## 3 Results and Discussion
### Physical Characterisation of Mortars  
Figures 9-11 show the evolution of apparent specific gravity, dry density and bulk density as a function of UFS replacement and W/C ratio.  
Figure 8: Mortar workability: slump table test results. Comparative example for a fixed W/C ratio.  
Figure 9: \(Cont\).  
Figure 10: Dry density (g/cm\({}^{3}\)) of the mortar mixes (**a**) and dry density decrease (%) (**b**).  
Figure 9: Specific gravity (g/cm\({}^{3}\)) of the mortar mixes (**a**) and decrease in the specific gravity (%) (**b**).  
Figure 9a shows that increasing the UFS decreases the specific gravity of the mortars, which is because, as previously stated, to increase the workability, the W/C should be increased. This increase generates air voids within the matrix, generating a loss of density. In general, it can be observed that mortars with UFS of 100% and W/C of 0.7 showed the lowest densities compared to the control mortar. Figure 9b shows the percentual decrease in the specific gravity between the mortar mixes with UFS 25, 50, 75 and 100% compared with the reference mix (UFS 0%). It is observed that there is a minimal difference between the decrease in UFS 25% and UFS 50% mortars, but as the UFS increases up to 100%, it is evident that the decrease in the specific gravity is higher because the higher W/C ratios needed to manufacture the mortars.  
Figure 10a shows the influence of increasing UFS and W/C on dry density. As in the previous figure, it can be observed that the mortar with UFS of 100% presents the lowest densities with a W/C of 0.7. On the other hand, mortars with UFS of 25% and W/C up to 0.57 present dry densities similar to those of the control mortar. Figure 10b shows the percentual decrease in the dry density as the UFS increases; this is because the higher W/C ratios form voids within the mortar matrix, creating a more porous and lighter structure.  
Figure 11: Bulk density of the mortar mixes (g/cm\({}^{3}\)) (**a**) and bulk density decrease (%) (**b**).  
Figure 11a shows that mortars with UFS of 25% and W/C up to 0.53 achieved similar bulk saturated densities to the control mortar. For mortars with UFS of 50% and W/C between 0.55 and 0.57, a similar behaviour is observed, while for the mortars with UFS of 100%, a decrease in bulk density is observed. Figure 11b shows a similar behaviour as the previous plot; the decrease difference between UFS 25% and UFS 50% is minimal, but as the UFS increases, the percentual decrease in the bulk density is higher because of the higher W/C ratio.  
Figure 12 shows the results for the porosity of mortars with different percentages of UFS and W/C ratios. It is observed that, compared to the control mortar, the porosity of mortars with UFS of 25 and 50% is very similar even with higher W/C ratios; this may be due to the fact that FS contains a higher proportion of fines than SS, creating a denser matrix as stated by other authors [46]. On the other hand, mortars with UFS of 75 and 100% present a higher porosity than the control mortar; this is because although the UFS presents more fines, increases in the W/C ratio increase the air void content within the matrix, making it more porous.  
Figure 13 presents the absorption results in relation to the UFS and W/C ratio. It is well known that porosity is directly related to absorption, so an explanation similar to the one presented in the previous figure is applied in this case. In general, mortars with UFS replacement up to 50% and W/C ratios up to 0.58 show absorptions similar to those of the control mortar.  
Figure 12: Accessible porosity of mortars with UFS (% vol.).  
Figure 13: Absorption coefficient of mortars with UFS (% wt.).