Modeling and optimization of thermal performance in concrete incorporating coal bottom ash as sand replacement using response surface methodology

Concrete exposed to elevated temperatures suffers severe degradation in strength and microstructure. However, limited research has systematically quantified the thermal mechanical behavior of concrete incorporating coal bottom ash (CBA) as a fine aggregate replacement under different temperature exposures. This study addresses this gap by developing a Response Surface Methodology (RSM) model based on the Central Composite Design (CCD) to evaluate and optimize the effects of CBA content (0–40%), temperature (100–800 °C), and curing age (7–28 days) on the compressive strength of concrete. A comprehensive experimental program was conducted, including workability, compressive strength, and thermal conductivity tests, alongside XRD, and FESEM analyses to correlate chemical composition with microstructural performance. The developed quadratic model exhibited excellent predictive accuracy (R² = 0.99). Temperature exerted a significant negative effect on compressive strength (p < 0.0001, coefficient = –12.64), while CBA content and curing age were also identified as statistically significant parameters influencing strength development. The highest compressive strength was recorded as 63.4 MPa at 10% CBA and 100 °C, attributed to enhanced internal curing and secondary hydration. Even at 40% CBA, concrete maintained 48.4 MPa at 450 °C and 46 MPa at ambient temperature, demonstrating strong thermal stability. The thermal conductivity decreased from 1.83 to 1.62 W/m·K, indicating improved insulation performance. The RSM method proved effective in both predicting responses and identifying the key parameters that influence concrete performance. It also successfully determined the optimal combination of variables for achieving the desired strength and thermal stability.