## 3 Results and Discussion
### Mechanical Properties of the Concrete Mixes  
The evolution of the compressive strength over time for the different mixes is shown in Figure 5. Regarding the SCM's, comparing the 3 mixes that use natural aggregates (identical proportions), it is observed that the cupola slag (SCC-SC-CS and SCC-DC-CS mixes) provides more strength than the limestone filler and the fly ash (SCC-DC-LF and -SCC-DC-FA), both at short and long ages, despite the instability of the mix. This demonstrates the pozzolanic character of the cupola slag, showing a 13% greater compressive strength at 28 days and 11% greater at 360 days than both mixes and showing once again the importance of the promotion of the pozzolanic reaction by the SCM's in high-performance self-compacting concrete [39]. Of these last two (SCC-DC-LF and -SCC-DC-FA), it is surprising that with the fly ash similar strength is achieved as with the limestone filler, despite the greater porosity of the first, due to the high alkalinity of concrete, which favors the reaction of the SCM. In the comparison of the SCC-SC-CS with the rest of the mixes, great improvements in compressive strength are observed at all ages due mainly to three factors: better compressive strength of EAFS, higher coefficient of friction between particles due to the EAFS roughness, and a better bond between paste and aggregates. Comparing SCC-SC-CS with SCC-DC-CS, there is an increase in resistance of 5% at 28 days and 13% at 360 days due to the incorporation of the EAFS aggregate and a slight reduction in the w/c ratio. Comparing SCC-SC-CS with SCC-DC-LF and SCC-DC-FA, the increase is 20% at 28 days and 25% at 360 days. Figure 6 (obtained by scanning electron microscopy) shows the appearance of the cupola slag particles at 5000\(\times\) after reacting, generating compounds in the form of parallel hexagonal plates, perfectly integrated in the paste and in combination with the cement hydration products. The composition of these plates, determined by EDX, corresponds to hydrated calcium aluminosilicates. Figure 5: Evolution of the compressive strength for the different mixes: the first at 28 days (left) and the second at 360 days (right). In absolute terms, the mixes that incorporate cupola slag exceed 100 MPa at 28 days while the rest of the mixes are over 90 MPa, all of which can be considered high-performance concretes. From 28 to 360 days, the most evolved mix is SCC-SC-CS (16.5%) followed by SCC-DC-FA (12%) and SCC-DC-LF (7.5%) and SCC-DC-CS (7.5%). Also, at 360 days, the mix with cupola SCM and EAFS reaches 130 MPa. Comparing Figures 4 and 5, a remarkable increase in strength can be observed when cupola slag is used as a filler, rather than a cement substitute, which indicates cupola slag's potential as a reactive filler. In Figure 7, the appearance of some of the cracking of the cubic specimens tested at 365 days is presented. As can be seen, due to the high stress applied to these concretes and the excellent paste-aggregate interface, the crack planes have propagated through the paste and aggregates, releasing all their energy through explosive cracks. Figure 6: Appearance of the cupola slag after reacting on the cement paste (5000\(\times\)). Figure 7: Detail of the compressive strength cracking (365 days). The analysis of the cracking behavior of these materials is important, and in addition to the visual analysis, there are methodologies to determine the cracking load from deflection-load curves in flexural tests [40]. The evolution of the elastic modulus over time is shown in Figure 8. Among the mixes that use diabase aggregate, it is observed that SCC-DC-FA has the smallest elastic modulus (37 and 40 GPa at 28 and 360 days), which is due to the great porosity of the mix, allowing greater deformations. The SCC-DC-CS mix shows a modulus 10% higher than SCC-DC-LF, having a slightly lower porosity, due to the pozzolanic character and the greater Blaine surface of the cupola slag SCM.