## 3 Results and Discussion
### Absorption, Density, and Voids in Hardened Concrete  
Focusing first on the absorption characteristics after immersion and subsequent boiling, as shown in Figures 12 and 13, a comparison across different water-binder ratios indicates  
Figure 11: RCPT at different W/B ratios and cement replacement levels for (**a**) fly ash; (**b**) FGCBA. that the absorption capacity of concrete tends to increase with the percentage of cement replacement. For instance, at a W/B ratio of 0.50, samples with no replacement exhibit absorption values of 9% after immersion which slightly increase after boiling. However, with the introduction of FA and FGCBA, there is a discernible increase in absorption, particularly in the samples with higher replacement rates. This suggests that while FA and FGCBA may contribute to a denser microstructure, as evidenced by the lower absorption rates at lower replacement levels, higher concentrations may result in the introduction of more voids or channels that facilitate moisture ingress. Through the bulk and apparent density results, as shown as Figures 14 and 15, it can be seen that the introduction of FA and FGCBA generally leads to a slight decrease in these values compared to the control samples. This reduction could be associated with the lower specific gravity of the replacement materials compared to that of cement. Despite the decrease, the densities remain within a range that suggests the mixes maintain a solid matrix, which is crucial for structural integrity. It is noteworthy that the samples with higher percentages of FGCBA tend to exhibit a more pronounced decrease in density, which could be a factor in the increased absorption seen in these samples. Figure 12: Water absorption of concrete with FA as cement replacement at different W/B ratios of (a) 0.4; (b) 0.45; (c) 0.5. The volume of permeable pore space within concrete is a crucial indicator of durability, reflecting the material's vulnerability to harmful agents. As depicted in Figure 16, increasing amounts of both FA and FGCBA as cement replacements correspond to a rise in voids, a pattern that is especially prominent at the 0.40 water-binder ratio. This elevation in void percentage may heighten the potential for degradation from environmental factors, including freeze-thaw cycles, chemical exposure, and reinforcing steel corrosion. At the 0.40 W/B ratio, the void content for the control mix is lower compared to those with replacements. The voids increase from 15.36% to as high as 18.40% as the replacement level of FGCBA rises to 60%. This increase in void content correlates with the absorption data, reinforcing the notion that higher replacement levels can adversely affect concrete's resistance to the penetration of water and, potentially, chlorides. The trends observed in the 0.45 and 0.50 W/B ratio mixes are consistent with those at 0.40, suggesting that the relationships between replacement material content, absorption, density, and void content are not significantly altered by changes in the W/B ratio within the tested range. However, it should be noted that the overall absorption values and void content are slightly lower for mixes with W/B ratios of 0.45 and 0.50, indicating that the mix's ability to resist moisture ingress improves with increasing water content. In summary, while the use of FA and FGCBA as partial replacements for cement can positively influence certain aspects of concrete's durability, such as reduced chloride permeability and improved long-term strength, there are trade-offs in terms of increased absorption and void content, especially at higher replacement levels. These findings  
Figure 13: Water absorption of concrete with FGCBA as cement replacement at different W/B ratios of (**a**) 0.4; (**b**) 0.45; (**c**) 0.5.  
underscore the importance of optimizing the amount of replacement material to balance benefits against potential drawbacks.