Synergistic design of a fully solid waste-based geopolymer: Mechanical performance and microstructural mechanisms

The sustainable utilization of industrial by-products such as fly ash (FA), steel slag (SS), and phosphogypsum (PG) presents both an environmental imperative and a technical challenge. This research assesses the potential for developing alkali-activated geopolymer composites based on FA, while partially replacing it with SS and PG in controlled proportions. Physical properties (setting time, fluidity, density, and viscosity) and mechanical performance were systematically examined. Additionally, XRD, SEM, FTIR and MIP tests were conducted to analyze the microstructural evolution and hydration mechanisms of geopolymer composites. The results demonstrate that the incorporation of SS and PG markedly modifies the rheological and setting behavior of the geopolymer pastes. While increasing SS-PG content improves fluidity and density, it concurrently reduces viscosity. The optimal mix ratio of 80% FA, 10% SS, and 10% PG maximizes the synergistic effect, achieving 28-day compressive and flexural strengths of 5.96 MPa and 1.7 MPa, respectively. This strengthening effect results from the release of Ca²⁺ and SO₄²⁻ from SS and PG, which accelerates the formation of C-S-H, C-A-S-H, and AFt. These reaction products contribute to structural densification by filling internal pores and microcracks. However, excessive SS or PG adversely affects matrix integrity and leads to strength degradation. The findings highlight the potential of integrating multiple waste materials into high-performance, sustainable geopolymer systems.