## 4 Discussion of the Results
### Influence on Compressive Strength
This inverse correlation between the percentage of replacement and the final strength suggests that while organic materials can be used as replacements for fine aggregates in concrete, there is a trade-off between the environmental benefits and the structural integrity of the concrete. This observation aligns with the findings of [PERSON] et al. [49] and [PERSON] and [PERSON] [50], who compared the performance of coconut shells and palm kernel shells as substitutes for coarse aggregates in concrete. Furthermore, the w/c ratio directly affected the porosity of the concrete mixture. A higher w/c ratio led to increased porosity, which resulted in the lower strength of the concrete specimens. Hence, it could be inferred that the compressive strength results for a 0.4 w/c ratio would be greater than those obtained for a 0.5 w/c ratio. Our findings indicated that the choice of water-to-cement (w/c) ratio significantly influenced the compressive strength of the concrete specimens. Specifically, specimens with a 0.4 w/c ratio consistently exhibited higher strength compared to those with a 0.5 w/c ratio. This observation could be attributed to the well-established principle that a lower w/c ratio results in a denser and more compacted concrete matrix, which generally leads to a higher strength. However, the addition of hazelnut shell (HS) content introduces complexity to this relationship. For instance, a lower w/c ratio means that there is less water in the mix relative to the amount of cement [51; 52]. This results in a more densely packed and compacted concrete matrix. A denser matrix generally leads to higher compressive strength because there is less porosity and a greater amount of cementitious material available for bonding. Moreover, with a lower water content, there is less excess water to create voids in the concrete. This reduced porosity limits the potential pathways for crack formation, increasing the overall strength of the concrete [53; 54]. Additionally, lower w/c ratios often require more efficient curing techniques to prevent the concrete from drying out too quickly. Lastly, hazelnut shells introduce unique physical properties to the concrete mix. The size, shape, and surface characteristics of the HS particles can affect their distribution in the mixture (i.e., HS particles may interact differently with the cement matrix based on their size and shape). As a consequence, the overall packing and distribution of the solid mate rials in the mix can be influenced. Interestingly, the correlation between the compressive strength and air content of the mixture for the 0.5 w/c ratio at 28 days was moderately inverse, with an \(r_{s}\) coefficient of \(-0.6\). It can be observed that the compressive strength was higher for lower w/c ratios (w/c = 0.4). This trend suggests that the density is directly proportional for conventional concretes, and similar conclusions can be found for other organic materials used as replacements for fine aggregates [55]. However, this relationship did not hold for concretes with higher percentages of hazelnut shell, which is lighter and more porous than regular aggregates. Consequently, the density of the concrete with a higher hazelnut shell content was lower, resulting in an increased air percentage and a reduction in concrete strength. For instance, in a study conducted by [PERSON] et al. [56], polypropylene fibers and waste slag aggregates (WSAs) were employed as partial substitutes for cement in concrete. The results indicated a reduction in compressive strength and an enhancement in flexural strength upon the inclusion of polypropylene fibers. Conversely, the incorporation of WSAs led to a densification of the microstructure, attributed to its pozzolanic activity. [PERSON] et al. [57] obtained similar patterns when incorporating sugarcane bagasse ash as a filler in concrete. It was observed that the density of the hardened concrete increased when the replacement level reached 20%, despite a decrease in compressive strength at that particular replacement level.