## 3 Results and Discussion  
### Characterization of Samples  
The use of bottom ash instead of silica fume allowed the realization of lighter porous sample, as shown in Figure 1. The colorimetric analysis defines a color space through CIELAB coordinates: L (luminance) and two-color channels (a and b). L ranged from 0 (dark) to 100 (white), while a and b changed from negative to positive values (green \(<\) a \(<\) red and blue \(<\) b \(<\) yellow). SUNSPACE changed its color from dark grey (L = 39.83) to light grey (L = 81.97) using bottom ash instead of silica fume, as reported in Table 1. The improvement of the color represents an advantage for the development of the material as plaster on urban buildings. SUNPACE SF spectrum, as already reported in [21], was characterized by a large halo between 15-30\({}^{\circ}\) (2\(\Theta\)) due to the presence of amorphous phase, such as silica fume. Cristobalite (SiO\({}_{2}\)) and sodium iodidate hydrate (NaIO\({}_{3}\)-H\({}_{2}\)O) peaks were identified in the spectrum. This last phase was formed during the material synthesis. On the contrary, SUNSPACE BA was characterized by calcite (CaCO\({}_{3}\)), quartz (SiO\({}_{2}\)), and ettringite (Ca\({}_{6}\)Al\({}_{2}\)(SO\({}_{4}\))\({}_{3}\)(OH)\({}_{12}\)-26H\({}_{2}\)O) due to the presence of bottom ash and calcium hydroxide used in the material synthesis [28]. In addition, as in the SUNSPACE SF spectrum, some sodium iodidate hydrate peaks were identified, generated during the material synthesis. Finally, the main crystalline phases of the cement sample were calcite (CaCO\({}_{3}\)) and calcium silicate oxide (Ca\({}_{3}\)(SiO\({}_{4}\))O). Some peaks could be attributed to ettringite (Ca\({}_{6}\)Al\({}_{2}\)(SO\({}_{4}\))\({}_{3}\)(OH)\({}_{12}\)-26H\({}_{2}\)O), gypsum (CaSO\({}_{4}\)-2H\({}_{2}\)O), grosavil (Ca\({}_{3}\)Al\({}_{2}\)(SiO\({}_{4}\))\({}_{3}\), and iron oxide calcium oxide (Fe\({}_{2}\)O\({}_{3}\)(CaFigure 2 shows the XRD spectra of SUNSPACE SF, SUNSPACE BA, and cement samples. Figure 3 shows the optical microscopy images collected on all samples. The presence of macro pores on SUNSPACE SF (Figure 3a) and SUNSPACE BA (Figure 3b) was evident, while cement material (Figure 3c) showed a uniform surface with smaller pores in dimensions. The image of _Hedera Helix_ L. (Figure 3d) surface showed no evident pores, very likely due to the low magnification of the image, but the presence of voids between the long tubules that promote fine PM capture was already reported in a previous work [20]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**SUNSPACE SF**} & \multicolumn{2}{c}{**SUNSPACE BA**} \\ \cline{2-5}  & **SCI** & **SCE** & **SCI** & **SCE** \\ \hline L & 39.83 & 39.61 & 81.97 & 81.49 \\ a & \(-0.92\) & \(-0.87\) & 0.77 & 0.80 \\ b & \(-2.33\) & \(-2.36\) & 6.34 & 6.22 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Results of colorimetric analysis made on SUNSPACE SF and SUNSPACE BA. Data were evaluated including SCI and excluding SCE specular component. Figure 2: X-ray diffraction (XRD) patterns of SUNSPACE SF, SUNSPACE BA and cement samples. Figure 4 shows SEM images of SUNSPACE BA at different magnifications (a and b), SUNSPACE SF (c), and cement material (d). As reported in [PERSON] et al. [28], bottom ash with particle size lower than 300 \(\upmu\)m was mainly characterized by amorphous phase (60%), suggesting its high reactivity with calcium hydroxide. Indeed, SUNSPACE BA, unlike the other two samples, showed a fibrillar matrix, clearly visible in the Figure 4b, due to the C-S-H formation typical of pozzolanic reaction [29]. This branched morphology may have a positive influence on PM adsorption, promoting particles entrapment in the sample. According to [21], SUNSPACE SF is characterized by spherical particles of silica, with size dimensions from 20 to 700 nm, agglomerated together. Macro pores with dimensions of some \(\upmu\)m were clearly identified on the SUNSPACE SF surface, as reported in Figure 4c.