Performance evaluation of ternary solid waste-based geopolymer subgrade materials

This study employed three industrial by-products-cement kiln dust (CKD), fly ash (FA), and phosphogypsum (PG)—as precursors to develop one-part geopolymer stabilized subgrade materials. The principal objective was to quantify how binder proportioning influences the mechanical and durability performance of the resulting mixtures. The findings support the large-scale recycling of solid wastes and the development of alternative low-carbon cementitious materials for pavement engineering. Unconfined compressive strength (UCS), water stability, freeze-thaw (F-T) cycle, and ultrasonic pulse velocity (UPV) tests were conducted on ten mixtures. The total CKD-PG dosage ranged from 42% to 90%. The results indicate that increasing the total CKD-PG dosage resulted in a denser internal structure, which in turn improved both mechanical performance and durability. When the total CKD-PG dosage exceeded 70%, the geopolymer exhibited substantially higher UCS, enhanced water stability, and a compressive strength loss of only 2.6% after 15 F-T cycles. These results were markedly better than those for mixtures with a total CKD-PG dosage below 70%. UPV measurements corroborated that this proportion yielded the greatest internal compactness. The optimal formulation was 46% CKD, 44% PG, and 10% FA (C46P44). This mix delivered the best combination of strength, frost resistance, and density and satisfied requirements for highway subgrade applications. The study provides experimental evidence for large-scale reuse of CKD, FA, and PG and identifies a key mix proportion for geopolymer use in highway subgrade construction.