## 3 Results and Discussion  
### Workability  
Figure 6a shows the effect of both the W/C ratio and the UFS on the slump table result test. The fitting equations for each UFS replacement percentage show that, in general, there is a linear relationship between increasing W/C ratio and slump. If the methodology is replicated in another study, validated results can be guaranteed for each replacement percentage. It also shows that the higher the UFS replacement, the higher the W/C ratio to obtain the same workability Figure 6b shows an example of the visual appearance of two of the tests carried out, namely the 50% UFS case, but for W/C ratios of 0.52 and 0.57, respectively.  
As shown in Figure 6b, the high absorption of the UFS influences the workability of the mortar. It can be observed that with a W/C ratio of 0.52, the slump diameter is 100 mm, i.e., the lower diameter of the cone. For a W/C ratio of 0.57, a higher workability is obtained, resulting in a slump diameter of 150 mm. It is also possible to conclude that to obtain similar workability, increasing the UFS requires an increase in the amount of water. In particular, to obtain workability similar to that obtained in the reference mortar with a W/C ratio of 0.5 in the case of a UFS replacement equal to 50%, the W/C ratio should be 0.57, and 0.7 in the case of a UFS replacement of 100%.  
From the results shown in Figure 7a, it is worth noting that there are no data for high UFS values and low W/C ratios. This is because these mixes were too dry, and it was not possible to achieve acceptable workability. The reduction in workability with the  
Figure 5: Sample preparation for SEM analysis: oven drying at 70 °C (a); samples after metallisation (b).  
increase in UFS was also observed in other studies [1; 22; 31]. In this regard, Figure 7b shows that for the same W/C (0.5), the slump diameter decreases with higher UFS replacement; from UFS 50 onwards, there is no slump, so the diameter is the same that the flow mould (10 cm). This reduction in workability for high UFS replacement is justified because UFS has a much higher absorption than SS [3; 44]. From Figure 7a, it is also possible to see that, in general, a high W/C ratio and low UFS values increase workability. However, the maximum workability values are found with a UFS of 25%. This is because the smaller UFS particle size facilitates particle mobility. Therefore, it is possible to conclude that the presence of UFS has two opposing effects: on the one hand, the particle geometry improves the workability of the mixes, as stated by [PERSON] et al. [45], and on the other hand, the higher absorption of UFS means that, especially for high UFS replacements, larger quantities of water are required to obtain similar workability.  
Figure 6: Mortar workability: slump table test results in cm (**a**) and comparative example for a fixed UFS value (**b**).  
In Figure 8, the effect of modifying the UFS replacement is observed; as mentioned in the previous paragraph, the UFS has a strong influence on the mortar workability. The reduction in the workability is because of the high absorption of the UFS. This prevents the proper workability of mortars with low W/C ratios; in consequence, higher W/C ratios are necessary to obtain the same workability as the control mortar. This loss of workability is more evident as the percentage of replacement increases.  
Figure 7: Mortar workability: slump table test results in cm (**a**); effect of UFS in mortar workability with W/C = 0.5 (**b**).