Toward Sustainable Construction: Modeling the Strength Development and Microstructural Mechanisms of Fly Ash–Metakaolin-Modified Coal Gangue Concrete

To enhance the utilization of industrial coal gangue, response surface methodology was used to optimize the concrete mix proportions based on three key factors: the mass ratio of fly ash (FA) to metakaolin (MK) (A), the combined dosage of FA and MK (B), and the water-to-binder ratio (C). A quadratic regression model was established, and the optimal mixture was characterized using FT-IR, XRD, and SEM. The model exhibited high statistical significance (p < 0.001) and an excellent fit (R2 > 0.95), confirming its predictive reliability. Single-factor analysis revealed that the order of influence on 28 d compressive strength was C > A > B, indicating that the water-to-binder ratio had the most significant effect on later-age strength. The optimal mix proportions were determined as follows: fly ash-to-MK ratio of 0.65, admixture dosage of 20% by mass of total binder, and C of 0.475. Under these conditions, the measured 28 d compressive strength reached 35.9 MPa, which was within 5% of the model-predicted value, thereby validating the model’s accuracy. Microstructural analysis demonstrated that the appropriate incorporation of FA and MK promoted the formation of C-S-H gel, refined the pore structure, and improved the quality of the interfacial transition zone, which collectively enhanced the mechanical performance. A systematic understanding of the strength and microstructural mechanisms of concrete incorporating coal gangue, fly ash, and metakaolin is currently lacking, which hinders the design of more robust and durable structures. This study addresses this gap by systematically clarifying the individual and combined effects of the key variables on the strength of coal gangue concrete. The findings reveal the underlying mechanisms, providing a scientific basis for the sustainable, large-scale application of coal gangue concrete in construction.