## 6 Results and Discussion
### Hardened Tests
#### 6.2.3 Compressive Strength  
Figure 16 displays the average compressive strength results at 7 and 28 days for group A mixtures (SF effect) and group B mixtures (optimized SF with varying w/b ratios and SNF effects). Based on Figure 16a, the compressive strength decreased at 7 and 28 days with higher SF replacement. For series 1 mixtures, increasing SF from 15% to 20% resulted in approximately 11% and 5% decreases in compressive strength at 7 and 28 days, respectively. The decrease in compressive strength with the increase in SF from 15% to 25% was also shown in series 1 mixtures, resulting in a 7% decrease in strength at 28 days. In series 2 mixtures, as the SF substitution rose from 15% to 20%, the compressive strength increased by 3% and 1% at 7 and 28 days, respectively. However, surpassing 20% SF replacement led to a decrease in strength. Elevating SF substitution from 15% to 25% resulted in a roughly 12% decrease in compressive strength at both 7 and 28 days. Series 3 mixtures showed a 6.5% strength reduction at 7 days and about 1.5% at 28 days when SF substitution increased from 15% to 20%. Furthermore, raising SF substitution from 15% to 25% in series 3 mixtures led to an 11% decrease in compressive strength. An excessive concentration of SF in concrete can adversely affect its compressive strength through multiple mechanisms. The incorporation of a high dosage of SF necessitates augmented water content to preserve workability, engendering a dichotomy [102; 103; 104; 105]: (1) the addition of water elevates the w/b ratio, undermining the concrete's integrity, and (2) the omission of water compromises the mix's efficiency, culminating in inadequate compaction and increased porosity. Elevated levels of SF can induce heightened viscosity within the mix, impeding cohesive properties and leading to the entrapment of air voids [106]. Although SF is known to enhance the microstructural compactness of concrete, an overabundance may result in over-densification, thereby obstructing the hydration process and the structural evolution of the concrete, consequently diminishing its strength [107; 108]. Based on the  
Figure 15: Flexural strength under the influence of (**a**) changes in the w/b ratio; (**b**) changes in the dosage of SNF. compressive strength (7 and 28 days) of mixtures in series 1, 2, and 3, the ideal SF dosages were determined to be 15%, 20%, and 15%, respectively. Figure 16 depicts the compressive strength of the mixtures at 7 and 28 days, considering the effects of the w/b ratio and SNF. The efficacy of SF in enhancing compressive strength is demonstrably linked to the w/b ratio. Generally, a lower w/b ratio translates into higher strength. However, improper adjustment of the w/b ratio when incorporating SF can compromise its effectiveness, leading to reduced compressive strength [109]. Increasing the w/b ratio from 0.18 to 0.22 resulted in a decrease in compressive strength. Comparing SF15 WB18N08 and SF15 WB22N08 mixtures revealed a reduction in compressive strength of approximately 13% for series 1, around 8% for series 2, and about 5% for series 3. Cement plays a crucial role in reacting with water to initiate a hydration reaction. This process results in the formation of a robust network of C-S-H gel, essential for concrete strength. A higher w/b ratio indicates more water relative to the binder (cement and other pozzolans). Excess water dilutes the cement paste, reducing the material for the vital C-S-H gel. With more water, not all cement particles may fully hydrate, leading to unreacted cement and weaker bonds in the concrete structure [100]. Increasing the SNF dosage from 0.7% to 0.9% also increased compressive strength. Comparing SF15 WB20N07 and SF15 WB20N09 mixtures reveals a 1.5 to 3.5% increase in compressive strength due to higher SNF substitution. SNF acts in the cement mixture by dispersing and separating cement particles and reducing internal friction.