## 3 Results
### LWA Characterization
#### 3.2.2 Mineralogical and Microstructural Analysis  
XRD patterns of the two LWA formulations (Figure 5) indicated that there were no particular differences between the two from the qualitative point of view. The crystalline phases identified were the same and derived from the fact that the clay was rich in iron and the carbonic phase, demonstrating that char introduced in the clayey matrix did not change the type of mineralogical composition.  
SEM was used to analyse the microstructure of the biochar obtained after pyrolysis (Figure 6). The pores formed in the biochar during the pyrolysis process were visible on the surface of the material and were attributed to both the nature of the original input material and to the release of volatile and organic substances during the thermal process. The concentration of C in the biochar was determined using EDS.  
Regarding the LWAs obtained, the cross-section images for Char 7.5% (Figure 7) show a particular morphology. The images (Figure 7a,b) show how the porosities were homogeneously distributed, with the presence of very small pores, between 1-5 \(\upmu\)m. EDS (Figure 7c) was used for the elementary semi-quantitative analysis, which confirmed a silicate matrix given the strong presence of Al, Si, K, Ca, Mg and Fe.  
On the external surface (Figure 8), there was both a lower porosity and a smaller average pore size (1-2 \(\upmu\)m) compared to the internal section. This was certainly due to the formation of a glassy phase during the flash-firing, which improved the sintering. From the SEM images (Figure 8b), it is possible to notice the presence of small, white crystals of about 1 \(\upmu\)m in size, rich in iron and probably related to the ferruginous clay used.  
Figure 4: LWA material obtained with 15 wt% Char in the composition.  
Figure 5: XRD patterns of Char 7.5 wt% and Char 15 wt% samples. Q = quartz: SiO\({}_{2}\); S = sanidine: (Na, K)(Si\({}_{3}\)Al)O\({}_{8}\); H = hematite: Fe\({}_{2}\)O\({}_{3}\); C = Calcite: CaCO\({}_{3}\).  
Figure 6: EDS spectrum (**a**) and SEM micrograph (secondary electrons) (**b**) of Char pyrolysed at 420 \({}^{\circ}\)C.  
Figure 7: _Cont._  
Concerning the Char 15% sample, it is possible to see from the SEM images at 600\(\times\) (cross-section) the presence of geometric formations with long-limbed development, probably due to areas of accumulation of char, and which had porous holes of the order of 1-5 \(\upmu\)m (Figure 9a). These conformations with particular geometries were composed mainly of clay minerals, confirming the complete decomposition of char, which resulted in porosity slightly more irregular than the Char 7.5% sample.  
The SEM image at higher magnification (Figure 9b) shows pores with dimensions slightly smaller than those of the Char 7.5% samples and a more vitrified nature, although  
Figure 8: SEM micrographs (backscattered electrons) of LWA Char 7.5% external surface with magnification of 600\(\times\) (**a**) and 2400\(\times\) (**b**), along with the relative EDS spectrum (**c**).  
Figure 7: SEM micrographs (backscattered electrons) of a cross-section of the LWA Char 7.5% sample with magnification of 600\(\times\) (**a**) and 2400\(\times\) (**b**), along with the relative EDS spectrum (**c**).  
the composition remained clay. For the external surface, similar conclusions as for the Char 7.5 sample can be drawn.  
The differences between the two LWA samples, such as the different homogeneities of the porosities and the sizes of the pores, were attributed to the much greater volume of char in the Char 15% sample rather than to the different percentages of char used. Indeed, it is possible to see how cork char behaved similarly to coffee as a pore-forming agent, but the difference between the two materials lay in the specific weight and, therefore, in the lower density of the cork, which led to the higher volume used.