## 3 Results and Discussion
### Air Content and Unit Weight of Freshly Mixed Concrete  
Analyzing the data from Table 7, the effects of cement replacement with fly ash (FA) and finely ground coal bottom ash (FGCBA) on the air content and unit weight of fresh concrete are presented, taking into account different water-binder (W/B) ratios.  
At a W/B ratio of 0.4, a unit weight of 2301.81 kg/cm\({}^{3}\), featuring an air content of 1%, was recorded for concrete without any replacement. As FA was introduced, there was a slight decrease in unit weight and a corresponding increase in air content with higher percentages of replacement, suggesting that FA contributes to a lighter and more aerated concrete mix. For FGCBA, the unit weight decreased to 2149.29 kg/cm\({}^{3}\) at a 60% replacement rate with a lower air content of 1.4%, indicating a more substantial impact on the density and air content compared to FA.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & & & & **Unit: cm** \\ \hline
**W/B** & **0.4** & & **0.45** & & **0.5** \\ \hline
**Replace (\%)** & **FA** & **FGCBA** & **FA** & **FGCBA** & **FA** & **FGCBA** \\ \hline
**0** & **FA** & **FGCBA** & **FA** & **FGCBA** & **FA** & **FGCBA** \\ \hline
**20** & **FA** & **FGCBA** & **FA** & **FGCBA** & **FA** & **FGCBA** \\ \hline
**40** & **FA** & **FGCBA** & **FA** & **FGCBA** & **FA** & **FGCBA** \\ \hline
**60** & **FA** & **FA** & **FGCBA** & **FA** & **FGCBA** & **FA** & **FGCBA** \\ \hline \hline \end{tabular}
\end{table}
Table 6: Effect of replacing cement with FA and FGCBA on the slump of fresh concrete.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & & & & **Unit: kg/cm\({}^{3}\), (\%)** \\ \hline
**W/B** & **0.4** & & **0.45** & & **0.5** \\ \hline
**Replace (\%)** & **FA** & **FGCBA** & **FA** & **FGCBA** & **FA** & **FGCBA** \\ \hline
**0** & **FA** & **FA** & **FGCBA** & **FA** & **FGCBA** \\ \hline
**0** & **FA** & **FA** & **FA** & **FGCBA** & **FA**With a W/B ratio of 0.45, the control mix demonstrated a unit weight of 2267.73 kg/cm\({}^{3}\) and an air content of 1.2%. The introduction of FA saw a decrease in unit weight across all replacement levels, with the air content rising to 1.8% at 60% replacement. FGCBA replacements exhibited a similar trend in reducing unit weight, with the air content peaking at 1.7% at a 40% replacement rate and then slightly decreasing to 1.6% at 60% replacement. This trend indicates that both FA and FGCBA may contribute to the formation of a lighter concrete mix, with varied impacts on air entrainment.  
For the 0.5 W/B ratio, the control mix started at 2235.36 kg/cm\({}^{3}\) with an air content of 1.5%. The mixes with FA used as replacement showed a consistent reduction in unit weight, reaching 2133.61 kg/cm\({}^{3}\) at 60% replacement, and an increase in air content to 1.8%. The FGCBA mixes followed a comparable pattern, ending at a unit weight of 2097.38 kg/cm\({}^{3}\) and an air content of 1.7% at the same replacement level. This suggests that at higher W/B ratios, both FA and FGCBA lead to a more pronounced decrease in unit weight with a modest increase in air content, contributing to a less dense concrete mix with a slightly higher air content.  
In conclusion, the data from Table 6 highlight that replacing cement with FA or FGCBA can reduce the unit weight and moderately increase the air content of fresh concrete. The extent of these changes depends on the type of replacement material and the W/B ratio. FA consistently demonstrated an increase in air content with a reduction in unit weight across all W/B ratios. Meanwhile, FGCBA had a variable impact on air content but also contributed to a lighter concrete; this impact was particularly noticeable at higher replacement rates and higher W/B ratios. These findings suggest that both materials can be effectively used to tailor fresh concrete properties for specific structural or construction requirements. Similar results were obtained when using coal bottom ash to replace fine aggregate [33] and cement [34].