# Self-Organizing Maps to Assess the Recycling of Waste in Ceramic Construction Materials
## 1 Introduction  
The implementation of the circular economy through the incorporation of residual raw materials into material cycles is a high priority for the Green Deal (Circular Economy Action Plan adopted by the European Commission on \(11\) March 2020). Wastes addition to manufacture building materials plays a key role in the circularity of construction sector through the development of innovative solutions of materials efficiency, as saving of raw materials by substitution of primary raw material with recycled materials from urban and industrial wastes [1]. Specifically, one of the most extended construction materials is fired clay products [2, 3]. Although clay brick and tile industries are showing innovative solutions to add to the circular economy in Europe, regulatory and technical barriers still exist to scale up current initiatives or to start new ones. Some of them are the differentiated interpretations of end-of-waste and by-products status among Member States, the lack of a well-functioning European market for secondary raw materials, as well as the need for an efficient system for collection, sorting and separation of waste that includes the adaptation of technical requirements of installations [4]. Some recently published research works have supported these limitations for an evolution towards a circular economy. [PERSON] et al. [5], analysed the effects of different types of waste on technological behaviour, technical performance and environmental impact of ceramic tiles in the prospect of an industrial transfer. [PERSON] et al. [6], shows the conflict between resource efficiency and market trends in the ceramic tile industry, and how it reflects on the supply chain raw materials. The environmental impact of Italianc ceramic tiles production is analysed and discussed by [PERSON] et al. [7], highlighting the larger number of technological innovations incorporated. On the one hand, many scientific works have been dedicated to the incorporation of wastes into clay ceramics during the last decade. Some researchers have also shown, not only that the use of residues contributes to the reduction of natural resources, but that certain residues can even produce several improvements regarding the manufacturing process and properties of fired clay products [8; 9; 10; 11; 12; 13]. Fuel or organic wastes contribute to savings in firing energy, while fluxing or inorganic wastes improve the ceramic properties [12; 14; 15]. Specifically, inorganic wastes containing metals and minerals can melt and react with the clay minerals [16; 17; 18]. Despite these advantages, these trials have not yet been scaled up to mass production in industrial facilities. Among other reasons, processing differences between laboratory and real factory lead to high uncertainty in the compulsory final product properties, particularly when waste materials are incorporated in the raw materials formulation [9]. As is highlighted in the review by [PERSON] et al. [5], a correct design of experiments is crucial to achieve useful results for a technological transfer to the industrial practice. In this sense, aspects such as the use of experimental conditions as close as possible to the actual industrial technology, the scale factor, the use of industrial references without waste addition and the analysis of the appropriate amount of recyclable waste in each experimental step are critical. Waste additions and processing conditions effect on the physico-chemical, technological and environmental properties of the final product have been extensively studied using classical multivariate statistical techniques [19; 20; 21; 22; 23; 24; 25; 26; 27]. Otherwise, Artificial Neural Networks (ANN) and Self-Organizing Maps (SOMs) have thus far been used in a more limited way as an alternative to assess the abovementioned relationships.