# 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
Mixtures grouped in cluster IV are C3-based products and therefore high firing temperature samples, with variable Waelz slag and low foundry sand dust content. As technical properties, these samples exhibit the highest values of density and the lowest of weight lost, water absorption and open porosity among the four clusters. Highest values of Cd, Cu and Zn release are characteristic of this cluster, intermediate values of Mo and low release values for the rest of the studied elements. Contrary to cluster III, Cr in cluster IV show low release due to the high firing temperature because of the inverse correlation between Cr release and firing temperature. Samples with higher content of WS are located at the bottom side of the map (Figure 3a), and most of the C3-based samples (fired at high temperature) are located at the right side, C2-based samples fired at medium temperature at the centre, and C1-based samples fired at low temperature to the left side. The addition of Waelz slag reduced the sintering temperature because it adds network modifying species and improved the physico-chemical and technological characteristics of the laboratory pieces. However, the addition of WS in the mixtures causes a severe increase of Ba, Cr and Mo concentrations in leachate, which are also dependent of the type of clay that determine the firing cycle and the maximum temperature reached. The increased mobility of Cr and Mo observed in Waelz slag containing pieces has been previously described as consequence of the speciation change, from elemental form to more soluble molybdates and chromates during the firing process. These species are weakly retained on the sintered matrix of the final product, leading to high releases in products fired at low temperatures [40; 60]. In relation to the product properties, the strong inverse correlation value obtained for the water absorption (WA) with the temperature can be interpreted along with the WA component plane in the SOM, as well as with the addition of waste [15]. Highest values of water absorption correspond to C1-based samples (low firing temperature clay), while the lowest are located in the right side of the map, where most of the C3-based samples are grouped. Open porosity (OP) evidences the same trend (Figure 3b) with highest values in C1 based samples. The highest values of bulk density (D) are shown by samples with high Waelz slag additions (bottom side) due to its high iron content. In contrast, the weight loss (WL) shows a mirror-like behaviour respect to the density. Linear firing shrinkage (LFS) displays the highest values in cluster II, where content of WS is relatively low or nonexistent, whereas the highest MOR values are in samples located in the centre of the map (Figure 3a) with relatively high FSD content. An increased mobility of Cr and Mo has been observed from the laboratory pieces with Waelz slag incorporation. Arsenic, despite its low content in the raw materials, increases its mobility due to the thermal process, particularly in two of the three types of studied clays (C2 and C3), although it remains below the inert landfill limit. Barium mobility is high in Waelz slag [61], although the thermal process reduces its mobility considerably, below the inert landfill limit as a result of metal incorporation to the generated silica-based matrix. For these reasons, As, Ba, Cr and Mo are considered critical pollutants whose leaching behaviour needs to be further studied in the next semi-industrial and industrial phases. Laboratory scale ceramic pieces show the feasibility of incorporating WS and FSD into fired ceramic materials. Industrial waste additions lead to analysed property values ranging above and below those obtained from the reference pieces (LC1, LC2, LC3), except for the open porosity, whose values are systematically 30% higher after the additions. By-products could adversely affect the sintering process, especially FSD that present a very high LOI.