*3.1. Characterization of the Aggregates and Rheological Tests of the Mortars*

Figure 1 shows the scanning electron micrographs (SEM) of tyre rubber grains and of PG beads. An intrinsic micro-scale texture of the elastomeric aggregates can be observed (Figure 1A), while the glass aggregates (Figure 1B) show a large open porosity together with a large closed porosity (inset Figure 1B). The properties of these secondary raw materials can explain the properties of the resulting mortars. In this respect, Table 2 shows that the TR and PG samples are lighter and with a much higher porosity than the references, while, among the lightweight composites, PG mortars were the most porous.

**Figure 1.** (**A**) Tyre rubber (TR) grain and (**B**) porous glass (PG) bead with evidenced porosity (in the inset: inner porosity).

Flow-test measurements were carried-out on the mixtures in order to determine the consistency of the fresh specimens (Figure 2).

Sample 2 (sand sample) showed a higher flow (+56%) than the control (normalized mortar (norm), with a plastic behavior) due to the lack of fines in the sand aggregate. The sample with bare fine tyre rubber aggregates (sample 3) showed a flow decrease in the range of 44.5% with respect to the reference [46], as opposite to sample 4 with bare coarse tyre rubber aggregates, which showed approximately the same flow as the control (+9%). The former result is ascribed to the absence of

the fine aggregates, with a higher specific surface, which contributes to the decrease of plasticity and increase of cohesiveness of the specimen. Sample 6, with fine TR and coarse sand, showed a plastic behavior as the flow was similar to the control (+8%), while the PG composite (sample 5) showed a flow increase in the range of 30% because of the absence of fine aggregates. Finally, the presence of the finer TR fraction is associated with the decrease of workability of the samples 7 and 8 (−25%).

**Figure 2.** Flow-test results with respect to the normalized mortar (norm sample).
