*3.3. Thermal Tests*

TR-based mortars (samples 3, 4 and 7) showed lower thermal conductivities and diffusivities (80–85%) as compared to the sand equivalent controls (Figure 9) because of the lower specific weight of the specimens due to the polymer characteristics (low specific weight of the aggregate) and also to the voids (entrapped air) at the TR/cement paste interface, which limit heat transport through the material (see Figure 5A) [50,68]. The best results were obtained in the case of the PG mortar (0.2 W/mK, sample 5). Specifically, a corresponding decrease (~90%) of the thermal conductivity with respect to the controls was observed, a result ascribed to the large porosity of the glass beads, which induces a

further increase in the thermal insulation (see Figure 1B). In addition, the TR/PG mixture affected the thermal insulation. Average values (60–65%) were obtained in samples with the presence of 50% of sand (sample 6). An exponential decrease in the conductivity and diffusivity data was observed with the decrease in the conglomerates specific weight. Table 3 also shows the results obtained from the thermal tests.

**Figure 9.** (**A**) Thermal conductivity and (**B**) thermal diffusivity of the cement mortar specimens.

#### *3.4. Wettability Tests*

An investigation on the wettability of the tyre rubber specimens was carried out in a previous work [52]. In the present paper, it was completed with the comparison of the properties of the porous glass mortars. As known, the wettability is the ability of a liquid to maintain contact with a solid surface, thus a hydrophobic behavior is associated with surfaces that repel water (poor wettability), while a hydrophilic behavior is associated with a favorable wettability of the surfaces [69]. A surface is considered hydrophobic if the water contact angle is higher than 90◦, whereas a surface is considered hydrophilic if the water contact angle is lower than 90◦. As has also been formerly observed [52], the surface and the bulk of the sand-based samples (norm and sand) showed an average fast water absorption and a hydrophilic behavior (water contact angle lower than 90◦) due to the hydrophilic porous domains of the cement paste (Figure 10A). Similar results were obtained in the case of the

porous glass mortar (PG), a result ascribed to the hydrophilic nature and the high porosity of the soda-lime aggregate (Figure 5C), together with the presence of the hydrophilic porous domains of the cement paste, which determine a fast penetration of water (Figure 10B). Tyre-rubber specimens (TRf, TRc, TRf/TRc) showed a strong reduction in water penetration both on the surface and on the bulk and a hydrophobic behavior (water contact angle higher than 90◦ [52]), although these samples were more porous than the references (poor adhesion of the aggregate to the cement paste, see Figure 5A). These results were totally ascribed to the hydrophobic nature of the organic aggregate. Maximum hydrophobic performances were obtained in the presence of the finer tyre rubber grain size distribution (TRf, Figure 10C). In addition, in the case of the rubber/sand sample (TRf/Sand) the water absorption was significantly lower than the reference samples (~15% on the side surface, ~25% on the fracture surface), but higher than the TR-mortars due to the halved volume of rubber, which dramatically reduced the net force for water penetration and thus stabilized the deposited drops on the surface [52]. Interestingly, the rubber/glass sample (TRf/PG) showed a low water absorption (~10% on the side surface, ~15% on the fracture surface) ascribed to the contribution of the organic hydrophobic aggregate in spite of the contribution of the hydrophilic and porous glass (Figure 10D). Therefore, in this case, the rubber contributed to the reduction of the net force for water penetration, thus stabilizing the deposited drops on the surface.

**Figure 10.** Wettability tests on the side surface of (**A**) Norm (sample 1) at t = 0 s (left) and at t = 60 s (right), on the fracture surface of (**B**) PG (sample 5) at t = 0 s (left) and at t = 2 s (right), on the side (left) and fracture (right) surface of (**C**) TRf (sample 3) at t = 150 s, on the side (left) and fracture (right) surface of (**D**) TRf/PG (sample 8) at t = 150 s.

It is worth saying that all materials showed a similar behavior because the surface and bulk of all the specimens were almost the same, which means that the mortar features cannot be modified by eventual wear or damage events of the surface.
