<p>To address the high cost and high carbon emissions of traditional cemented backfill and the poor performance of alkali-activated FA-BFS binders, this work investigates the influences of SAP content, particle size, and sodium silicate modulus on the workability, strength and drying shrinkage of FA-BFS paste, and reveals its internal curing mechanism using NMR, FTIR, and SEM&amp;EDS. SAP swelling capacity increases with a lower sodium silicate modulus and finer SAP particles. Higher SAP content and a lower sodium silicate modulus reduce fluidity and shorten setting time, while finer SAP particles only extend the final setting time. Early strength declines with increasing SAP content, whereas the 28 d strength first increases and then decreases, with 0.2% SAP being the optimal content. Finer SAP particles and a higher sodium silicate modulus effectively improve compressive strength. SAP significantly reduces drying shrinkage through water retention and sustained water release. Excessive SAP introduces a large number of macropores, while hydration products partially refine the pore structure at later ages. SAP reacts with Ca²⁺ and A³⁺ through chelation and ion exchange to promote matrix hydration. This study provides insights for developing eco-friendly and low-carbon solid waste backfill materials.</p> Graphical Abstract <p></p>