## 4 Discussion  
The main objective of the study was to search for a method of valorisation of industrial cork production waste and obtain the recovery of material to produce two different materials: lightweight aggregates and porous ceramics. The project activities suggest the possibility of reusing cork waste in a sustainable way, and the fine grain size of the waste makes it possible to directly reuse the material, which therefore does not require prior grinding. In light of its possible application as a weight-lightening agent in clay-based materials, the neutral pH of the char derived from the pyrolysis of cork dust is important, giving the final product (the lightweight aggregates) a value in the optimal range for plant comfort (6-8), favouring pH and conductivity levels that are appropriate for agronomic use. Use in bricks would also lead to a strong weight-lightening effect, with a significant reduction in the bulk density of the materials and an increase in porosity. The pyrolysis of cork residue was proven to be possible and involved a reduction in volume, mass (60%) and pollutant load; the SEM/EDS analysis also showed the absence of residual heavy metals and high porosity. Analysing all these results concerning the pyrolysis performed at different temperatures, we opted for the use of the char produced at 420 \({}^{\circ}\)C (not fluxed), since that produced at 640 \({}^{\circ}\)C did not have different characteristics that would justify the higher energy consumption needed to reach this temperature. We also opted for the char deriving from non-fluxed pyrolysis since the greater complexity of the flushing system did not guarantee concrete advantages in heat distribution during the process. For lightweight aggregates and porous ceramic materials, we concentrated on a characterization that could give information on the porosity of the material, its lightness and its microstructure, as well as its chemical stability. The tests on the aggregates showed how the organic matter decomposed by combustion could obtain a high porosity value, which in turn increased the water absorption value, allowing the retention of water which is so useful for plants. The XRD and SEM/EDS analyses carried out on the prototypes also confirmed that the inclusion of this char in the composition did not significantly affect the crystalline structure of the aggregates, which remained almost the same as that of the waste-free mixture (the red clay (Char 0%)). Comparing the aggregates formed with (Char 7.5%) and without the use of coffee waste (Char 15%), it was seen that the pyrolysed cork waste behaved similarly to the coffee. It can therefore be said that the use of this waste in the production of light and porous ceramic aggregates containing clay is a valid means to  
Figure 11: XRD pattern of porous ceramic sample containing char 5 wt%. Q = quartz: SiO\({}_{2}\); S = sanidine: (Na, K)(Si\({}_{3}\)Al)O\({}_{8}\); H = hematite: Fe\({}_{2}\)O\({}_{3}\). enhance the waste from the cork processing industry, offering the possibility of obtaining a resource for agronomic and construction purposes, as well as for roof gardens, green roofs and hydroponic crops. Positive results were also found regarding the porous ceramic materials (char \(<\) 1000 \(\upmu\)m); however, they highlighted the impossibility of using the materials created with char percentages higher than 5% (10% and 15%) due to their fragility. In the second formulation of these samples, in which pyrolysed and sieved cork powder smaller than 250 \(\upmu\)m was used, characterization results comparable to those for the first formulation were obtained, but with better visual characteristics for the specimens that would allow their use with char percentages up to 10%. However, the fraction below 250 \(\upmu\)m represented only 8% of the total char created by pyrolysis; therefore, this path is not recommended because there would be too high a percentage of unused waste, which would not justify the better properties or the increase of up to 10% char in the material.