Mix-Proportion Design and Properties of Steel Slag-Based Composite Cementitious Materials Using a Constrained Four-Component Mixture Design

High-volume utilization of steel slag in cementitious materials requires a proportioning method that accounts for coupled effects among multiple components. This study investigated a four-component system comprising steel slag (SS), blast furnace slag (BFS), ordinary Portland cement (OPC), and desulfurized gypsum (DG), using 17 formulations within a constrained four-component mixture design. Unlike previous studies focused mainly on ternary binders or empirical proportioning, this work combined four-component formulation screening with age-dependent hydration characterization in one SS-BFS-OPC-DG framework. The 7 d compressive strength was selected as the modeled response because it provided a consistent early-age basis for comparing all 17 formulations within a practical testing period, whereas flexural strength was retained as a complementary experimental response. The compressive-strength mixture model was significant (p = 0.0044), with R2 = 0.692, adjusted R2 = 0.589, predicted R2 = 0.435, and adequate precision = 8.545. Its performance was evaluated using prediction indices, residual diagnostics, and an independent confirmation batch. The selected mass fractions of SS, BFS, OPC, and DG were 0.322, 0.429, 0.131, and 0.116, respectively. The confirmation batch achieved a 7 d compressive strength of 24.80 MPa, compared with the predicted value of 24.93 MPa. The model was therefore used only for formulation screening and interpolation within the investigated component ranges. The selected formulation reached compressive and flexural strengths of 32.6 and 7.6 MPa at 28 d, respectively. XRD, SEM, TG, and FTIR results collectively supported continued formation of hydration products and progressive matrix densification from 3 d to 28 d. The C-S-H gel index increased from 2.07% to 6.15%, while the number of large visible voids decreased and the continuity of the matrix improved. These results provide a constrained mixture-design route for screening high-solid-waste SS-BFS-OPC-DG binders.