## 4 Conclusions  
The conclusions that may be drawn from this study are set out below:  
1. The incorporation of up to 10% of recycled concrete powder (RC) slightly decreases (\(\sim\)4%) workability, as compared to a conventional mortar, due to a greater demand for water because of the greater porosity of the adhered mortar present in this material. The simultaneous incorporation of 10% of RC and 20% of fly ash (FA) improves workability by up to 11.8% compared to mortars with only the incorporation of 10% of RC, being an effective solution to reduce the higher water demand caused by the use of RC;
2. In general, the fresh-state density of the binary mortars with RC or with FA is lower than that of the conventional mortar. However, the simultaneous incorporation of both additions (RC and FA) leads to a higher density compared to their counterparts with only RC or only FA;
3. The compressive and flexural strengths of mortars are reduced (compared to conventional mortar) by the incorporation of RC (5-10%). They are between \(-\)9.8% (5%) and \(-\)14.3% (10%) for compressive strength, and between \(-\)2.6% (5%) and \(-\)5.0% (10%) for flexural strength, at 365 days of curing. The addition of FA in the mortars with RC is beneficial, improving their mechanical performance. In this way, the ternary mortars that jointly incorporate 5% of RC and 10% of FA show compressive and flexural strengths similar to those of the conventional mortar;
4. The dynamic modulus of elasticity was not significantly affected by the incorporation of RC, nor by the simultaneous incorporation of RC and FA, at 365 days of curing. All the mortars, regardless of the RC and/or FA content, showed a dynamic modulus of elasticity with values between 37.5 GPa and 38.3 GPa;
5. The exponential relationship between compressive strength and ultrasonic pulse velocity (established for other cementitious materials) was not affected by the incorporation of RC and/or FA;  
Figure 9: Quantification of synergistic effects on the 7-day and 365-day energy performance of the mortars.  
The energy consumed per MPa of strength at 365 days of curing of the mortars with 5% of RC was similar to that of the conventional mortar. The simultaneous incorporation of both additions (RC and FA) reduces the energy consumed per MPa of strength (in comparison to the conventional mortar) by up to 13.2% when using 10% of RC and 20% of FA. These results reveal that the simultaneous incorporation of RC and FA can help achieve higher levels of cement replacement;  
In light of the results obtained in this work, and from a mechanical point of view, the optimal percentage of simultaneous replacement of cement is 10% of RC and 20% of FA.  
Conceptualization M.B. and B.C.; performed the experiments, analysed the data, and wrote the paper; M.B. and B.C.; supervised the research work and revised the paper, I.F.S.d.B., C.M. and J.d.B. All the authors contributed to experiment design and analysed and discussed the findings. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Portuguese Foundation for Science and Technology (FCT) through project PTDC/ECI-COM-31138/2017 (DecarbonCrete).  
Not applicable.  
This study benefitted from the Santander Iberoamerica mobility grant 2020/21, awarded in connection with pre-doctoral university professor scholarship 16/02693, under which [PERSON] conducted research at the Lisbon University--Instituto Superior Tecnico's construction laboratory. The support of the Foundation for Science and Technology, Civil Engineering Research and Innovation for Sustainability (CERIS) research centre, and the Instituto Superior Tecnico and a Government of Extremadura, under grant GR 18122 awarded to the MATERIALS research group is also acknowledged.  
The authors declare no conflict of interest.