The Effect of Fly Ash on the Microstructural Evolution and Mechanical Properties of Geopolymers Made from Phosphorus Tailings and Ground Granulated Blast-Furnace Slag

Severe environmental contamination from accumulated phosphorus tailings and their low recycling rate remain critical bottlenecks in solid waste treatment. Furthermore, geopolymers fabricated using single or binary solid wastes generally suffer from insufficient mechanical properties and poor durability, while their synergistic activation mechanisms have not been fully clarified. To address these issues, this study developed a binary geopolymers composite using phosphorus tailings and ground granulated blast furnace slag (GGBS) as raw materials through alkali activation (sodium hydroxide and water-glass), and prepared a ternary geopolymers composite incorporating fly ash. The effects of solid waste blending ratios on flowability, compressive strength, drying–wetting and freeze–thaw resistance as well as drying shrinkage were systematically investigated, and the synergistic geopolymerization mechanism was characterized by SEM-EDS, XRD and FTIR. Experimental results show that the binary system achieves optimal performance at 40% GGBS substitution, yielding a 28-day compressive strength of 23.4 MPa with lower shrinkage and better anti-damage capacity than pure phosphorus tailings specimens. Introducing 20% fly ash into the optimized binary system forms a ternary geopolymer with a 28-day strength of 37.8 MPa, rising by 61.5%. Its drying shrinkage, mass loss from drying–wetting cycles and freeze–thaw erosion are separately reduced by 33.3%, 39.5% and 40.0%. Microscopic analyses verify that GGBS and fly ash collaboratively provide active Ca, Si and Al species to stimulate abundant C-S-H gel formation and compact the matrix microstructure. This ternary geopolymer realizes efficient collaborative utilization of three industrial solid wastes, providing theoretical support and technical guidance for large-scale resource utilization of phosphorus tailings and the fabrication of geopolymer construction materials.