# Cleaner Design and Production of Lightweight Aggregates (LWAs) to Use in Agronomic Application
## 3 Results and Discussion
### Sintered Materials  
#### 3.2.1 Chemical and Physical Properties  
According to [PERSON] et al. [36], if the SiO\({}_{2}\)/\(\sum\)Flux ratio \(\geq\) 2, the chemical composition of the sample is suitable to produce expansion when subjected to sudden heating at a high temperature. In Table 3, it can be seen how the RC met this condition, as well as  
Figure 1: Differential scanning calorimetry and thermogravimetric analysis (DSC-TG) graphs of the clays (white clay (WC), black clay (BC), and red clay (RC)) and wastewater sludge (BS). the BC. However, the Spanish mixture of clay using WBC no longer complied with this condition as it had a value in the parameter of 1.9, slightly lower than 2. [PERSON] [34] and [PERSON] [55] indicate in their works that lightweight aggregate is formed due to the development of a very viscous phase that is capable of trapping the gases released by the thermal alteration/decomposition of some mineral and/or organic species. Therefore, on the basis of the investigations of [PERSON] [34] and [PERSON] [56], we were able to determine a zone of chemical composition in a triangular diagram, which responded to the conditions of viscosity and bloating, i.e., to the conditions of expansion. In accordance with this, the sample located outside this region would in principle not be suitable for expansion. On the other hand, it is likely that mixtures located inside and therefore having the right chemical composition to form the necessary viscosity will not expand because the viscosity developed after the gases escaped to the outside. Therefore, Figure 2 shows how the RC clay and the \(\mathrm{RCB_{15}}\) sample were located within the expansion area, while the rest remained outside it. Some of them, such as the \(\mathrm{RCF_{FG}}\) and the \(\mathrm{RC_{CBA}}\), were very close to the limits of the area. Indicating the fact that the samples did not meet the Riley and Conley criteria or the \(\mathrm{SiO2}/\bigtriangledown\mathrm{Flux}\) condition is not a decisive process. Recent work indicates that these criteria are not totally reliable, and in many cases, it has been proven that lightweight aggregates are obtained from samples with compositions located outside the expansion zone [57; 58]. The European standard EN-13055-1 [29] states that an aggregate is considered light if its \(\mathrm{\rho_{b}}\) is \(<\)1.20 g/cm\({}^{3}\) or its \(\mathrm{\rho_{rd}}\) is \(<\)2.00 g/cm\({}^{3}\). On this basis, only the \(\mathrm{WB_{FG}}\) and \(\mathrm{WB_{CBA}}\) aggregates complied with the standard (Table 5), and therefore the rest of the mixtures tested would not be suitable to obtain lightweight aggregates according to the protocol established in the present investigation. However, although the \(\mathrm{WBB_{S15}}\) sample did not comply with the limit requirement for \(\mathrm{\rho_{b}}\) and the \(\mathrm{RC_{CBA}}\) and \(\mathrm{RC_{FG}}\) with that of \(\mathrm{\rho_{rd}}\), the fact that their densities were very close to the established values allows these materials to continue to be considered interesting. The WBC, RC, and \(\mathrm{RCB_{S15}}\) samples would not be  
Figure 2: Representation of the aggregates studied in the Riley diagram [20]. suitable for the manufacture of lightweight aggregates under the conditions established in the study. [PERSON] et al. [28], after examining the results published by different authors, proposed a standard classification of light aggregates on the basis of their \(\rho_{\text{b}}\). Using this classification, it can be established that WB\({}_{\text{CBA}}\), RC, and RC\({}_{\text{CBA}}\) aggregates showed a low density, while RC\({}_{\text{FG}}\), RCBS\({}_{\text{15}}\), WBC, WBBS\({}_{\text{15}}\), and WB\({}_{\text{FG}}\) were classified in the medium density group. According to [PERSON] et al., low-density lightweight aggregates are the most common, representing about 40% of the aggregates considered in their study. On the other hand, low-density values are related to high open porosity values (Table 6).