*3.2. Mechanical Tests and Microscopical Characterization of the Mortars*

Figures 3 and 4 report the flexural and compressive strengths of the samples. Reference sand-based mortars (samples 1 and 2) showed the best mechanical performances. In fact, the flexural and compressive strengths of all the unconventional and lightweight mortars were lower than the references, based on the more resistant sand aggregate.

**Figure 3.** Flexural strengths of the cement mortars. On the top: discrete cracks after rupture in the TR specimens (evidenced by the arrow), with the two parts of the sample still connected by the tyre rubber.

**Figure 4.** Compressive strengths of the cement mortars.

As for specimens 3, 4 and 7, characterized by bare tyre rubber, the addition of the elastomeric waste disrupted the mineral skeleton of the mortars after the formation of voids in the composite, which sensibly reduced the specific weight of the samples.

This effect depends on the organic/inorganic interface properties. In fact, an unfavorable adhesion of the aggregate to the cement paste [46–52] was observed and ascribed to the hydrophobic nature of the tyres rubber and to the completely different chemistry of the compounds present in the polymeric structure and in the inorganic matrix (Figure 5A). This evidence was not observed in the sand-based references where a good aggregate/cement paste adhesion was present (Figure 5B).

Accordingly, after total replacement of the sand volume, a decrease in the mechanical performances of these composites was ascribed to the low density of the TR grains and to the voids (entrapped air) created by the aggregate at the cement/TR interface during mixing, which can explain the decrease of the specific weight and an almost double porosity of these samples with respect to the references [46–52] (Table 2).

Specifically, the flexural resistances of samples 3, 4 and 7 were 60–70% lower than the references, while the compressive resistances were ~85% lower than the references (Figures 3 and 4). Replacement of 50% of the sand volume with TR grains (sample 6) led to an increase in the mechanical resistances with respect to the composites with 100% sand replacement, due to the presence of the more resistant sand aggregate. In fact, in this case, the flexural strength decrease was approximately 40–55% with respect to both references, while the compressive resistances were ~70% lower than the reference samples [51,53–57]. The specimen with fine tyre rubber aggregates (samples 3) showed higher mechanical resistances than the coarse tyre rubber type (samples 4), as was also observed in previous works [53–56], a result ascribed to the higher surface area of the fine type elastomeric materials, which, as observed in rheological measurements, improve the cohesiveness of the mixture. Mortars based on PG and PG/TR aggregates (samples 5 and 8) showed an increase in the flexural and compressive strengths with respect to tyre rubber composites, in particular the flexural resistance of sample 5 (PG specimen) was almost double the other tyre rubber samples (samples 3, 4 and 7) together with a three times higher increase of the compressive strength. Moreover, the PG composites showed a decrease in the flexural resistances in the range of 25–50% (sample 5) and 50–60% (sample 8) with respect to the references, and a decrease in the compressive resistances in the range of ~65% (sample 5) and ~80% (sample 8) with respect to the references [58–60]. The presence of tyre rubber was detrimental for the mechanical strengths, which conversely were interesting when bare PG was used because glass showed higher stiffness and better adhesion to the cement paste due to the high roughness of the beads

and to a chemical composition (silicates, aluminates) similar to the ligand matrix (Figure 5C) [11,61]. For this reason, the lowest specific weight of the glass samples was exclusively ascribed to the intrinsic porosity of the aggregate and not to the porosity of the composite at the interface. Table 3 shows the results obtained from the mechanical tests.

**Figure 5.** SEM images of: (**A**) cement/TR interface, (**B**) cement/sand interface, (**C**) cement/PG interface.

The flexural failure mode of the mortars containing bare TR aggregate did not exhibit the typical brittle behavior observed in the conventional sand-based samples (samples 1 and 2), indeed a separation of the two parts of the specimens was not observed but only discrete cracks, ascribed to the tyre tensile strength (Figure 3, on top) [53,62].

Similar to flexural strength observations, the compressive failure observed in the case of TR mortars was more gradual and the specimens showed a high-energy absorption capacity because of the load retention after failure without collapse. It was also observed that many aggregates of the TR specimens sheared off along the failure plane, and accordingly, the bond between the TR aggregate and the cement paste was stronger than the failure strength of the aggregate granules. Reference samples and sample 5 (based on bare PG) showed a typical brittle failure [60,63–65]. A semi-brittle failure was observed in the case of the Sand-TR and of the PG/TR samples (sample 6 and 8). The Rc values of the PG, TRf/Sand and TRf/PG samples were in the CS IV conformity range for plasters, while the Rc values of the TRf, TRc and TRf/TRc samples were in the CS III conformity range for plasters [66]. The Rc values of the PG and TRf/Sand samples were in the M10 conformity range for masonries, while the Rc values of TRf/PG, TRf, TRc and TRf/TRc were in the M5 conformity range for masonries [67].


**Table 3.** Main results from flexural (Rf), compressive (Rc), impact resistance (IR) and thermal tests (λ).

Figure 6 shows a picture of the sections of the specimens after the mechanical tests, where the nature of the aggregates can be observed together with a good distribution of the tyre rubber grains, porous glass beads and conventional sand. The samples, although characterized by different types of aggregates and with different grain size distribution, were extremely homogenous without any form of segregation.

**Figure 6.** Sections of the specimens after the mechanical tests. (**A**) Norm (sample 1), (**B**) Sand (sample 2), (**C**) TRf (sample 3), (**D**) TRc (sample 4), (**E**) PG (sample 5), (**F**) TRf/Sand (sample 6), (**G**) TRf/TRc (sample 7), (**H**) TRf/PG (sample 8).

The temporal evolution of the flexural and compressive resistances of samples 3, 4, 5 and 8 is reported in Figure 7. An increase in the strengths can be observed for every composite with a final stabilization in the range of 60–90 days. This result may demonstrate a stability of the materials, in consideration of the adopted curing/conservation conditions of the conglomerates (in water).

**Figure 7.** Temporal evolution of the (**A**) flexural and (**B**) compressive resistances of sample 3 (TRf), sample 4 (TRc), sample 5 (PG) and sample 8 (TRf/PG).

From the impact compression tests (Figure 8A), it was observed that the references and the PG mortars (samples 1, 2 and 5) were extremely fragile, with a relatively low number of blows necessary for fracture formation (Figure 8D,E).

**Figure 8.** (**A**) Impact resistance of the cement mortars, (**B**) TRf (sample 3), (**C**) TRf/TRc (sample 7), (**D**) Norm (sample 1), (**E**) PG (sample 5).

The best results were obtained with the TR specimens because of the load retention of these composites, ascribed to the super-elastic properties of the elastomeric material and evidenced by a deep groove before complete failure (Figure 8B,C) [46,68]. Specifically, sample 3 with finer grains showed the highest energy absorption capacity ascribed to a better compaction with respect to the other similar composites. As for the flexural and compressive tests, average values were observed in samples with 50% of TR (samples 6 and 8) because of the presence of brittle materials as sand and PG.
