# Cleaner Design and Production of Lightweight Aggregates (LWAs) to Use in Agronomic Application
## 3 Results and Discussion  
### Materials Characterization  
Elemental analysis (CNH-S) results of the three different types of clays are reported in Table 2. RC showed an almost complete absence of carbon while BC and WC contained low carbon content. Only black clay presented traces of sulfur. On the other hand, the wastewater sludge had a high carbon content, in line with other wastes of this type, and traces of S below those reported by other authors [49; 50]; thus, the combustion process of BS will not generate SO2 formation, which is an environmental benefit. The chemical analysis (XRF) results reported in Table 3 show that white, black, and red clays contained high silica (SiO2) and alumina (Al2O3) quantities, typical for these minerals, as well as iron oxide (Fe2O3), with red clay being the one with the highest values of the three oxides mentioned. A high amount of calcium oxide (CaO) in WC and BC in comparison to RC was observed, being attributable to the presence of carbonates, with white clay being the one with the highest percentages (20%). BS and CBA showed high amounts of calcium oxide (CaO), 18.52% and 53.89%, respectively. Regarding the other raw materials, we found that BS showed a high LOI (53.80%), probably due to organic matter content and carbonates (18.52% de CaO), making it suitable as a poring agent, while CBA and FG had high P and K content, respectively, confirming their use as a nutrient source. The results of the XRF analysis showed the absence of toxic substances for possible agricultural use. As far as the mineralogical composition of the clays is concerned, it is worth highlighting the enormous similarity between Spanish clays due to their common origin. They presented the same majority, minority, and trace phases, with the exception of the illite, which is the minority phase of the WC, with plagioclase being present in the case of the WC (Table 4). The different origin of RC, comprising one-third of the clays in this study, was evident in the majority phases present in this clay, illite and kaolinite. It should also be pointed out that this clay had chlorite as a minority phase. The thermal behavior of the LWA raw materials (clays and BS) was analyzed by DSC-TG [37]. As can be seen in Figure 1, at up to 600 \({}^{\circ}\)C, no significant differences were observed between the clays studied in terms of weight loss. Up to this temperature, in all \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Clays** & **Major** & **Minor** & **Traces** \\ \hline WC & Cal, Sm & Q, Dol, Ill & Kao \\ BC & Cal, Sm & Q, Dol, Pg & Kao \\ RC & Ill, Kao & Q, Dol, Ch & – \\ \hline \hline \end{tabular}  
\(\mathcal{Q}\)= quartz; Cal = calcite; Dol = dolomite; Pg = plagioclase; Sm = smetic; Ill = illite; Kao = kaolinite; Ch = chlorite.  
\end{table} 
Table 4: Mineralogy composition of the clays under study. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **\% N** & **\% C** & **\% H** & **\% S** \\ \hline WC & 0.04 & 4.35 & 0.45 & - \\ BC & 0.05 & 3.33 & 0.35 & 0.73 \\ RC & - & 0.92 & 0.69 & - \\ BS & 3.02 & 23.45 & 3.32 & 0.09 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Elementary CNH-S analysis of clay materials and wastewater sludge. cases, there was a loss of mass of approximately 3-6% due to the following processes in ascending order of temperature:  
* hygroscopic water loss (up to 150 \({}^{\circ}\)C);
* interlayer water loss in illite and smectic (100-200 \({}^{\circ}\)C);
* organic matter decomposition (between 200-550 \({}^{\circ}\)C);
* dihydroylation of illite (400-550 \({}^{\circ}\)C; [51]), kaolinite (530-590 \({}^{\circ}\)C), and/or chlorite (470-650 \({}^{\circ}\)C) [52];
* around 800 \({}^{\circ}\)C, endothermic event, decomposition of carbonates;
* between 900 and 1000 \({}^{\circ}\)C, exothermic event, peaks associated with the formation of calcium silicates;
* around 1200 \({}^{\circ}\)C, endothermic event corresponding to melting.