Preparation of steel slag-based cementitious materials compounded with GBFS and phosphogypsum: mix design, hydration, and microstructure characteristic

Steel slag and phosphogypsum are industrial wastes produced during steelmaking and the wet phosphoric acid process, respectively, contributing to significant environmental challenges due to their low utilization rates. This study explores their combination with GBFS to develop a new ecofriendly cementitious material based on steel slag. The effects of initial factors on early properties were systematically investigated using an orthogonal test. Hydration and microstructural characteristics were obtained through analysis of hydration processes, product characteristics, microstructure, and pore structure, and hydration behaviors were postulated. The findings reveal that a lower steel slag-GBFS ratio leads to a higher strength of the composite system. A higher water-binder ratio leads to a longer setting time and greater fluidity. The composite exhibits a low exothermic reaction, characterized by three exothermic peaks on its hydration heat release curve. The primary hydration products are Ettringite and C-(A)-S–H gel. Gypsum acts as an intermediate product. The main reaction in the composite system is that bassanite first transforms into gypsum. Concurrently, the [SiO 4 ] 4− , and [AlO 4 ] 5− in steel slag and GBFS gradually depolymerize. Once the conversion of hemihydrate gypsum is completed, the gypsum begins to dissolve, forming the primary hydration products. As hydration progresses, the system gradually forms a structure where ettringite intertwines, with gel filling and encapsulating the ettringite. The composite exhibits a loose pore structure in the early stages, leading to lower mechanical performance. Strength increases rapidly from 3 to 7 d. In the later hydration stages, the volume of harmful pores increases again, which subsequently moderates the strength gain. Nevertheless, under optimal conditions, compressive strength reaches 34.1 MPa at 28 d.