Optimization of humidification stabilization for SRF co-firing ash: Balancing expansion inhibition and drying shrinkage

Amidst the global transition toward carbon-neutral infrastructure, circulating fluidized bed boilers co-fired with solid recovered fuel represent critical nodes for circular resource recovery. However, the resulting residues exhibit volumetric instability due to unreacted free calcium oxide and anhydrous calcium sulfate, which compromises durability in cementitious composites. This study conducts a multi-scale investigation into the competitive volume dynamics – the trade-off between expansion mitigation and the induction of drying shrinkage. Using integrated characterization (X-ray diffraction, thermogravimetry, and scanning electron microscopy with energy-dispersive spectroscopy), we elucidate the micro-macro correlation in dimensional stability. Experimental results demonstrate that raw ash exhibits a destructive 7-day linear expansion of 1.62% (±0.12%) under accelerated testing in a 70°C water bath. Controlled humidification stabilizes the atypical ash by converting reactive precursors into stable calcium hydroxide and ettringite (Aft). Crucially, a suitable balance is identified: while 100% humidification eliminates expansion, it triggers a significant 1.21% (±0.08%) drying shrinkage. This is attributed to excessive Aft formation that refines pore structures, thereby elevating capillary tension governed by the Kelvin–Laplace effect. A 30% humidification protocol for 1 day strikes an optimal balance, controlling expansion to 0.39% (±0.04%) while mitigating shrinkage risk and ensuring the environmental sequestration of heavy metals (leachable lead < 0.12 mg/L). This research provides insights into the safe integration of atypical residues in sustainable binders.