# 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  
#### 3.1.1 Mineraled Changes and Zeolitization  
The aggregates obtained for each formulation have been designated by the following code: K-xP-600, where K means kaolin, x is the % of MPF added, and 600 is the muffle firing temperature (in this case 600 \({}^{\circ}\)C). For hydrothermally treated samples (see specimens in Figure 3), the term H has been added to the end of the name (K-xP-600-H). It is worth noting that the high plastic content of the specimens containing 20 wt.% MPF led to the crumbling of the specimens after firing in the muffle, so that only the formulations with 0, 5, and 10 wt.% MPF could be studied. The mineralogical changes that occurred during firing and hydrothermal treatment can be seen in Table 2 and Figure 4. The diffractograms are shown in Figure S1 in Supplementary Material. Firing at 600 \({}^{\circ}\)C affected the three formulations in a similar way, irrespective of the added MPF content. A very noticeable increase in the amorphization of the sample (82-87%) stands out, mainly associated with the dehydroxylation of kaolinite (endothermic peak with mass loss at \(\sim\)525 \({}^{\circ}\)C for kaolin in Figure 2), which was reduced from 85% to 2-6%. However, the hydrothermal treatment of the fired samples has affected the mineralogy in the opposite direction, highlighting a very notable decrease in such an amorphous phase in favor of the crystallization of minerals of the zeolite group (zeolite A, zeolite P1, and analcime) and feldsparboids (cancrinite, hydrosodalite, and nepheline). In the case of the feldsparboids detected, the International Zeolite Association integrates them into its zeolite database as they present a zeolite-type framework [28], so they will be considered as such in this study (Table 2). The percentage of plastic added to the starting mixture is a variable that has significantly affected the mineralogy. From a general point of view, the overall zeolite content increases with the MPF content, going from 51.7% zeolite in the sample without plastic, to 58.3% in the sample with 5% MPF, and almost 80% in the materials with 10% plastic added initially (Table 2, Figure 4b). This would be because the pores left by the plastic particles when burning in the muffle would allow (1) a better permeation of the NaOH solution through the whole body of the specimen, and (2) more space for the nucleation and crystallization of the zeolites, both aspects leading to a more effective zeolitization [21]. Zeolite crystallization occurs mainly from the amorphous phase, whose content decreases proportionally with the increase in zeolite, although it is also nourished to a lesser extent by the remnants of kaolinite, illite, and quartz that had not amorphized during firing. Regarding the mineral species neoformed in each sample during the hydrothermal treatment, according to Table 2 and Figure 4a, the presence of cancrinite and nepheline stands out (20 and 17%, respectively) in the sample without MPF, followed by zeolite A (\(\sim\)10%), with the rest of the zeolites presenting lower proportions. However, zeolite A (\(\sim\)20%) is the major mineral in the material to which 5 wt.% plastic was added, followed closely by nepheline (\(\sim\)17%), and to a lesser extent by hydrosodalite (\(\sim\)8%), cancrinite, and zeolite P1 (5-6%). However, in the material with 10 wt.% of MPF, no zeolite A has been detected, and as with the sample without plastic, its dominant mineral phases are cancrinite  
\begin{table}