# Hydrothermal Manufacture of Zeolitic Lightweight Aggregates from Clay and Marine Plastic Litter
## 3 Results and Discussion
### Zeolitic Lightweight Aggregates Obtained from Kaolin and Different Proportions of Marine Plastic
#### 3.2.1 Mineralogical Changes and Zeolitization  
The results of mineralogical composition corresponding to the materials obtained in the mixtures of rejected Mg-clay and kaolin are shown in Table 4. The diffractograms are shown in Figure S1 in Supplementary Material. Based on the findings of the first part of the study, it was decided to add an amount of 10 wt.% MPF to maximize the zeolitization of the samples. The formulations containing 67% and 100% rejected Mg-clay do not appear in Table 4, because the specimens obtained after the hydrothermal treatment tended to crumble in aqueous medium, so they were discarded. Firing at 900 \({}^{\circ}\)C did not lead to significant changes in the mineralogy of the sample with 100% kaolin with respect to its counterpart fired at 600 \({}^{\circ}\)C, beyond the complete amorphization of the little kaolinite that remained (Tables 2 and 4). In the case of the granules with 33% rejected Mg-clay, firing has led to the thermal decomposition of the calcite and dolomite of the starting mixture, as well as the destruction of all the phyllosilicates, with the exception of illite, although its content is very low (\(\sim\)2%). The initial feldspar and plagioclase content (4 and 8%, respectively) has also been reduced to \(\sim\)1%. The decomposition of the original phases has led to a significant amorphization of the material (70%), as well as the neoformation of two new phases: pigeonite (4%) and mullite (13%). The DSC-TGA graphs in Figure 2 reveal that these new minerals would not come from kaolin since the peak of mullite formation is located at 1000 \({}^{\circ}\)C, which is higher than that applied. However, the endothermic-exothermic peak system between 800 and 850 \({}^{\circ}\)C in the rejected Mg-clay indicates, first, the structural decomposition of smectite and sepiolite and, subsequently, the crystallization of these two new phases [24]. Regarding the hydrothermal treatment, it has eliminated all the mineral species from the fired specimens, leaving only a few traces of quartz. The amorphous phase content was also reduced, more markedly in the sample with only kaolin (23% amorphous) than in the one containing 33% rejected Mg-clay (52%). This entails a different degree of zeolitization, being much more noticeable in the former (77% zeolite) than in the latter (48%). This suggests that the addition of the rejected Mg-clay (rich in magnesium smectite and sepiolite) could inhibit the zeolitization process under the conditions studied, which could explain why the formulations with 67% and 100% of this clay did not present structural stability. One possible reason for this is such a high Mg content (Table 1), so that for future research, an acid pretreatment could be considered to reduce its concentration. Regarding the developed zeolite species, with respect to the sample with only kaolin, it can be seen that firing at 900 \({}^{\circ}\)C has led to slight changes, compared to the same formulation fired at 600 \({}^{\circ}\)C and then hydrothermally treated (Table 2 vs. Table 4). It stands out that cancriine continues to be the dominant phase in the material fired at 900 \({}^{\circ}\)C (45.4% compared to 33.5% at 600 \({}^{\circ}\)C), followed by nepheline (11.8% compared to 33.2%). Likewise, no nepheline or hydrosodalite appears, but phillipsite (13%) and, to a lesser extent, sodalite (5%) and zeolite A (1.7%), which were not detected in the aggregate fired at 600 \({}^{\circ}\)C. This would indicate that the firing temperature could be a variable that can greatly affect the mineralogical changes during hydrothermal synthesis. With regard to the zeolitic content of the material with rejected Mg-clay, the data in Table 4 reveal that, although the contents of cancriine and zeolite A are very similar to those of the sample with only kaolin, sodalite would not have been formed. In the case of phillipsite and nepheline, their content would be lower than that of the aggregate with 100% kaolin (1.4% and 4.2%, respectively).