracted as its composition is very similar to that of clay, SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and Fe\({}_{2}\)O\({}_{3}\) with some minor constituents such as CaO and MgO and other inorganic elements. Amorphous material and crystalline phases as well as unburned or partially burned carbon residues represent its mineralogical composition [23]. CFA is considered \"non-hazardous\" waste according to different regulations (EPA and EU) despite containing metals and metalloids such as, As, Pb, Zn, Ni, Cu, Mn, Cd, Cr, and Se at trace levels [24]. However, due to serious environmental accidents, agencies such as the EPA have questioned the need to include them in a new classification such as special waste (Resource Conservation and Recovery Act, 2008) [25]. Therefore, CFA disposal is an important issue worldwide due to its huge volume and to its potential negative environmental impacts, which requires great disposal fees and the consequent negative Public Relations (PR) of the sector. On the other hand, unburned carbon in CFA is an indicator of defective combustion of the original fuel, mainly due to the ageing of the boilers [26]. CFA can be valuable as a source of activated carbon or a source of coal to return it to the boiler. However, an unburned carbon content greater than 10% considerably reduces its pozzolanic capacity, preventing its recovery in cement and concrete industries [20].