Abstract

The sealing behavior of fracture-filling minerals in the near-field of the deep geological repository (DGR) is critical for the safe disposal of high-level radioactive waste (HLW). In granite host rocks, natural fractures are often filled with polymineralic assemblages of calcite, quartz, and clay minerals; however, their coupled compaction–pressure solution mechanisms under thermal–hydraulic–mechanical–chemical (THMC) conditions remain poorly understood. In this study, 12 fracture sealing tests were conducted on Beishan granite and its typical fracture fillings at 90 °C and 15 MPa effective stress, using different pore fluids and systematically varying grain size (75–250 μm), mineral proportions, and clay content. The results indicate that stress-assisted dissolution–precipitation of calcite in saturated CaCO3 solution is a key process contributing to porosity reduction and chemo-mechanical densification of the fracture filling, achieving a compaction strain of 24.6%—substantially higher than those obtained in deionized water (20.6%) and under dry conditions (14.8%). Fine-grained calcite compacts more effectively than its coarse-grained counterpart, reaching a porosity as low as 4.8%; rigid quartz locally redistributes contact stress at quartz–calcite interfaces, promoting preferential deformation or dissolution of adjacent calcite, although increasing quartz abundance reduces the bulk compaction efficiency. A moderate amount of clay minerals (~20 wt%) further reduces porosity to 2.1% through lubrication and micropore filling. The study reveals a multi-stage process transitioning from mechanical compaction to chemo-mechanical sealing, and a synergistic mechanism dominated by calcite compaction–pressure solution, augmented by quartz stress redistribution and clay lubrication. These findings provide direct experimental evidence for the progressive chemo-mechanical densification of mineral-filled granite fractures, and offer quantitative constraints for long-term THMC modeling of fracture sealing behavior in HLW repositories.

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