## 4 Conclusions  
The conclusions that may be drawn from this study are set out below:  
* The use of GRC to replace 25% of OPC and MRA to replace 50% of NA lowered concrete fresh-state density by 6.2% and raised its air content by 30%. All the recycled material concrete mixes studied had an air content under 4.5%, the upper limit recommended by the ACI committee for structural concrete;
* Porosity rose by 39.7% in the mixes with 25% GRC and 50% MRA and by 13.2% in the mixes with 25% GRC and 100% NA relative to the reference mix made with 100% OPC and 100% NA;
* The decline in dry density and the rise in electrical conductivity were associated with the incorporation of GRC in mixes with 100% NA as well as those with 50% MRA due to the higher porosity in the recycled materials;
* The use of 25% GRC in conjunction with 50% MRA reduced thermal conductivity by 11.8% and raised specific heat capacity by 9.1%, whilst the values for 25% GRC with 100% NA were a 6.9% reduction in thermal conductivity and a 4.1% rise in specific heat capacity, both relative to concrete with 100% natural aggregate (NA);
* Due to their greater porosity, the new recycled materials concrete may provide better thermal insulation and greater thermal inertia than conventional concrete;
* Cross-referencing concrete properties showed that, whereas using GRC and MRA as replacement materials had an adverse effect on concrete's compressive strength, it improved its thermal properties;
* Mixes with 25% GRC and 100% NA and those with 10% GRC and 50% MRA, with compressive strength values of \(>\)25 MPa, are not only apt for use in building construction, but afford greater energy efficiency than conventional concrete.  
[PERSON] performed the experiments; [PERSON] and [PERSON] analysed the data and wrote the paper; [PERSON], [PERSON] and [PERSON] supervised the research work and revised the paper. All the authors contributed to conceive and design the experiments, and to analyse and discuss the results. All authors have read and agreed to the published version of the manuscript.  
Figure 10: Variation in electrical conductivity/thermal conductivity and electrical conductivity/specific heat capacity relationships for all concrete mixes.  
**Funding:** This research was funded by Spanish Ministry of Science, Innovation and Universities mobility grant ETS18/00313, awarded in connection with pre-doctoral university professor scholarship 16/02693, under which [PERSON] conducted research at the Lisbon University Instituto Superior Tecnico construction laboratory. European Regional Development Fund (ERDF) funding was also received for an Interreg-POCTP research grant (0008_ECO2 CIR_4_E, Project for cross-border cooperation to introduce the eco- and circular economy by reducing waste generation and enhancing waste recycling, management and valorisation in Central Spain, Extremadura and Alentejo) and, in conjunction with the Government of Extremadura, under grant GR 18,122 awarded to the MATERIALS research group.  
**Acknowledgments:** The support of the Foundation for Science and Technology, Civil Engineering Research and Innovation for Sustainability (CERIS) research centre and Instituto Superior Tecnico is also acknowledged.  
**Conflicts of Interest:** The authors declare no conflict of interest.