Compaction Characteristics and Expansion Mechanism of Circulating Fluidized Bed Bottom Slag for Subgrade Filler

Circulating fluidized bed combustion bottom slag exhibits sand-like particle gradation and self-cementing potential, yet severe vertical expansion restricts its broad engineering application. This study investigates the compaction characteristics and expansion mechanisms of CFBBS using Proctor compaction, unconfined compressive strength (UCS), California bearing ratio (CBR), and expansion tests, combined with X-ray diffraction (XRD), scanning electron microscopy (SEM), and particle-size analysis. Results show that CFBBS has a high optimum moisture content of 15.4–19.6% due to its porous microstructure, and soil incorporation improves its compaction performance. Expansion behavior is synergistically governed by f-CaO and SO3 content, curing regime, compaction degree, and soil proportion. Seven-day standard curing, 93% compaction degree, and 20% blended soil provide favorable comprehensive performance with satisfactory mechanical properties and mitigated expansion. A 400 m field trial validates this integrated solution, with no expansion-induced cracks observed and reduced construction cost. This work provides theoretical guidance for the safe reuse of CFBBS as subgrade fill.