# Self-Organizing Maps to Assess the Recycling of Waste in Ceramic Construction Materials
## 3 Results and Discussion
### Semi-Industrial Extruded Clay Mixtures with WS-FSD Incorporation  
In the development stage carried out in a pilot-plant, at a semi-industrial level, it was decided to develop a brick prototype, named common brick, corresponding to the family of \"bricks and blocks to cover\", that have the highest marketable production in Spain [3]. In this pilot-plant trial, mixtures of the illicit-calcareous clay C1, WS (0-40%), FSD (0-35%) and a fixed amount of 8% wood pulp have been used. The moulding system of extrusion has been used as well as a firing cycle that is completed after drying, in an industrial furnace reaching required temperatures up to 850 \({}^{\circ}\)C (Figure 1) to obtain common bricks at semi-industrial level. The same technological properties have been measured on the fired products obtained, and the critical pollutants that have been determined on the laboratory scale have been analysed in the leachates. Prototypes obtained in this semi-industrial phase were evaluated through two leaching protocols, UNE-EN 12457-1 and 2, using the liquid/solid ratio 2 and 10, which simulates closed and open landfill conditions, respectively. The data set results of the 14 variables obtained in the 22 semi-industrial prototypes shown in Table S3 (Supplementary material) can be grouped in an SOM trained map (Figure 4a) based onto \"range\" normalization of data, resulting in a 6 \(\times\) 4 (24 neurons) map, similar to the size of the 23-unit map proposed by the algorithm [59] with QE = 0.548 and TE = 0. The application of the k-means algorithm to the trained map classifies the samples into four clusters (clusters I-IV). Figure 4b shows the component planes of the SOM for the 14 properties of the clay fired prototypes. Cluster I is characterised by ternary mixtures with low WS (5-20%) and high FSD (20-35%) content. Cluster I is defined by the lowest values of metals release, for both L/S = 2 and L/S = 10, except for As with high release values at L/S = 2 and the highest release values at L/S = 10. These samples showed low values of WA, OP, MOR and WL, high values of LFS and intermediate values of D. Cluster II grouped the higher number of mixtures (40% of the samples), including the reference: three binary mixtures with low values of WS content (0-25%) and five ternary mixtures with a broad range of both WS as well as FSD (5-30%) content. Cluster II, which groups samples with the lowest clay substitution rates (0-30%), shows low and intermediate values of metals release, with the lowest values of As at L/S = 2. These prototypes exhibit the highest values of WA, MOR, WL, intermediate values of OP and LFS and the lowest values of D. Cluster III groups only the ternary mixtures with a higher content of WS (5-30%) and a broad range of FSD (0-30%) and clay substitution rate (20-40%). The metals release of this sample is high to intermediate with high values of As releasing mainly at L/S = 2. The properties reach intermediate values. Cluster IV includes only three samples, with the highest content of WS (30-40%) in all of them. Here, the highest values of metal (Ba, Cr and Mo) release are found for both leaching tests. Mo component planes, Mo2 and Mo10, whose neurons surpassing the threshold values for non-hazardous landfill at L/S = 2 and L/S = 10, are highlighted in Figure 4b. Values of WA, OP and LFS in prototypes with industrial wastes additions are 31%, 20% and 25% as average, lower than the reference values (SC1 located in Cluster II). The obtained values of LFS below 8% and WA below 16% classify the prototypes as high quality bricks [48; 49]. Density (D) values increases 25% as average reaching values in the range 1.5 to 1.9 g/cm\({}^{3}\) typical of good quality bricks [62]. Samples with the higher WS additions (Cluster IV) developed the higher density but the lower WA, MOR, WL and LFS. On the other hand, the higher relative additions of FSD (samples in cluster I) lead to intermediate values of technological properties. Metal release behaviour is similar in both of the leaching tests at L/S = 2 and L/S = 10 (Figure 4b).