# Self-Organizing Maps to Assess the Recycling of Waste in Ceramic Construction Materials
## 3 Results and Discussion
### Industrial Extruded Clay Mixtures with WS Incorporation  
In this demonstration stage, at an industrial level, two types of hollow brick are manufactured according to the current market demand in Spain: large-format hollow brick (or lightweight brick) and perforated brick (or ceramic block). In these industrial trials, mixtures of illicit-calcareous clay C1, a fixed amount of 8% wood pulp and a sample with 30% WS (IC1W30) as well as a reference without WS (IC1) have been developed for lightweight brick. The second products, the ceramic blocks, have been manufactured with an illitic-calcareous clay C4 and 20% WS (IC4W20) as well as a reference without WS (IC4). The moulding system of extrusion has been used coupled to a drying and firing cycle in industrial furnaces reaching temperatures up to 850 \({}^{\circ}\)C and 930 \({}^{\circ}\)C adequate to obtain a batch of each product at industrial level at Cerabric (La Rioja, Spain) and Utzubar (Navarra, Spain) companies (Figure 1). In the previous semi-industrial stage, the obtained prototypes showed linear firing shrinkage (LFS) and weight loss (WL) values well below the reference sample values (SC1) (Table S3) (Supplementary Material). An undefined trend has been observed with the incorporation of slag, both in binary mixtures and in ternary mixtures. In addition, these properties are not required in the Declarations of Performance (technical specifications) for this type of products [44]. For this reason, these properties are not measured on the industrial tests. The data set of values of the 12 analysed variables in the four industrial samples is shown in Table S4 (Supplementary Material). The WS incorporation increases the modulus of rupture (MOR) and the density in the two types of industrial bricks tested. Additionally, IC4W20 decreases water absorption (WA) and maintains the open porosity (OP) constant respect to his reference IC4, while in contrast, IC1W30 increases WA and OP respect to the IC1 reference brick. Although the release of Zn, Ba and As are below the limits for the Inert landfill, experimental results showed that none of the commercial bricks (nor reference bricks nor bricks with waste) complied with the criteria established by the European Union for inert wastes (the most stringent), and being classified as a non-hazardous waste (due to Cr and Mo release). Incorporation of Waelz slag into the clay mix results in higher leaching of chromium (Cr) and molybdenum (Mo) from the fired ceramics. This is due primarily to the higher concentration of these elements in the slag but may also be affected by microstructural changes caused by the presence of these elements in the clay mix. The leaching of Cr and Mo from light bricks containing 30% Waelz slag (IC1W30) reached values close to the thresholds established for non-hazardous wastes (at L/S = 2 test). Lower release values were observed in the case of standard bricks containing Waelz slag (IC4W20), which may be attributed to the lower proportion of slag incorporated into the mix or to microstructural variations derived from the small differences in the firing conditions and raw materials. Due to the reducing conditions of the Waelz process, most of the Mo and Cr present in the slag are in their elemental form [63; 64]. It should be expected that this state of oxidation changes during the brick firing process, resulting in formation of molybdates and chromates that release more readily from the final product. Figure 4: (**a**) Clustered map with labels of the studied pilot-plant clay mixtures with WS-FSD incorporation. The four clusters (I–IV) have been derived from the k-means algorithm applied to the trained SOM. The labels in the hexagons of the map show the BMU corresponding to each sample. (**b**) Component planes of the SOM for the 14 input variables with Mo highlighted along with neurons that exceed non-hazardous landfill limits.