**4. Conclusions**

End-of-life tyre rubber (TR) and recycled waste porous glass (PG) were employed for the production of lightweight eco-sustainable cement conglomerates specifically for indoor applications. A cheap and environmentally friendly process was used because the aggregates were not pre-treated. The secondary raw materials were added as partial and/or total replacement of the conventional sand aggregate, which was made on a volume basis rather than on a weight basis due to the low specific weight of both waste materials. The specimens were characterized by rheological, mechanical, thermal, microstructural and wettability tests.

TR and PG were added as partial and/or total replacement of the conventional aggregate, which was made on a volume basis rather than on a weight basis due to the low specific weight of both waste materials. In the present case, the total volume of aggregate was set at 500 cm3 in order to preserve an acceptable workability of the mixture. The samples were prepared with a water/cement ratio equal to 0.5, a value that was chosen according to the standard protocol for the normalized mortar preparation, showing a plastic behavior.

The main results showed that:

(a) Fine TR aggregates affected the cohesiveness of the mixtures as opposite to coarse TR and PG types.

(b) The flexural and compressive strengths of the unconventional and lightweight mortars were lower than the references, based on the more resistant sand aggregate. Specifically, a decrease of ~60% and ~85% respectively of the flexural and compressive strengths was observed in the case of TR samples, whereas a lower decrease was observed in the case of the PG specimens (~25% and ~65% decrease respectively of the flexural and compressive strengths). The flexural and compressive strengths of the PG samples were higher than the TR samples because of the higher stiffness and good adhesion of the glass to the cement paste, ascribed to the roughness of the aggregate surface together with a chemical composition (silicates, aluminates) similar to the ligand matrix. On the contrary, an unfavorable adhesion of the rubber aggregate to the cement paste was observed and ascribed to the hydrophobic nature of the organic material and to the completely different chemical composition of the polymer and of the inorganic matrix.

(c) The specimen with fine tyre rubber aggregates showed higher mechanical resistances than the coarse tyre rubber type, a result ascribed to the higher surface area of the fine type elastomeric materials, which, as observed in rheological measurements, improves the cohesiveness of the mixture.

Replacement of 50% of the sand volume with TR grains 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.

(d) 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 and in the bare PG samples, indeed a separation of the two parts of the specimens was not observed but only discrete cracks were noticed and ascribed to the rubber tensile strength.

(e) From the impact compression tests it was observed that the references and the PG mortars were extremely fragile, and 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.

(f) From thermal measurements it was observed that the thermal conductivity and diffusivity decrease of the lightweight materials (tyre rubber and porous glass specimens) was in the range of ~80–90% with respect to the sand-based samples. This was ascribed to the large porosity of the glass

beads and, in the case of the TR specimens, to the voids at the TR/cement paste interface, which limit heat transport through the material.

(g) Suitable applications in the construction industry as non-structural artifacts may be found for all the samples, as acceptable compressive data for plasters and masonries were obtained. Specifically, after wettability investigations, bare PG specimens may be suitable for indoor applications, while TR specimens may be also suitable for outside elements exposed for example to water flowing and capillary rise. The latter application may also be indicated for the glass/tyre rubber mortars characterized by hydrophobic behavior and low water absorption, ascribed to the presence of the organic aggregate, and by interesting mechanical resistances and high thermo-insulation, mainly ascribed to the inorganic aggregate.

Finally, it is worth considering that these waste/cement composites are cost-effective and environmentally sustainable construction materials because they are prepared through a cheap and eco-friendly process where the aggregates were not pre-treated and the mixture preparation did not require complex manufacturing processes or expensive procedures.

**Author Contributions:** Conceptualization, M.N.; methodology, R.D.M.; software, F.T.; validation, C.L.; formal analysis, A.P.; investigation, A.P.; data curation, S.D.G.; writing—original draft preparation, A.P.; writing—review and editing, A.P. and M.N.; supervision, M.N.

**Funding:** This research received no external funding

**Acknowledgments:** The authors wish to thank Pietro Stefanizzi and Stefania Liuzzi for thermal analysis. The authors also thank Adriano Boghetich for SEM-EDX analysis. Regione Puglia is gratefully acknowledged for financial support (Micro X-Ray Lab Project–Reti di Laboratori Pubblici di Ricerca, cod. n. 45 and 56). The Department of Civil, Environmental, Land, Construction and Chemistry (DICATECh) of the Polytechnic University of Bari is gratefully acknowledged for SEM analyses.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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