## 3 Results and Discussion
### Mechanical Characterisation of Mortars  
Figure 14 shows the flexural strength results as a function of the W/C ratio and the UFS replacement, while Figure 15 shows the values of compressive strength as a function of the same two parameters. From the results shown in Figures 14 and 15, it can be concluded that for high W/C ratios and/or high UPS replacements, the mechanical properties of the mortars are reduced. The effect of increasing the W/C ratio on the mechanical properties is clearly documented. From these results, robust conclusions cannot be drawn on the effect of UFS replacement on compressive strength, as there are no case studies with the same W/C ratios and different UFS values. For this reason, these results were completed with the second batch of mixes, which enables the comparison of test specimens (Figure 16) with the same W/C ratio and different UPS replacements. These results are shown in Figure 17a. Figure 16: Mortar specimens with UFS, from 0 to 100%. Figure 14: Flexural strength (MPa) as function of W/C ratio and UFS replacement. Figure 15: Compressive Strength (MPa) as function of W/C ratio and UFS replacement. As previously stated, Figure 17a confirms that increasing the W/C ratio generally decreases the mechanical performance of the mortars. An exception occurs for a UFS replacement of 100%, where the lowest compressive strength is observed for a W/C ratio of 0.6, and it increases with the W/C ratio. This is because, in the case of 100% UFS and a W/C ratio of 0.6, the workability of the specimens was very low, the mortar was too dry, and the specimens had defects that were detrimental to their strength. Regarding the effect of the UFS replacement on the compressive strength for constant W/C values, there is no uniform trend for all cases. This is because, as previously stated, there are two phenomena that have opposite effects on the strength of the mortars. In general, given a fixed W/C ratio, an increase in UFS replacement reduces the mechanical properties of the mortars, as also reported by other authors [1; 22; 31; 47]. However, as UFS has a higher absorption than the SS, this water absorption can, in some cases, lead to an increase in the mechanical properties of mortar and concrete, as reported by other authors [30; 48], because the finer particles of the UFS fill the pores of the mortar matrix, resulting in a denser material. This is especially seen in cases of very high W/C ratios and UFS replacements. This phenomenon is similar to recycled concrete, where recycled aggregates from concrete also have a higher absorption [49]. Figure 17: Compressive strength of mortars (MPa) (**a**) and W/C ratio influence on compressive strength of UFS mortars (**b**). Figure 17b shows the influence of the W/C ratio on the compressive strength of mortars with UFS. In general, it is observed that there is a trend in the loss of compressive strength as the UFS replacement increases, as reported in the literature. The fitting equations for each UFS replacement percentage show the relationship between increasing W/C ratio and decreasing compressive strengths. It is observed that a small variation in the W/C ratio has a major influence on compressive strength when higher percentages of UFS are used. Moreover, it is observed that the slope increases with increasing UFS. For low W/C ratios (0.5-0.52), UFS 25% mortars present an initial tendency to increase their compressive strength up to 5%, but at high W/C ratios (0.55 to 0.7), there is a loss of compressive strength of 17, 19, 23 and 49%, respectively. The UFS 50% mortars show similar behaviour, and it is observed that with a W/C ratio of 0.52, the compressive strength is slightly higher than that of UFS 25% (2%); at a W/C ratio of 0.55, this increase is 22%. However, as the W/C increases from 0.6 to 0.7, the compressive strength losses are 32 and 53%, respectively. In the case of UFS 75% mortars, as in the other two cases, there is a tendency to lose compressive strength.