## 3 Results and Discussion
### Compressive Strength Development  
The experimental data presented in Figures 7 and 8 offer a nuanced perspective on the role of supplementary cementitious materials such as FA and FGCBA within the concrete matrix. The interaction between these materials and the hydration process is critical for understanding the mechanical properties of concrete, particularly its compressive strength across various curing times. Figure 6: Effect of different FGCBA replacement ratios on setting time at W/B ratios of (**a**) 0.4; (**b**) 0.45; (**c**) 0.5. Starting with a water-binder ratio of 0.4, the control mix without any cement replacement sets a benchmark with a 7-day compressive strength of 40.66 MPa. This establishes a point of comparison for concrete mixes in which cement has been partially replaced. When 20% of cement is substituted with FA, there is an observable decline in early compressive strength, registering 30.53 MPa at 7 days. This initial reduction may be attributed to the slower reactive nature of FA compared to Portland cement. Despite this initial setback, the FA mix exhibits remarkable recovery in the long term, with the compressive strength escalating to 67.89 MPa at 180 days. The data reveal that the pozzolanic reaction of FA, although delayed, continues to contribute to strength gain over time, suggesting its viability for structures for which long-term performance is crucial. Since the pozzolanic activity index test also involves a 20% replacement, it can be observed from Table 5 that with a 180-day activity index at 166.88% and a 7-day activity index at 91.23%, the trend in compressive strength bears a strikingly similar relationship to the activity index. When the cement replacement level increases to 60% with FA, the early strength at 7 days further drops to 17.93 MPa. This significant reduction could be due to the dilution of cement content, which initially limits the formation of calcium silicate hydrate (C-S-H), the primary strength-contributing phase. However, over time, the pozzolanic action of the FA continues to form additional C-S-H, which is reflected in the considerable strength of 49.14 MPa achieved by 180 days. This indicates that while high volumes of FA can impede early strength, they do not necessarily compromise the long-term strength potential of concrete. Figure 7: Compressive strength trends with FA as a cement substitute at W/B ratios of (**a**) 0.4; (**b**) 0.45; (**c**) 0.5. Conversely, mixes with FGCBA as replacement at a 0.4 W/B ratio present a different pattern of strength development. At a 20% replacement level, there is an initial 7-day strength of 25.84 MPa, significantly lower than that of the control mix. The strength gain over time is less pronounced with FGCBA, reaching 50.24 MPa at 180 days which, although representing an increase, is not as substantial as that seen with FA. At a 60% replacement level, the initial strength is markedly low at 12.88 MPa, gradually increasing to 32.04 MPa at 180 days. This modest increase points to the less reactive nature of FGCBA, which may limit its effectiveness in enhancing the long-term strength of concrete. At a water-binder ratio of 0.45, the influence of cementitious replacements on compressive strength exhibits similar trends but with notable differences in magnitude. The control mix shows an initial 7-day strength of 31.07 MPa, which marginally increases to 49.67 MPa at 180 days. This is indicative of the inherent curing capacity of concrete with this particular W/B ratio. At this ratio, FA replacement at the 20% level starts at 22.88 MPa and impressively increases to 63.87 MPa at 180 days, reaffirming the material's long-term beneficial effects on strength development. For 60% FA replacement, there is also an increase from a lower initial strength of 14.30 MPa to 46.21 MPa at 180 days. For FGCBA replacement at a 0.45 W/B ratio, the initial strength is 25.51 MPa at 20% replacement, reaching 50.43 MPa at 180 days. At 60% replacement, the initial strength is only 11.37 MPa, increasing to 36.27 MPa by 180 days.