Elevated-temperature behavior and degradation mechanism of alkali-activated back-filling grout for shield tunnel prepared from full industrial solid wastes

Backfill grouting layer is the only continuous external structure surrounding shield tunnel linings, making its thermal resilience critical to structural safety during tunnel fires. This study investigated the high-temperature behavior and deterioration mechanisms of newly developed alkali-activated grout (AAG) prepared from multi-source solid wastes. Two representative AAG were benchmarked against a typical OPC-based grout. Specimens were heated to 200, 400, 600, and 800 °C, and their appearance, mass loss, flexural/compressive strength, deterioration coefficient, phase assemblage, morphology, and pore structure were characterized. The results showed that the mass loss increased monotonically with temperature and was well described by an empirical mass-temperature relationship for both AAG and OPC-based specimens, indicating predictable release of free/bound water and other volatiles. Although AAG showed more pronounced steam-induced surface pitting and localized flaking due to their denser matrix and restricted vapor release, their cross-sections after severe heating exhibited less internal cracking and disruption than OPC-based specimens. Strength results also demonstrated better flexural and compressive strength retention for AAGs at elevated temperatures. OPC-based grout deteriorated mainly through dehydration/decomposition of hydration products, causing shrinkage cracking, pore coarsening, and internal damage. In contrast, AAG degradation was governed by a coupled pressure-shrinkage mechanism, where vapor-pressure cracking dominated at intermediate temperatures and gel dehydration/dehydroxylation drove shrinkage and pore coarsening at higher temperatures. More importantly, the AAG gel system can partially accommodated thermal damage through polymerization-dehydration-sintering type structural evolution.