Multiscale study on thermomechanical behaviour of shield muck-slag-based engineered geopolymer composites (SS-EGC) for shield tunnel segments

The shield method is widely recognized as a superior method of tunneling for its advantages of safety, high efficiency, and adaptability to the stratum. However, the manufacturing of shield tunnel segments requires substantial cement and carries a huge carbon footprint, while the engineering waste of shield muck occupies territory and poses a serious threat to local ecological safety. To achieve the goal of green tunnels, a sustainable shield muck-slag-based engineered geopolymer composite (SS-EGC) for segments was proposed in this paper. Meanwhile, systematic tests were conducted to investigate the multiscale behaviour of SS-EGC subjected to elevated temperatures. The macrostructural performances after heat exposure were evaluated through visual appearance, weight loss, ultrasonic velocity, compressive, flexural, and tensile strength, as well as microstructural characteristics of SS-EGC were examined through X-ray powder diffractometer (XRD), nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), scanning electron microscope (SEM), and mercury intrusion porosimetry (MIP). Results indicate that SS-EGC exhibits excellent high-temperature resistance, and no explosive spalling occurs in any specimen during heating, due to the interconnected channels left by the melted PE fibers and the porous structure of gel matrix. As the temperature rises, high cross-linked and porous N-A-S-H gels gradually become the dominant reaction products in gel matrix. Besides, the mechanical properties of SS-EGC at elevated temperatures are highly affected by the phase composition, the pore structure and the fiber bridging action. The strength gain mechanisms can be ascribed to further geopolymerisation and viscous sintering that refines the pore structure, while the strength degradation mechanisms are related to the damage caused by pore pressure accumulation and thermal gradient, melting and evaporation of fibers, dehydroxylation and recrystallisation of gels.