## 3 Results and Discussion
### Rcpt  
The RCPT test results are shown in Figure 11. At the W/B ratio of 0.5, concrete samples with 20% FA replacement exhibit a marked improvement in durability, with a total charge passed that is significantly lower than the control mix. This improvement highlights the pozzolanic activity of FA, which results in the refinement of the pore structure and reduced permeability. However, as the FA replacement level increases to 40% and 60%, the total charge passed increases, indicating that there may be an optimal threshold for FA replacement at which durability is maximized without compromising the other properties.  
Figure 10: Drying shrinkage of various proportions with age for different amounts of FGCBA replacing cement at W/B ratios of (**a**) 0.4; (**b**) 0.45; (**c**) 0.5.  
In contrast, the concrete samples with FGCBA replacements at a W/B ratio of 0.5 present an intriguing trend. With increasing replacement percentages, there is a consistent decrease in the total charge passed which is particularly notable at the 60% replacement level, which suggests that FGCBA may be particularly effective in producing a more impermeable concrete matrix at higher replacement ratios. This could be attributed to the physical characteristics of FGCBA, which may lead to a denser microstructure, thus enhancing the material's resistance to chloride ion ingress.  
When examining the results at a W/B ratio of 0.45, the trend in charge passed for concrete samples with FA replacements shows an initial decrease followed by an increase at the 60% replacement level. The increase in total charge passed at higher FA contents could potentially be due to an imbalance in the pozzolanic reaction kinetics or a limit to the amount of calcium hydroxide available for reaction with FA.  
The FGCBA replacements at a W/B ratio of 0.45 again demonstrate their superior performance, with the total charge passed decreasing as the replacement percentage increases. The results indicate that FGCBA might be particularly advantageous for use in concrete when high resistance to chloride penetration is required, making it a promising option for structures exposed to aggressive environments.  
At the lower W/B ratio of 0.4, the effectiveness of both FA and FGCBA as cement replacements is reaffirmed in terms of enhancing concrete durability. The samples with FA replacement show a considerable decrease in the total charge passed, suggesting that a lower water content coupled with FA leads to lower permeability. The FGCBA replacements mirror this trend, with even lower total charges passed at higher replacement levels, further underscoring the potential of FGCBA in reducing concrete permeability.  
Across all W/B ratios, the decrease in the total charge passed with the incorporation of FA and FGCBA highlights the potential of these materials to significantly enhance the durability of concrete. Although the replacement ratios of finely ground coal-fired bottom ash are different in the study by [PERSON] et al. (2018), the results of the RCPT test are related to the overall proportion of concrete. The RCPT results indicate the better performance of using finely ground coal-fired bottom ash to replace cement compared to pure cement. The results suggest that there are optimal replacement levels for both FA and FGCBA for which the highest resistance to chloride penetration is achieved, which is vital for extending the service life of reinforced concrete structures, especially those susceptible to chloride-induced corrosion. These findings have important implications for the design of durable concrete mixtures in which both sustainability and performance are key considerations.