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Optimization of shield tunnel spoil via solid waste-based binders: Microstructural evolution and solidification mechanisms
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DOI：10.1016/j.cscm.2026.e06393.png)
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Abstract
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The solidification of shield tunnel spoil has attracted growing interest for large-scale reuse of construction waste. However, existing approaches based on solid waste-derived binders are largely confined to alkali-sulfate activation, leaving the roles of residual organic foaming agents and the interaction among multiple activators largely unexplored. This study develops a quaternary binder system composed of fly ash, desulfurized gypsum, carbide slag, and sodium persulfate and proposes a synergistic framework in which oxidative pretreatment serves as a critical precursor that primes subsequent hydration. The solidified spoil was characterized by unconfined compressive strength (UCS), X-ray diffraction, thermogravimetric-differential scanning calorimetry, scanning electron microscopy, and mercury intrusion porosimetry (MIP), complemented by low-field nuclear magnetic resonance (NMR) for water distribution analysis. The low-field NMR results showed that sodium persulfate achieved 72.18% degradation of residual organics and altered pore water distribution, converting free and capillary water into interstitial water and increasing its proportion from 95.27% to 97.05%. This redistribution modified the moisture environment prior to hydration, which correlated with the subsequent microstructural evolution. Carbide slag provided alkalinity to activate fly ash, while gypsum supplied sulfate ions, jointly promoting concurrent formation of ettringite and C-S-H gel. Over 28 days, the MIP-derived fraction of pores smaller than 10 nm increased from 13.88% to 43.93%, capillary pores decreased to 32.30%, and total porosity dropped from 40.12% to 20.62%, with a corresponding UCS of 1179.41 kPa. The contribution establishes the sequential causality from physicochemical conditioning to microstructural densification, offering a sustainable route for spoil reuse, with preliminary evidence of reduced carbon footprint compared with ordinary Portland cement.
Keywords:
Shield tunnel spoil
Solid waste-based binder
Microstructural evolution
Pore structure
Solidification mechanism
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