Compressive Strength–Water Absorption Behaviour of Concrete with Coal Bottom Ash as Sand Replacement across Water–Cement Ratios

Coal bottom ash (CBA) is produced in large quantities by coal-fired power plants. It offers a viable opportunity for sustainable utilisation as a partial replacement, especially for fine aggregate in concrete. However, its porous morphology and surface characteristics complicate controlling water demand and optimising concrete performance. In this research, the synergistic effects of water–cement (WC) ratio (0.40, 0.45, and 0.50) with CBA contents (0%, 10%, and 20% from mass of sand) on compressive strength and water absorption of concrete at 28 and 56 curing ages. Increasing the WC ratio and CBA content generally reduced compressive strength and increased water absorption, whereas extended curing improved strength while lowering absorption. Target performance was achieved with up to 20% CBA when the WC ratio was maintained within 0.40–0.45, defining a practical mix-design window. A strong inverse correlation was observed between compressive strength and water absorption at WC = 0.40–0.45 (R2 ≈ 0.92–0.95), whereas the relationship weakened at WC = 0.50 (R2 ≈ 0.82–0.83) due to increased pore connectivity and variability associated with excess mixing water. The reliability of these correlations was further confirmed through statistical error analysis, with low RMSE, RAE, and RRMSE values, particularly at WC = 0.45, indicating high predictive accuracy and minimal deviation between measured and predicted strengths. In contrast, higher error metrics at WC = 0.50 reflect reduced model robustness. These findings establish design boundaries that can be adopted in practice to valorise CBA while safeguarding performance, thereby informing greener specifications and guiding future standards for the use of industrial by-products in concrete.