Optimization of Fly Ash-Based Composite Cementitious Material Proportion Using Box-Behnken Response Surface Methodology

To realize high-value synergistic utilization of the three major solid wastes from thermal power generation (fly ash-FA, coal-fired slag-CS, desulfurization gypsum-DG), a Box-Behnken response surface model was established with CS, DG, and cement as factors and FA as the matrix. Unlike existing research focusing on single or binary solid waste composites, this study systematically optimized the synergistic blending ratios of the three wastes without additional activation. The 7d/28d strength models showed significant statistical validity (R2 = 0.9918/0.9979, p < 0.001). The optimal mix ratio (CS 21.38%, DG 10.96%, cement 16.15%, FA 51.51%) achieved 7d strength of 13.60 MPa and 28d strength of 19.07 MPa, with a model deviation rate below 2%. The statistical model results are deeply correlated with the mechanisms of hydration and microstructural evolution: cement and DG drive early-stage hydration reactions to form rapid-strength products, while CS continuously generates hydration gel through slow pozzolanic reactions to develop late-stage strength. XRD/SEM analysis confirmed significant formation of calcium-aluminum-silicate hydrate (C-(A)-S-H), calcium hydroxide (CH), and ettringite (AFt), verifying full activation of pozzolanic substances in FA and CS. This study innovatively overcomes bottlenecks in the simultaneous high-value utilization of three thermal wastes, providing a scientific pathway for optimizing cementitious materials from multi-source solid wastes.