Study on the variation rules of the pore structure of phyllite under high temperature

• First systematic study on phyllite pore structure evolution up to 1000°C. • Phyllite porosity peaks at 700°C, 4.13 times its initial value. • Integrated NMR, SEM, and 3D microscopy to characterize thermal damage. • Pore formation and blockage mechanisms at different temperatures are revealed. • Provides guidance for tunnel safety and geothermal development in phyllite. In recent years, frequent domestic mining activities and tunnel construction have generated substantial quantities of phyllite by-products. At ambient temperatures, phyllite has been widely reused as a subgrade filler in roadbed engineering. Under high temperatures, phyllite can serve as a raw material and tempering agent for ceramic production. However, research on the evolution of pore structure in phyllite under high-temperature conditions remains limited. Phyllite samples were subjected to heat treatment within the range of 25 to 1000°C, aiming to investigate the patterns of pore structure change at different temperatures. This study offers theoretical guidance for geothermal development in phyllite reservoirs and the construction of phyllite tunnels. Techniques including Nuclear Magnetic Resonance (NMR), Scanning Electron Microscopy (SEM), and 3D Microscopy were employed to systematically examine the damage characteristics of phyllite at various heat treatment temperatures. The results indicate that both the total porosity and micropore porosity of phyllite initially increase and subsequently decrease with rising heat treatment temperature, reaching peak values at 700°C. At this temperature, they were 4.13 times and 4.07 times higher than those at 25°C, respectively. When the heat treatment temperature was below 700°C, continuous evaporation of free and bound water, oxidation of pyrite, and phase transformation of quartz within the phyllite led to a progressive increase in porosity. When the heat treatment temperature exceeded 700°C, partial mineral decomposition and the onset of sintering and recrystallization occurred within the phyllite. These processes resulted in reduced pore connectivity and effective porosity, leading to a stabilization of the phyllite's properties.