## 3 Results
### Technical Performance Results
#### 3.1.7 Mechanical Performance  
The flowability and the 1-, 7-, 28- and 91-day compressive and flexural strength results of the mortar mixtures are shown in Table 9. An important observation was the increase in the flowability by replacement of cement with slags, which is favourable from a practical point of view. This is mainly due to the coarser particle size (in the case of TS1 and TS3) and less reactivity of the slags compared to Portland cement. The compressive and flexural strength results are plotted in Figure 11 as well. It is observed that all mixtures containing slag gained less strength compared to both P_PC up to the age of 91 days. However, the strength ratios were found to continuously increase over time. The strength activity index (SAI) values are reported in Table 10, which suggest that the strength gain in 1 day in the case of all slags was less than 50% of the control mixture, while that of the P_QP mixture was 50%. This is an indication of the retarding effect of the slags on cement hydration and early-age strength development. This confirms the TGA findings on the pastes at 1 day. The P_QP paste developed more bound water (see Figure 8a) and generated slightly more heat (see Figure 4a) compared to P_TS1, P_TS2 and P_TS3. Nonetheless, the compressive strength values of slag-blended mixtures reached 65-70% of the reference mortar at 7 days, which suggests the later-age reactivity of the slags. The 28-day strength values, which are benchmarks for the assessment of the performance of slags as SCMs, were all similar and slightly above 70%. As such, all slags met the minimum 28-day SAI requirements of EN 15167/EN 450-1. In comparison with P_QP, it is worth noting that while the slag-blended mixtures showed similar compressive strength results at 1 day, they slightly exceeded the compressive strength of Q_QP at 7 days. This is in agreement with the calorimetry findings (Figure 4a). It was observed that P_QP initially showed a higher rate of heat flow up to 19 h and then fell below the slag pastes up to the age of 3 days. This might account for the strength development pattern of slag pastes compared to P_QP. Similar strength values compared to slag-blended mixtures were observed for P_QP at later ages. Interestingly enough, P_QP showed lower flexural strength values compared to slag mixtures at all ages. The reason behind the generally higher compressive strength of P_QP compared to slag pastes (despite the higher reactivity of the slag) and its lower flexural strength could have to do with the higher mechanical strength of quartz and its lower chemical reactivity compared to the slags, respectively. As shown in Figure 1, granulation of the slags resulted in the amorphisation of their mineral structure. Test results were also indicative of the fact that such amorphisation further reflected itself in the density of the slags as well. The same slags slowly cooled in air were found to have density values of 3.23, 3.78 and 3.65 kg/m\({}^{3}\), which were considerably higher than those of the granulated slags used in this research (see Table 2). The amorphisation of the microstructure resulted in increased entropy, which often translates to lower mechanical strength. This was while P_QP contained fully crystalline quartz particles that better resist compressive loading compared to granulated slags. Due to the chemically inert nature, however, quartz cannot form chemical bonds with the surrounding cement matrix and thus acts as a point defect at the bottom half of the bending specimen (i.e., the tensile region), causing failure at lower stress levels. In the slag-blended mixtures, in contrast, the formation of hydration products at the interface between slag