Influence of Composition on Differential Evolution of Pore–Fracture Systems and Micro-mechanical Properties of Lignite During Drying-Wetting Cycles.

The cracking behavior of lignite during drying–wetting cycles impacts the efficiency of coal mining, underground coal gasification, and coalbed methane development. Lignite is known to have high water content, and it exhibits strong hydrophilic properties, resulting in a weak and rigid gel structure caused by water interactions with coal matrix molecules. Even at ambient temperatures, dehydration causes violent shrinkage of the lignite matrix and numerous fractures. Due to the shallow burial depth of lignite, it is often mined in open pits, and it often undergoes cycles of dehydration and rewetting in the open-air environment, resulting in significant changes to its physical structure and mechanical properties. Thus, a detailed characterization of this evolution process is necessary. In this study, lignite samples with different compositional properties were collected from the Shengli Coalfield in the Erlian Basin, and their physical structure and mechanical properties were characterized using nondestructive testing methods such as nuclear magnetic resonance, environmental scanning electron microscopy, X-ray computed tomography, and atomic force microscopy. The results provide insights into the impact of material composition on the syneresis behavior and destruction process of lignite. Upon removal of water from the coal matrix, rapid shrinkage occurs, resulting in the simultaneous formation of numerous fractures. Significant differences were observed in the contraction characteristics and fracture propagation patterns among the various lignite lithotypes. Xylite lignite exhibited the highest degree of contraction following dehydration, with the macropore reduction rate being the most significant. Matrix lignite showed a lower degree of syneresis compared to xylite lignite, while fusain-rich lignite demonstrated the weakest syneresis ability. The homogeneous xylite lignite shrinks as a whole after dehydration, and its fractures are long and straight, with good orientation. Fractures formed in the detrital humic groundmass of matrix lignite are short, convoluted, and poorly oriented. Fusain-rich lignite fractures are thin, and straight, with the best orientation. The expansion of fractures can cause geological disasters such as land subsidence, collapse, and landslides. Fusain hinders fractures' expansion and acts as a "skeleton" to support the lignite structure. The presence of fusain is advantageous to maintaining the steadiness of the lignite. The differential evolution of the physical structure of lignite during the drying–wetting cycles, which aids to avert and mitigate disasters linked to dehydration cracking in different lignite-related engineering geological fields, is discussed in this paper. [ABSTRACT FROM AUTHOR]