<p>To achieve the utilization of solid waste and address the issue of insufficient strength of ground granulated blast furnace slag (GGBS) and phosphorus slag (PS) as single admixtures, and fill the gap that the synergistic effects among multiple solid wastes remain poorly understood, ultra-high performance concrete (UHPC) was prepared by combining fly ash (FA) with a ternary system of PS-GGBS-FA (PGF). Compressive strength and microstructural tests were performed to assess mechanical properties and elucidate the hydration mechanisms across different mix ratios. The optimal 28-day compressive strength of 143.3 MPa was achieved with a mix containing 30% total admixture (PS:GGBS:FA = 6%:16%:8%). This result exceeded that of the pure cement group. Microstructure analysis revealed the synergistic staged hydration mechanism of the PS-GGBS-FA ternary system. In the early stage, cement hydration provided Ca²⁺ and alkalinity, accelerated by PS. Subsequently, accumulated Ca(OH)₂ activated GGBS. In the later stage, the residual constituents of FA undergo slow reactions within the weakly alkaline environment conditioned by PS, continuously yielding C-(A)-S-H gel in association with Ca²⁺. Concurrently, sulfate from PS rapidly formed AFt crystals with silicate and aluminate hydrates. These crystals offered heterogeneous nucleation sites, forming a spatiotemporally synergistic composite structure with C-(A)-S-H. Moreover, inert particles in the PGF system significantly reduced porosity. Their surfaces served as substrates, guiding the uniform attachment of C-(A)-S-H and AFt to prevent defect-forming local accumulations. This study demonstrates a sustainable path to high-performance concrete with reduced cement demand.</p>