<p>Iron tailings, a by-product of iron ore processing, can be recycled as pavement subgrade fill through proper solidification. However, using conventional cementitious materials, such as ordinary Portland cement (OPC) or sulphoaluminate cement (SAC), with hydration products dominated by crystal or gel results in insufficient solidification due to the complex multi-scale pore structure of tailings. Inspired by the “bionic swallow nest” principle, this study proposes a “multi-scale pore regulation” strategy by developing a novel sulphoaluminate composite cementitious material (SCCM) with a “skeleton-filling-cementation” concept. Experimental results show that SCCM demonstrates notably higher early-stage strength than OPC and superior later-stage strength compared to both solid waste-based sulphoaluminate cement (WSAC) and OPC, resulting from the synergy of their hydration products (i.e., ettringite and calcium-(aluminum)-silicate-Hydrate (C–(A)–S–H)). Consequently, SCCM significantly improves the strength, water stability, and impermeability of solidified tailings compared to WSAC and OPC, while effectively immobilising heavy metals to meet regulatory standards. This can be attributed to the densified structure, as SCCM is more effective in reducing porosity, pore connectivity, and average pore size. Specifically, the flaky C–(A)–S–H and needle-like ettringite interweave to form a homogeneous network that fills the spaces between tailing particles, creating a multi-level pore structure refinement. This “multi-scale pore regulation” strategy overcomes the “single-phase filling” limitation of traditional materials, providing an efficient solution for sustainable tailing utilisation in pavement construction.</p>