## 6 Results and Discussion  
### Fresh Tests  
#### 6.1.1 Slump Flow Test  
Figure 7 shows the results of the slump flow test. The slump flow test indicated acceptable filling ability; the higher the slump flow, the greater the concrete's ability to fill the mold. Examining the mixtures of group A shows that the increase in SF substitution caused a decrease in slump flow. A decrease in workability with an increase in SF content has also been reported in other studies [68; 69]. In series 1 mixtures, increasing SF from 15% to 20% caused a decrease in slump flow by almost 3%, and increasing SF to 25% resulted in a reduction of about 6%. Examining the mixtures of series 2 also shows a 2.92% and 5% decrease in slump flow when SF was increased to 20% and 25%, respectively. The series 3 mixtures also experienced 3.32% and 5.62% reductions in slump flow when SF was increased to 20% and 25%, respectively. SF, an exceptionally fine material (approximately 100 times smaller than cement particles) [70; 71], effectively fills the voids between cement grains and larger aggregates when its dosage is increased. This phenomenon enhances the packing density of the concrete mix [70]. While this improvement translates to increased strength and durability, it can also lead to a stiffer mix. Furthermore, SF undergoes a chemical reaction with calcium hydroxide within the concrete matrix, resulting in the formation of additional calcium silicate hydrate gel (C-S-H). This reaction consumes free water in the mixture, potentially leading to a further reduction in slump flow [72; 73; 74; 75].  
Increasing the w/b ratio from 0.18 to 0.22 increased the slump flow. In this regard, the comparison of the SF15 WB18N08 and SF15 WB22N08 mixtures showed that in the mixtures of series 1, an increase in slump flow of about 11.9% was recorded; in the mixtures of series 2, an increase of about 13% was recorded; and in the mixtures of series 3, an increase of about 9.4% was recorded. The presence of water in a mixture acts to diminish interparticle friction, facilitating enhanced particle mobility [76]. This phenomenon translates to a reduction in cohesiveness and an increase in fluidity, consequently resulting in a higher slump flow value [76].  
Figure 6: Displaying inputs and outputs in the FL model.  
Increasing SNF from 0.7 to 0.9 increased the slump flow. Therefore, slump flow was increased by 10.9%, 8.9%, and 8.6%, respectively, in the mixtures of series 1, 2, and 3 due to the increase in SNF. Superplasticizers, a class of surface-active agents, are proficient at dispersing cement particles. Their mechanism of action is twofold. First, due to their inherent electrostatic charge, superplasticizers adsorb onto the surface of cement particles, inducing electrostatic repulsion that hinders particle agglomeration [76; 77; 78]. Second, superplasticizers effectively reduce the surface tension of the water layer naturally surrounding cement particles. This phenomenon facilitates enhanced particle mobility by minimizing resistance to interparticle movement [76; 77; 78].  
The main trends observed in the slump flow results indicate that increasing silica fume (SF) content reduces workability across all mix series, with greater SF levels (20-25%) leading to up to a 5-6% reduction in slump flow. Conversely, increasing the water-to-binder ratio from 0.18 to 0.22 improved slump flow by up to 13%. Additionally, increasing the superplasticizer dosage (SNF from 0.7% to 0.9%) led to an 8-11% rise in slump flow.