# 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
#### 3.1.2 Technological Properties of the ZLAs  
The main characteristics of the materials obtained are shown in Table 3. First, it can be seen that the average size of the fired aggregates is between 10.5 and 10.7 mm. The alkaline treatment did not result in a substantial change in the diameter of the specimens without MPF; however, it favored an increase of approx. 5% in the two samples with the additive. This could mean that the zeolite neoformation has meant a certain volumetric expansion in these samples (crystal growth in small pores and fissures may have \"pushed\" the internal structure), also promoted by the growth of microcrystalline crystals on the surface of the material. As explained in the previous section, zeolitization has occurred most intensely inside the pores (Figure 5b). This has led to a drastic decrease in total porosity (Table 3 and Figure 6a), a phenomenon that has been more pronounced in the sample with 0 and 5 wt.% MPF, dropping from \(P_{\text{T}}\) = 45 and 49% to \(P_{\text{T}}\) = 24 and 26%, implying a decrease of 20-23 points, i.e., 45-47%, respectively. In the case of the sample with 10 wt.% added plastic, the addition of higher plastic content has helped cushion porosity depletion, so the total porosity has decreased 24.5%, from 48.6% to 36.7%. The open porosity is dominant in the fired samples, with values between 32% (sample with 10 wt.% MPF) and 37% (the other two), while the closed  
Figure 5: Images taken by SEM of zeolitized samples: (**a**) zeolite crystals along the entire section of sample K-10P-600-H, with rounded crystalline aggregates of carnitine on the left and aggregates of elongated crystals of nepheline and cancinrite on the right; (**b**) elongated crystals of nepheline and cancinrite within a pore and (**c**) detail view of these in K-0P-600-H; (**d**) tip of an acicular crystal of cancinrite with hexagonal cross-section and tip of a nepheline crystal showing characteristic Miller indices according to Hansen and Falth [29]; (**e**,**f**) analcime crystals with trapezoidal habit and, in (**e**) together with other nepheline tabular/columnars in K-5P-600-H; (**g**) cubic crystals of zeolite A in K-0P-600-H; (**h**) crystalline aggregate with cubic crystals zeolite A, pinecone aggregate of zeolite P1, and lepisheres of hydrosadalite or zeolite P1 in K-5P-600-H; (**i**) pinecone-like crystalline aggregate typical of zeolite P1 in K-10P-600-H.  
porosity increased 7, 12, and 16% with the addition of the plastic. Due to this dominance of open porosity over closed porosity, the former has been more affected by zeolitization, being reduced to values between 21 and 25%, while the closed pores represent between 3 and 12% after treatment, being the sample with 10 wt.% of MPF less affected (Table 3). The clogging of the open pores is reasonable since the alkaline solution would penetrate through it without difficulty, and the zeolites can crystallize on the walls of these pores and open fractures (Figure 5b). However, the decrease in the closed porosity is counter-intuitive, and could indicate that the alkaline treatment is capable of permeating a large part of the structure through the different physicochemical and mineralogical transformations that have taken place, even altering the walls of the closed pores, in which crystals could also have grown. \begin{table}
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This decrease in porosity is critical, as it has affected the technological properties of the material. Thus, while the water absorption in the samples only fired was around 23-27%, this figure has been reduced after zeolitization to 12% and 17% in the samples with 0 and 5% MPF and 10% MPF, respectively, representing a variation over the non-treated specimens of more than \(-\)50% in the former two and almost \(-\)30% in the latter (Table 3, Figure 6b).