## 3 Results and Discussion
### Reuse of the Metal Laden Perlite in the Construction Field  
After test no. 7 (4 g of perlite, 0.3 L\(\cdot\)h\({}^{-1}\) flow-rate, 10 mg of Pb\({}^{+2}\) L\({}^{-1}\)), exhausted beads were characterized by SEM analysis. No changes were observed on the structure of the sorbent, as shown in Figure 5.  
Figure 4: (**A**) Breakthrough curves for lead ion retention at 0.15, 0.2, 0.3, and 0.4 L\(\cdot\)h\({}^{-1}\) flow-rates (1–2 mm bead size range; 2 mg of Pb\({}^{+2}\) L\({}^{-1}\); 4g perlite; T = 298 K). (**B**) Time-dependent breakthrough curves. (**C**) Correlation of the half exchange time, t\({}_{0.5}\), vs. flow-rate. The silico-aluminate materials were finally reused in the construction field as lightweight aggregates in cement mortars, so as to minimize technical and economic problems related to regeneration processes [17; 23; 45; 46]. As observed in previous studies [47], metals leaching through the composite were few. Accordingly, the specimens were thermally and mechanically characterized, as shown in Table 2 and Figure 6. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & \(\varrho\) (**Kg m\({}^{-3}\)**) & \(\lambda\) (**W m\({}^{-1}\) K\({}^{-1}\)**) & **Rc (Mpa)** \\ \hline Normalized mortar & 1960 \(\pm\) 100 & 2.02 \(\pm\) 0.10 & 49 \(\pm\) 3 \\ Sand (100\%) & 1920 \(\pm\) 100 & 1.59 \(\pm\) 0.08 & 48 \(\pm\) 3 \\ Sand (75\%)/Perlite (25\%) & 1750 \(\pm\) 88 & 1.2 \(\pm\) 0.06 & 34.5 \(\pm\) 2 \\ Sand (50\%)/Perlite (50\%) & 1600 \(\pm\) 80 & 0.90 \(\pm\) 0.05 & 26 \(\pm\) 1 \\ Sand (25\%)/Perlite (75\%) & 1350 \(\pm\) 68 & 0.60 \(\pm\) 0.03 & 21 \(\pm\) 1 \\ Perlite (100\%) & 1050 \(\pm\) 53 & 0.39 \(\pm\) 0.02 & 16.4 \(\pm\) 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Thermal and mechanical properties of the controls and perlite-based composites. Figure 5: SEM image of a perlite bead, after the sorption process (**top**) and magnification (**bottom**). Thermal conductivity, \(\lambda\), is the property of a material to transfer heat. It indicates the temperature response of specimens to heat flow impulses [48; 49]. Perlite-based mortars showed lower thermal conductivities (from 40% to 80%), compared to the normalized mortar, taken as a control. An increase of perlite content induced an exponential decrease of conductivity, as shown in Figure 6A. The addition of lightweight aggregates creates voids in the composite matrix, thus sensibly limiting heat transport phenomena. These voids barely contributed to the extended internal porosity of perlite, as shown in Figure 1, as there was a strong adherence at the cement/aggregate interface, as shown in Figure 7. This is opposite to what was observed in composites characterized by cement and organic materials, such as tire rubber, where unfavorable adhesion was observed, due to the different nature of the constituents [50]. In the present example, this perfect adhesion may be ascribed to the similar nature of the compounds present in the ligand paste and aggregates (silicates, aluminates). As perlite volume increased, the thermal insulating properties of the samples improved, due to the decrease in the weight of the composites. In the case of the resistance to compression tests (after 28 days aging), the perlite-based composites showed a decrease in mechanical performance, with increase of the silico-aluminate aggregate volume, as shown in Figure 6B. This result is in accordance with the thermal measurements, as the weight decrease induced a decrease in mechanical strength, due to the large number of voids introduced into the cement conglomerates. It is also worth to noting that the mechanical performances were extremely interesting, specifically in the case of specimens with 50% and 75% perlite, which could be considered a good compromise between thermal insulation and compressive resistance. Applications of these eco-friendly materials may be useful for non-structural composites, such as plaster or panels [24; 50; 51], thanks to the presence of lightweight, insulating silico-aluminate aggregates.