## 4 Experimental Results  
First, the experimental results of preliminary AS tests (10 \({}^{\circ}\)C, 20 Hz) on binder-, mastic-, and fine-mortar are depicted in Figure 4a, and allow identification of the LVE limits used for FS analysis. Such LVE limits (calculated as average of the three test replicates) were equal to 0.40%, 0.20%, 0.08%, and 0.07% for _BIT_, _MAS-L_, _MOR-L_, and _MOR-S_, respectively. The results measured for _BIT_ were in accordance with those commonly found for a non-polymer-modified bitumen for road applications. As expected, the progressive inclusion of lithic fractions (passing from binder to mastic and fine mortars) led to a clear stiffening effect that determined progressively reduced LVE domains. Comparing limestone- and steel-slag-based samples, no substantial differences in LVE limits were detected at this scale of analysis, notwithstanding an expected higher stiffness for _MOR-S_. Frequency sweep test results, achieved within the identified LVE domain, are presented in terms of phase angle and complex shear modulus in the Black space (Figure 4b). Again, the above-mentioned stiffening effect due to the addition of the mineral particles was clear. Higher stiffness of mastic- and fine-mortar samples was even more marked in the central part of the curves which refer to the intermediate and high in-service test temperatures (approximately between 30 and 60 \({}^{\circ}\)C). At higher temperatures (where rutting potential is higher), such a stiffening effect was still significant. Within the whole investigated domain, _MOR-S_ showed higher stiffness with respect to the corresponding reference _MOR-L_. Moreover, this higher stiffness generally corresponded to a slightly lower elasticity of the material (higher phase angles \(\delta\)). This suggested that the addition of manufactured steel-slag fine aggregates to the binder, rather than conventional limestone, can affect the bitumen-aggregate interaction, and thus the overall rheological response of the blends. Furthermore, the alignment of the data series in the Black space demonstrates the validity of the time-temperature superposition principle also at this scale. With respect to the \(G^{\star}\)/_sin\(\delta\)_ parameter, the average values (three replicates) resulting from RTFO-aged bitumen analysis (Table 4) suggest a Performance Grade upper limit likely equal to 52 \({}^{\circ}\)C, which can be considered acceptable for a 50/70 penetration grade plain bitumen. Figure 4: AS and FS results: \(G^{\star}\) vs. \(\gamma\) for LVE limit study (**a**); Black diagrams \(\delta\) vs. \(G^{\star}\) (**b**). As far as MSCR test results are concerned, the experimental findings in terms of percentage recovery \(R_{\%}\), non-recoverable creep compliance \(J_{nr}\), and non-recoverable creep compliance to total creep compliance \(J_{nr}/J_{tot}\) are depicted in Figure 5. For the sake of brevity, only the results obtained for the more critical applied stress levels \(\tau\) (3.2 and 10 kPa) are plotted. As expected, the above-mentioned stiffness characteristics, hierarchically greater for _MOR-S_, _MOR-L_, and _MAS-L_, clearly reflect a gradually-increasing aptitude to recover deformations, even if at the higher tested temperatures (70 and 80 \({}^{\circ}\)C) such a strain-recovery ability significantly decreased. Similar considerations can be drawn based on \(J_{nr}\) experimental findings. Overall, mastic and fine mortars showed a behavior not dependent on test temperature and stress level up to 70 \({}^{\circ}\)C. As far as the influence of steel slag (_MOR-S_) on the non-recoverable creep compliance is concerned, a higher anti-rutting effect with respect to the reference case (_MOR-L_) was clearly observed (lower strain accumulation). According to specific literature [37], such results are likely related to the higher stiffness in the case of steel slag blends with respect to the corresponding traditional limestone-based material.