## 4 Discussion of the Results
### Influence on Compressive Strength  
Figure 5 shows the results of compressive strength tests at 7 days for different HS replacement percentages and water/cement ratios. The data indicate that the specimens prepared with a 0.4 w/c ratio had a higher strength than those prepared with a 0.5 w/c ratio. Furthermore, a decreasing trend in strength was observed as the HS replacement percentage increased for the 0.4 w/c ratio specimens, whereas an increase in strength was noted with an increasing HS replacement percentage for the 0.5 w/c ratio specimens. The maximum strength was achieved at a 2.5% HS replacement percentage for the 0.4 w/c ratio specimens, almost reaching the value of the standard. Figure 6 presents the compressive strength results at 28 days, with higher strength values compared to those at 7 days. The specimens with a 0.4 w/c ratio had a higher strength than those with a 0.5 w/c ratio, with a maximum increase of 9.5% observed for the 2.5% HS replacement percentage compared to the standard. This increase in strength may be attributed to the improved workability of the mixture, as reflected by the larger slump values observed for this HS replacement percentage in Table 5. The better workability may have reduced the difficulties encountered during the compaction by tamping, resulting in better strength. Additionally, a strong direct relationship was found between these two variables, with a correlation coefficient, \(r_{s}\), of 0.8 (Table 6). Figure 5: Average compressive strength results at 7 days for 0.4 and 0.5 water/cement ratios. Figure 6: Average compressive strength results at 28 days for 0.4 and 0.5 water/cement ratios. In Table 6, it can be observed that the \(r_{s}\) correlation coefficients between HS replacement percentage and compressive strength at 28 days were \(-0.6\) for both w/c ratios, indicating a moderate inverse relation. This suggests that an increase in the HS replacement percentage led to a decrease in the compressive strength. The maximum reduction in strength for both w/c ratios was around 23% compared to the standard specimens. The inverse correlation between the percentage of organic material replacement and the final strength of concrete when using organic material can be attributed to several key factors. For instance, organic materials are typically much weaker than traditional fine aggregates like sand. This inherent weakness in organic materials can lead to a reduction in the overall strength of the concrete. Also, they can absorb moisture from the concrete mix, leading to a reduction in the water/cement ratio. As the percentage of organic replacement increases, the water/cement ratio decreases, negatively impacting strength. Moreover, organic materials often contain voids or pores that can create weak points within the concrete. As the percentage of replacement increases, these voids become more prevalent, increasing the overall porosity of the concrete. A higher porosity can reduce the compressive strength of concrete. The suitability of different organic materials as replacements for fine aggregates in concrete has been explored in other studies. For example, [PERSON] [46] investigated the suitability of palm kernel shells as a partial replacement for coarse aggregates in asphalt concrete. [PERSON] [47] conducted a study on the behavior of concrete slabs incorporating sawdust and palm kernel shells as replacements for fine and coarse aggregates, respectively. The study found that the production of lightweight concrete slabs was possible with a lower replacement value of 25% sawdust and palm kernel shells. Similarly, [PERSON] et al. [48] investigated the use of granite fines as a replacement material for fine aggregates. They assessed the replacement of fine aggregates in five different proportions, ranging from 5% to 50%. Their study revealed that as the percentage of granite fines used as a replacement increased, there was an inverse correlation with the final strength of the concrete.