## 3 Results
### Mineralogical, MIP, and Microstructural Characterization
The decrease in cumulative intrusion is linked to the sintering effect accompanied with densification, which seems to reduce the large pores within the matrix justifying the good values in flexural strength as evidenced in Table 3. The sample fired at 1000 \({}^{\circ}\)C, 60 min showed a porosity of 8%. This trend is in line with the microstructural evolution against with heating temperature from SEM analysis. The samples at 800 and 900 \({}^{\circ}\)C were characterized by similar cumulative curves with a sigmoid trend that permits to hypothesize the cylindrical shape of the pores [41] (p. 171). Figure 5: Cumulative intrusion curves of V75 samples fired at different temperatures and 60 min of dwell time. Regarding the pore size distribution reported in the incremental intrusion curves it is possible to observe that the samples fired at 800 and 900 \({}^{\circ}\)C show similar pore size distributions in the range 0.3-10 microns with small differences in the height of the main peak (3.745 \(\upmu\)m) depending on porosity access to mercury. Interesting to note, for the sample fired at 1000 \({}^{\circ}\)C, that the size pore shift to diameters lower than 0.1 microns (Figure 6). This trend confirms the modification occurring in ceramic matrix when heated at 1000 \({}^{\circ}\)C, 60 min indicating the absence of open porosity due to the increase of a glassy phase and the presence of residual closed porosity as indicated in the SEM analysis. Microstructural study confirms the XRD analysis, besides evidences the effect of time and temperature on the sintering process and on the corresponding microstructure. Sintered V125 samples showed that thermal treatments up to 900 \({}^{\circ}\)C for 60 min is not enough to complete the densification. As can be observed in Figure 7a grains of starting materials are already observed corresponding to not completely sintered microstructure. Instead, after treatment at 1000 \({}^{\circ}\)C for 60 min the microstructure shows a high sintering degree with crystals of neo-formation phases embedded in a glassy phase with residual closed porosity (Figure 7b). Moreover, the investigation of sintered V75 samples at 900 \({}^{\circ}\)C, 60 min confirmed the positive effect of grain size on sintering (Figure 7c). The microstructure is similar to that of sample V125 at 1000 \({}^{\circ}\)C, 60 min. At 1000 \({}^{\circ}\)C the amount of glassy phase is increased closing the large pores and residual closed porosity (Figure 7d). This behavior agrees with the density and shrinkage data from which it is possible to observe an increase of the apparent density due to the viscous densification process (Table 3). Comparing fired samples at the same temperatures, the time increase highlights an improvement of the sintering process due to the reduction of porosity and both diffusion and growth of crystallites. In micrograph reported (Figure 8) the main crystalline phases as wollastonite (W) (from EDS analysis Ca = 34%, Si = 24% and O = 41 wt%), albite (A) (from EDS analysis Na = 9%, Si = 32%, Al = 10% and O = 49 wt%) and quartz (Q) (from EDS analysis Si = 45%, O = 55 wt%) and white grains of Zr silicate (derived from decoration components presents in the ceramic sludge) were identified. It is possible to observe the characteristic morphology (acicular and dendritic crystals, plates, and granules) of the different phases identified. Besides, the EDS analysis confirmed the composition of glassy phase (sodium  
Figure 6: Incremental intrusion curves of V75 samples fired at different temperatures and 60 min of dwell time. calcium silicate glass) and the presence of white agglomerates corresponding to unfused oxides of Cr and Fe (present in the ceramic sludge). These spectra are reported in the supplementary data.