3.1 Compressive Strength
Figure 3 illustrates the compressive strength of coal bottom ash (CBA) concrete at different water-cement (WC) ratios (0.40, 0.45, and 0.50) and CBA replacement levels (0%, 10%, and 20%) after 28 and 56 days of curing. At 28 days, concrete with WC ratios of 0.40 and 0.45 were higher compared to control specimens up to 3.4% and all specimens exceeds the target strength of 43.12 MPa, even with CBA replacements as illustrates in Fig. 3(a). However, at a WC ratio of 0.50, the compressive strength falls below the target (by about 11.3% compared to control concrete) for all CBA replacement levels. Notably, 20% CBA replacement with a WC ratio of 0.45 still achieves the target strength, indicating that a moderate WC ratio can accommodate higher CBA content while maintaining adequate strength. The optimal replacement of sand with CBA was determined to be 10% at WC ratios of 0.40 and 0.45, bearing compressive strengths of 52.8 MPa and 46.8 MPa respectively at 28 days, closely matching the performance of the control concrete.
Figure 3(b) shows all concrete mixes demonstrates significant strength gains by 56 days, with 0.40WC and 0.45WC consistently exceeding the target strength across all CBA replacement levels. Even 0.50WC specimens, which initially underperformed, approach or slightly exceed the target strength at higher curing periods. Compared to the control, these compressive strengths were reduced to 26.1% and 5.1% at 28 days, whereas 22.4% and 5.8% at 56 days for water-cement ratios of 0.40 and 0.45, respectively. Besides that, the compressive strengths were consistent at 49.6 MPa for both CBA replacement levels, indicating minimal strength reduction at a WC ratio of 0.45. However, replacing 10% and 20% of sand with CBA at a WC ratio of 0.50 showed no significant impact on compressive strength. After that, the increase in compressive strength was reliant on the WC ratio because a lower WC ratio contains large amount of cement contents. However, decreases of water content may resulted to increases porosities in concrete due to improper concrete handling, which leading to deteriorate of concrete strength [16].
CBA concrete with 10% CBA replacement and WC ratio of 0.40 and 0.45 were comparable to the control concrete. Nevertheless, this result was similar to that of Muthusamy et al. [10]. Its reported that 10% CBA substitution for sand achieved comparable concrete strength to the control (50 MPa) at 28 days. Another finding by Ghadzali et al. [17], the optimal level of 10% of CBA as sand replacement to achieved the target strength (35 MPa) was 0.55 for WC ratio following by increment up to 15% (58.3 MPa) after 56 days. Nevertheless, tt has been reported that replacing fine aggregate with CBA reduces compressive strength in 28 days of curing; however, its compression strength appears to be comparable to control concrete for longer curing times of up to 365 days [18]. Hence, the addition of CBA to concrete increased its compressive strength in long-term. In addition, CBA is high water adsorbent material as the CBA contents were higher may affects the workability of concrete during batching due to difficult to compact and might increases voids, leading to directly reducing specimen strength. As for suggestion, the replacement of from 10–20% of CBA contents with 0.40–0.50 of WC ratio can be used concerning to workability performance by neglecting superplasticizer. This suggestion were aligned with Faisal et al. [19], CBA replacement by 30% or less in concrete was deemed acceptable incorporated to workability and compression strength at 28 days. On the other hand, some researcher found that the compressive strength of specimen with CBA were lower than control concrete at 56 days due to high pozzolanic reaction of CBA occurred slower during curing process, resulting in the development of strong calcium–silicate–hydrate (C–S–H) gels with higher compressive strength [17].
In related to the chemical composition for this study, CBA were classified as Class C due to the percentage of SiO2 + Al2O3 + Fe2O3 were 55.53%. Thus, the silica oxide content in Class C was lower than Class F resulting slightly have lower strength performance in long-term characteristic strength development [20]. Thi et al. [20] carried out experimental tests comparing CBA sources of different classes (corresponding to Class F and Class C) and concluded that, higher silica oxide content affects the strength characteristics of the concrete. Apart from that, the CBA used in this research contains a high calcium oxide concentration (refer Table 2). Higher levels of calcium oxide lead to increased early strength due to the accelerated reaction with water and the resulting formation of calcium hydroxide (Ca(OH)2). CBA has high calcium oxide producing free Ca(OH)2, helps to strengthening of C–S–H gels. This is supported by the findings of Muthusamy et al. [10] found that substitution of CBA from Class C as sand replacement increases up to 30% the early strength at 7 days surpassed the control concrete.
These results underscore the importance of maintaining lower WC ratios to optimize the performance of CBA, demonstrating that with proper mix design adjustments, CBA can be effectively used to produce high-strength concrete, thereby promoting sustainable construction practices.
3.2 Water Absorption
Figure 4 illustrates the water absorption of CBA concrete with varying WC ratios and CBA replacement levels at 28 and 56 days of curing. Generally, CBA concrete with higher replacement levels and WC ratios exhibited increased water absorption compared to control concrete. At 28 days, control concrete showed water absorption rates of 5.33%, 5.96%, and 6.11% for WC ratios of 0.40, 0.45, and 0.50, respectively. The addition of CBA further increased these values, especially at 20% replacement and higher WC ratios, reaching up to 6.91% for a WC ratio of 0.50.
Over time, water absorption decreased for both control and CBA concrete, reflecting the ongoing hydration and pore refinement in the concrete matrix. By 56 days, the water absorption of control concrete dropped to 5.22%, 5.81%, and 6.03% for WC ratios of 0.40, 0.45, and 0.50, respectively. Similarly, CBA concrete showed reduced water absorption at 56 days compared to 28 days, indicating improved durability and reduced porosity with extended curing. However, the overall trend remained the same, with higher CBA content and WC ratios resulting in greater water absorption.
Despite the increase in water absorption with CBA replacement, the values remained below 10%, which is considered acceptable for good quality concrete% [10]. This suggests that while CBA increases water absorption due to its porous nature, the resulting concrete still falls within acceptable limits for durability. The findings highlight the importance of optimizing the WC ratio when using CBA to balance workability, strength, and durability. Lower WC ratios (0.40 and 0.45) are more effective in controlling water absorption and maintaining the concrete's performance, even with higher levels of CBA replacement. These insights are crucial for developing sustainable concrete mixes that incorporate industrial by-products like CBA while ensuring long-term structural integrity.
The observation that water absorption in CBA concrete increases with higher percentages of CBA replacement aligns with findings from other studies, including research on self-compacting concrete (SCC) using CBA as a sand replacement [21]. In these studies, water absorption ranged between 6.0% and 6.8% for CBA replacements of 10–30% at 28 days, indicating a consistent trend of increased water absorption with higher CBA content [10]. This increase is attributed to the porous nature of CBA particles (as shown previously in Fig. 2a), which inherently possess a higher capacity for water retention. Despite this, concrete mixtures with up to 40% CBA replacement were still considered acceptable in terms of water absorption, demonstrating that even with higher absorption rates, CBA concrete can meet the necessary performance criteria [22].
The reduction in water absorption over time, as observed in the study, is likely due to the ongoing pozzolanic reactions involving CBA [5]. The pozzolanic elements in CBA, particularly those with calcium-silicate bases, contribute to the formation of calcium-silicate-hydrate (C–S–H) gels. These gels fill the spaces between cement grains, reducing the overall porosity of the concrete and making it less permeable to water. This gradual densification of the concrete matrix explains the observed decrease in water absorption with age. The formation of C–S–H gels plays a crucial role in enhancing the durability and long-term performance of CBA concrete, effectively mitigating the initial increase in water absorption due to the porous nature of CBA [23].
3.3 Correlation between Compressive Strength and Water Absorption
Generally, correlation is a statistical term for measuring the strength of a relationship between two factors. It is described through the correlation coefficient (R2) which provides a numerical expression of the degree with values near 1 indicating a strong relationship between the factors. The correlation coefficient was performed with the aid of Microsoft 365 (Office) - Excel. According to Hasim et al. [8], R2 value greater than 0.85 indicates a strong relationship between the parameters. In general, when the R2 value is small, the points from the correlation line could be described as far apart and therefore, it reveals insignificant findings that indicate a less strong relationship between the factors.
This research conducted on the correlation between compressive strength and water absorption in concrete cube contains CBA (0%, 10%, and 20%) across different WC ratios (0.40, 0.45, and 0.50) yielded significant findings as shown Fig. 5. The study showed a strong negative correlation between these two properties, meaning that as water absorption increases, compressive strength decreases. This inverse relationship is crucial for understanding how CBA impacts the structural integrity of concrete. In addition, extending curing periods improved compressive strength whilst also decreasing water absorption. The correlation coefficients (R²) provided statistical validation for these observations, with values near 1 indicating a robust relationship, particularly at lower WC ratios.
Figure 4 (a) shows at a WC ratio of 0.40, the R² values were exceptionally high, with 0.9201 at 28 days and 0.9298 at 56 days, indicating a very strong negative correlation between compressive strength and water absorption. This suggests that concrete mixes with this WC ratio are highly predictable and consistent in terms of their structural performance, even when incorporating up to 20% CBA. The lower water absorption rates at this WC ratio contribute to higher compressive strengths, underscoring the importance of maintaining a low WC ratio for optimal concrete performance.
For the WC ratio of 0.45, the R² values were slightly higher than those for the 0.40 ratio, with 0.9449 at 28 days and 0.9491 at 56 days as shown in Fig. 4 (b). This indicates an even stronger correlation at this ratio, likely due to a balance between sufficient workability and reduced porosity. The findings suggest that a WC ratio of 0.45 is also effective for CBA concrete, providing a good compromise between ease of mixing and high strength. The consistency in the R² values over time further reinforces the reliability of this mix design for structural applications.
Conversely, at a WC ratio of 0.50, the R² values were lower, with 0.8229 at 28 days and 0.8313 at 56 days as shown in Fig. 5 (c). Although still indicative of a significant negative correlation, these values suggest more variability and less predictability in the relationship between compressive strength and water absorption. The higher water absorption rates at this ratio lead to increased porosity, which negatively impacts compressive strength. Another reason for this can be seen in Fig. 3, which illustrates the WC ratio of 0.50 with a CBA replacement of 10–20% had no significant effect on concrete strength. Moreover, this findings is similar with Zhang & Zong [24], who looked into the relationship between water absorption (surface and internal) and compressive strength. It was discovered that the scatter plots had a moderate correlation, indicating that there is no obvious relationship between these parameters. According to Ghewa et al. [25], the concrete with a higher WC ratio has large pores within the specimens, causing it to absorb a significant amount of water, which in turn reduces its strength.
Nevertheless, according to these findings, the R2 value of WC ratio of 0.40 and 0.45 was found to be close to 1, and so, it reveals the strong relationship between compressive strength and water absorption. This shows that the resulting strength values are exactly proportional to water absorption, which is also directly influenced by the proportion of CBA materials and WC ratio. These findings highlight the limitations of using a higher WC ratio in CBA concrete, where the trade-off between workability and strength becomes more pronounced.