## 6 Results and Discussion
### Hardened Tests
#### 6.2.2 Flexural Strength  
Figure 14 displays the flexural strength outcomes of the mixtures in group A (SF effect) and group B (optimal SF-based, investigating w/b and SNF effects). In Figure 14a, it is observed that as the SF content increases, the flexural strength of series 1 decreases. The highest bending strength in series 1 was recorded in the mix with 15% SF (6.46 MPa). Examining series 2 mixtures reveals that raising SF from 15% to 20% leads to a 13% increase in flexural strength. In contrast, elevating SF from 15% to 25% caused a 23% decrease in flexural strength. In series 3, increasing SF from 15% to 20% resulted in a 9% reduction in flexural strength, and increasing SF from 15% to 25% resulted in an 11% reduction in flexural strength. The reduction in bending strength due to higher SF doses can be attributed to several factors [96; 97; 98]: (1) SF particles are extremely fine, leading to a high specific surface area. This results in increased water demand. Insufficient water results in a drier cement mix. (2) Higher SF content may reduce efficiency, making it harder to achieve proper density. (3) Excessive SF can cause uneven particle distribution in the cement paste, hindering the formation of calcium silicate hydrate (C-S-H) gel. (4) SF, by absorbing excess water, may deplete the water available for concrete hydration, disrupting the process. (5) Increasing SF dosage can weaken the interfacial transition zone (ITZ) between aggregate and cement paste. For series 1, 2, and 3, the best SF percentages for achieving maximum flexural strength are 15%, 20%, and 15%, respectively.  
According to Figure 14b, the flexural strength of mixtures with increasing w/b ratios (from 0.18 to 0.22) shows a decreasing trend. Therefore, the rise in the w/b ratio in the mixtures of series 1, 2, and 3 led to 23%, 7.4%, and 10.4% reductions in flexural strength, respectively. A higher water-to-binder ratio results in more water in the mixture, potentially increasing porosity. Excess water also reduces the concentration of cement particles, decreasing the level of hydration. Consequently, fewer cement particles are accessible to create the robust bonds essential for achieving high flexural strength [99; 100].  
The increase in SNF from 0.7 to 0.9% enhanced bending strength (Figure 14b). Comparing SF15 WB20N07 and SF15 WB20N09 blends reveals a 17%, 4.8%, and 13% rise in flexural strength in series 1, 2, and 3, respectively, attributed to the higher SNF dosage. The improvement in flexural strength through SNF can be considered a result of the potential of SNF to create better alignment of the concrete microstructure [46; 101]. In other words, the  
Figure 14: Results of flexural strength (28 days): (a) effect of SF; (b) effect of w/b and SNF.  
complete hydration of cement particles is ensured by reducing water content, improving particle dispersion, and providing optimal particle packing [46; 101].  
Figure 15 illustrates the impact of the w/b ratio and SNF on flexural strength. As shown in Figure 15a, a rise in the w/b ratio from 0.18 to 0.22 resulted in a decrease in flexural strength. Series 3 mixtures exhibited the highest flexural strength relative to the w/b ratio. In Figure 15b, it is shown that as the SNF ratio increased from 0.7 to 0.9, the flexural strength also increased. Series 3 mixtures exhibited the highest flexural strength based on the SNF ratio.