## 4 Conclusions  
This work aims to develop more sustainable asphalt mixtures that fit within the circular economy objectives. Thus, it was designed to maximize steel slag aggregates (SSA) incorporation in asphalt mixtures. The following conclusions were drawn from this study:  
* The SSA characterization revealed the potential of this byproduct for incorporation in asphalt mixtures due to the excellent wearing and polishing resistance and good affinity with bitumen;
* The incorporation of SSA in asphalt mixtures should be limited to 75% for both the surface (AC 14) and the binder/base (AC 20) courses due to workability problems caused by the SSA roughness and porosity;
* Compared to natural granite aggregates, the high particle density of SSA has a significant impact on the design of asphalt mixtures with SSA. The effective binder content used in asphalt mixtures with 75% SSA should be reduced from an equivalent value of 5.0% to only 4.3% due to the density differential;
* The asphalt mixtures with 75% SSA still present lower workability, making it challenging to obtain the mixtures' air void content within the specified limits;
* The mechanical performance (water sensitivity and rutting resistance) of the asphalt mixtures with 75% SSA was excellent, surpassing that obtained for conventional asphalt mixtures with identical compositions produced with natural aggregates.  
Summing up, it was concluded that the developed asphalt mixtures with 75% SSA are sustainable solutions meeting the circularity and performance needs of the paving industry. However, the composition of these mixtures with SSA should be further studied before carrying out a trial pavement section to assess the in situ performance.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Asphalt Mixtures** & \multicolumn{2}{c}{**WTS\({}_{\text{AIR}}\)**} & \multicolumn{2}{c}{**PRD\({}_{\text{AIR}}\)**} & \multicolumn{1}{c}{**RD\({}_{\text{AIR}}\)**} \\  & \multicolumn{2}{c}{**(mm/10\({}^{3}\) Cycles)**} & \multicolumn{1}{c}{**(\%)**} & \multicolumn{1}{c}{**(mm)**} \\ \hline AC 14 surf 35/50 (BB) & 75\% SSA & 0.07 & 0.76 & 2.26 \\  & 100\% NA & 0.20 & 1.80 & 5.40 \\ AC 20 bin/base 35/50 (MB) & 75\% SSA & 0.05 & 0.71 & 2.13 \\  & 100\% NA & 0.24 & 1.73 & 5.20 \\ \hline \hline \end{tabular}
\end{table}
Table 12: Permanent deformation results for mixtures with 75% SSA and natural aggregates.  
Figure 9: Rut depths of AC 14 mixtures after the WTT test: (**a**) natural aggregates; (**b**) SSA.  
**Author Contributions:** Conceptualization, C.M., L.N., C.L., M.R., J.O. and H.S.; methodology, L.N., M.R., J.O. and H.S.; validation, J.O. and H.S.; formal analysis, C.M., L.N. and C.L.; investigation, C.M., L.N. and C.L.; resources, M.R.; writing--original draft preparation, C.M., L.N. and C.L.; writing--review and editing, M.R., J.O. and H.S.; visualization, J.O. and H.S.; supervision, J.O. and H.S.; project administration, M.R. and H.S.; funding acquisition, M.R., J.O. and H.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the PORTUGAL 2020 Partnership Agreement through the Competitiveness and Internationalization Operational Program (POCI) and the European Regional Development Fund (ERDF), under the R&D Project \"RENEw--Construction waste for a circular economy\", with reference POCI-01-0247-FEDER-033834.  
Not applicable.  
The presented data are available in the article.  
The authors would like to thank the technicians from all the institutions involved in this work, who contributed to achieving this study's objectives, and the DST group for providing the materials used in the laboratory work.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.