Effects of high temperature on pore structure and mechanical properties of phyllite

Studying the thermal damage mechanism of phyllite is critical for tunnel thermal hazard assessment. Due to the current lack of research on the thermal effects on the physical and mechanical parameters of phyllite, this study examined the pore and failure characteristics of phyllite after high-temperature treatment (25∼900°C) and elucidated the damage mechanisms from a microstructural perspective. The results indicate that 600°C is the threshold temperature for the rapid initiation and expansion of pores in phyllite, with the total porosity increasing from 2.08% at 25°C to 7.50% at 600°C. However, at 900°C, the porosity of mesopores and macropores decreases to 0.25% and 0.01%, respectively. The propagation of pores will weaken the mechanical parameters of phyllite. The uniaxial compressive strength (UCS) of phyllite follows a trend of "no significant change (25∼500°C)-decrease (500∼600°C)-increase (600∼900°C)". Due to the phase transition of quartz, the UCS decreases from 34.36MPa at 500°C to 19.65MPa at 600°C, accompanied by a transition from brittle to ductile failure characteristics. Between 600°C and 900°C, the UCS shows a negative correlation with the porosity of mesopores and macropores. This study fills the research gap on phyllite under thermal damage, providing a theoretical basis for stability assessment and repair of tunnels after fires. • High temperature alters the pore structure and mechanical properties of phyllite. • 600°C is the threshold temperature for the rapid decline in UCS. • Pore structure is a critical factor influencing the mechanical properties of phyllite.