## 2 Materials and Methods  
### Porous Inorganic Materials Formulations and Characterization  
The core of the porous inorganic materials (PIMs) was realized using Red Clay sieved below 1 mm from a clay pit in the province of Modena (Italy) and pumice in the form of quarry scraps provided by Europomice Srl (Milan, Italy) with d \(<\) 100 \(\upmu\)m at 1:1 rate. The first study conducted by [PERSON] et al. [12] demonstrated the feasibility of making PIMs with pumice scraps only with high porosity results, therefore, a mixed clay and pumice formulation was tested in this study. Spent coffee grounds collected from coffee bars of Modena, dried and ground to 1 mm, are a post-consumer waste and were used as a porous agent (15 wt%) following the results of previous studies [13]. Potassium and phosphorus were added to the formulation using, respectively, potassium carbonate (K\({}_{2}\)CO\({}_{3}\)) and cattle bone flour ash originated from the calcination of the flour (900 \({}^{\circ}\)C). The nutrients were used as they were (formulation APNUT) and in a fertilizer glass (formulation APV50) as 50 wt% of the core's elements. Vitrification has the benefit of providing a controlled release of nutrients, preventing their waste, leaching in the soil, and the consequent underground water contamination. For this reason, studies and applications of fertilizer glass have been growing in the last few years [14]. The vitrifying agent chosen was pumice scraps, since the chemical and mineralogical analysis showed the basic constituents to form a glassy matrix, as melting components (Table 1), and a high amorphous fraction (79.7 wt%). The main crystalline phases of pumice instead were quartz (1.1 wt%), sanidine (11.2 wt%), and anorthite (3.0 wt%). Fertilizer glass (FG) was obtained in a color factory mixing all the components in a rotatory mill to obtain a homogeneous glass containing P\({}_{2}\)O\({}_{5}\) = 18%; K\({}_{2}\)O = 18%, SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) = 3, and K\({}_{2}\)O + Na\({}_{2}\)O = 20%. The mixture was melted in an electric industrial kiln at 1200 \({}^{\circ}\)C. In this study, pumice was also added to nonvitrified samples. The flash firing of the granules at 1000 \({}^{\circ}\)C simulated in fact the glass melting process and could give similar behavior and controlled release of nutrients of the vitrified fertilizer but with lower energy consumption. Components were mixed manually adding distilled water to obtain a workable paste and then peltetized to form spheres with a diameter of 1-2 cm. The samples were then dried for 24 h at 105 \(\pm\) 5 \({}^{\circ}\)C to remove free water and to avoid the formation of cracks during the thermal treatment. Finally, PIMs were fired in a static oven for 1 h at 1000 \({}^{\circ}\)C to simulate the thermal shock of expanded aggregates in the industrial kiln, obtaining a porous ceramic matrix for the combustion of the organic compounds. Both formulations were analyzed for physical and chemical properties to confirm the possible application in the agricultural field as controlled-release fertilizers. The samples were weighed after the drying process (_Wi_) and after the firing (_Wf_) to measure the weight loss percentage (_WL_ (%)) as the first information of the stability of the matrix and loss of organic compounds. The formula is:  
\[WL(\%)=\frac{Wi-Wf}{Wi}\times 100 \tag{1}\]  
Static water absorption percentage (_WA_24 (%)) was calculated by dipping 10 PIMs in distilled water for 24 h, following the standard procedure UNI EN 772-21:2011 [15]. The absorption capacity represents a fundamental characteristic for the application in gardening. It is strictly connected to the open porosity of the material. The formula is:  
\[WA24(\%)=\frac{Ww-Wd}{Wd}\times 100 \tag{2}\]  
where _Ww_ is the weight of the wet samples after 24 h in distilled water and _Wd_ is the initial dry weight of the samples. PIMs used in soil must also guarantee good drainage capacity to release the absorbed water gradually over time, avoiding its excessive stagnation in the ground.