# Self-Organizing Maps to Assess the Recycling of Waste in Ceramic Construction Materials
## 3 Results and Discussion  
### Laboratory Pressed Clay Mixtures with WS-FSD Incorporation  
Laboratory ceramic pieces obtained by pressing at a wide range of raw materials composition and firing temperature (Figure 1) were analysed for their technological properties of bulk density (D), water absorption capacity (WA), open porosity (OP), flexural strength (MOR), linear firing shrinkage (LFS) and weight loss during firing (WL), as well as for As, Ba, Cd, Cr, Cu, Mo, Ni, and Zn in equilibrium leaching tests at L/S = 10 (Table S2) (Supplementary material). The data set results of the 14 variables obtained in the 30 samples of laboratory pieces can be grouped in a SOM trained map (Figure 3a) based on \"range\" data normalisation resulting in a 6 x 5 (30 neurons) map, similar to the size of the 27-unit map proposed by the algorithm [59], with QE = 0.519 and TE = 0. The application of the k-means algorithm to the trained map classifies the samples into four clusters (clusters I-IV). Figure 3b shows the component planes of the SOM for the 14 properties of the laboratory clay fired pieces. Cluster I is characterised by mixtures without or with relatively low content of Waelz slag, while foundry sand dust contents vary from 6 to 40%. The predominant type of clay found in this cluster is the low firing temperature clay, C1. This cluster is characterised for the highest values of water absorption, open porosity and weight loss, and the lowest of the module of rupture and density, probably related to the high content in carbonates of clay C1 (Table 1) and their decomposition during firing. In terms of leaching behaviour, this cluster outperform the others as the element release is relatively low for all studied elements. Cluster II is similar to the Cluster I in terms of mixtures composition with negligible Waelz slag and variable contents of foundry sand dust. C2 and C3-based clays, that means, mild and high firing temperature processes respectively, are mainly found in this cluster. Figure 3: (**a**) Clustered map with labels of the studied laboratory pressed clay mixtures with WS-FSD incorporation. The 4 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 (Best Matching Unit) 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. Each map corresponding to one variable (component) should be compared to the map representing the distribution of the laboratory pressed clay mixtures with WS-FSD incorporation presented in Figure 3a; hexagons in the same place on different component planes correspond to the same map unit. The colours indicate the value of the component in the weight vector of each unit of the map, according to the colour bars on the right. Technological properties associated to this cluster are the relatively low values of water absorption (WA) and open porosity (OP), while the linear firing shrinkage (LFS) is the highest. The cluster exhibits also intermediate values of weight loss (WL) and density (D). Leaching behaviour is characterised by the highest value of As, relatively high values of Cu and Zn release, while Ba, Cr and Mo exhibit the lowest values. Mixtures with high content of Waelz slag, higher than 20% and relatively low content of foundry sand dust are included in Cluster III. C1, C2 and C3-based clays are found in this cluster. Samples in this cluster show the lowest values of linear firing shrinkage (negatives) and low values of weight loss, with high values of density, due to the glassy phase formed by melted slag [27]. Ba, Cr and Mo release exhibit the highest values in this cluster. Mo component plane is highlighted because some values in neurons exceed non-hazardous landfill limits from the European Waste Landfill Council Decision (Table S2) (Supplementary material). Neurons that surpass those values are framed in the component plane (Figure 3b).