## 5 Results and Discussion
### Compressive Strength  
Figure 8 shows that increasing the SF content improves the compressive strength of HSC. [PERSON] and [PERSON] [49] showed almost the same results thus confirming those in the current study. They showed that pozzolanic activity of SP led to improving the compressive strength of concrete by up to 20%. In another investigation, [PERSON] et al. [52] found that, by using 2% NS in the mix, the compressive strength of concrete substantially improved. [PERSON] et al. [59] measured the mechanical characteristics of NS mortars and found that the compressive strength of those specimens was greater than that of the control sample with no NS. They stated that, if the NS is uniformly distributed in the cement paste, the microstructure of the concrete matrix could be improved, which results in improving the compressive strength. Besides, [PERSON] et al. [60] revealed that the compressive strength of concrete was improved by the use of NS, particularly at an early age.  
Figure 6: Impact of the fibres on the slump of HSC (**a**) SF and (**b**) PPF.  
Figure 7: Influence of SP on the slump of HSC containing various NS and fibres (**a**) SF and (**b**) PPF.  
On the other hand, raising the amount of SP led to enhancing the compressive strength. Finally, increasing NS at up to 1% also enhances the compressive strength of HSC; however, using NS at more than 1% has a negative impact on this property. Furthermore, raising the SP content has a substantial influence on the improvement of HSC, especially when 10% SP is used. The compressive strength of HSC raised by almost 15%, 20%, 30%, 32% and 36% when 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF were employed, respectively. Moreover, adding 1% NS and 10% SP with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF has the greatest impact on the HSC compressive strength: about 82%, 90%, 105%, 109% and 115%, respectively.  
In addition, the improvement effect of SF on the compressive strength decreased by more than 0.3% SF when NS was employed. Additionally, the substantial positive impact of PPF on the compressive strength of HSC with NS and SP is seen in Figure 8. The main reason for improving the compressive strength of concrete with the use of fibres could be attributed to the bridging role of fibres, which create a strong core in the mid-height of the specimens. [PERSON] and [PERSON] [61] performed a comprehensive evaluation of the influence of PPF on improving the compressive strength of concrete. They stated that, when a specimen is loaded by compression, a lateral expansion happens at the mid-height of the cylindrical sample. The bridging role of PPF improved the tensile strength of the matrix and enhanced the cohesion between concrete paste and aggregates and that limited lateral deformation of the sample, which enhanced the compressive behaviour of the specimen. Therefore, using 0.1%, 0.2%, 0.3%, 0.4% and 0.5% SF improved the compressive strength by about 15%, 22%, 34%, 43% and 50%, respectively. By comparing Figure 8a,b, increasing the value of SP did not have a negative effect on the improved performance of compressive strength when the value of PPF increased; however, using NS at more than 1% has a negative impact on this property. Besides, employing 1% NS and 10% SP together with 0.1%, 0.2%, 0.3%, 0.4% and 0.5% PPF enhances the compressive strength of HSC by approximately 66%, 81%, 99%, 113% and 123%, respectively.  
In Figure 9, the effect of fibres on the compressive strength of HSC is illustrated in a different manner. As seen in Figure 9a, there is no substantial impact on this property between 0.1% and 0.2% SF. In addition, a similar trend is observed when 0.4% and 0.5% SF are used.