The mechanism of synergistic hydration performance improvement of the ternary composite cementitious material composed of hemihydrate phosphogypsum, blast furnace slag and quicklime

To realize the resource utilization of solid waste hemihydrate phosphogypsum (HPG) and significantly enhance its mechanical strength and water resistance as a construction material, this study employs the synergistic activation of HPG with blast furnace slag (BFS) and quicklime. A high-strength and highly water-resistant HPG-based ternary composite cementitious material (HPG-CCM) was prepared. Systematic investigations on its leaching toxicity, hydration characteristics, and microstructural evolution were conducted through ICP-OES, XRD, SEM-EDS, and TG-DTG analyses. The results demonstrate that when the HPG-to-BFS mass ratio was 6:4 with 3% externally added quicklime, the 28-day compressive strength and softening coefficient of HPG-CCM reached 52.8 MPa and 0.97, respectively. Under this optimal formulation, the relative proportions of C-S-H gel and ettringite in the reaction system were effectively modulated, resulting in significantly enhanced structural compactness, suppressed volumetric expansion, and improved water resistance. Furthermore, when the external quicklime content exceeded 3%, the leaching concentrations of phosphorus (P) and fluorine (F) remained below the thresholds stipulated by the Chinese standard GB 8978-1996, confirming the material’s exceptional safety. Mechanistic analysis reveals that the hydration process of HPG-CCM comprises three distinct phases: Early Hydration Stage: Quicklime neutralized the inhibitory effects of soluble P and F within HPG on its hydration activity, facilitating the formation of interlocking CaSO 4 ·2H 2 O crystals that established the primary strength framework. Densification Stage: Active silicates and alumina from BFS reacted with quicklime and CaSO 4 ·2H 2 O to generate C-S-H gel and ettringite, which filled interstitial pores within the CaSO 4 ·2H 2 O matrix, yielding a dense microstructure. Encapsulation Stage: Continuous hydration of BFS produced C-S-H gel that progressively enveloped CaSO 4 ·2H 2 O crystals, thereby augmenting both mechanical robustness and water resistance.