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Article

Compaction and Pressure Solution of Mixed Mineral Assemblages: Implications for Granite Fracture Sealing in the Near-Field of High-Level Radioactive Waste Repository

by
Xiao Tian
1,2,*,
Ju Wang
1,2,*,
Jia-Wei Wang
1,2,
Jing-Li Xie
1,2,
Zhi-Chao Zhou
1,2 and
Ke Li
3
1
Beijing Research Institute of Uranium Geology, Beijing 100029, China
2
CAEA Innovation Center for Geological Disposal of High-Level Radioactive Waste, Beijing 100029, China
3
School of Resources and Safety Engineering, Chongqing University, Chongqing 400044, China
*
Authors to whom correspondence should be addressed.
Minerals 2026, 16(6), 603; https://doi.org/10.3390/min16060603
Submission received: 5 May 2026 / Revised: 23 May 2026 / Accepted: 2 June 2026 / Published: 3 June 2026
(This article belongs to the Special Issue Implications of THMC Processes on Long-Term Safety of Geological Disposal of Radioactive Waste, 2nd Edition)
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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.

1. Introduction

The safe disposal of high-level radioactive waste (HLW) constitutes a critical constraint on the sustainable development of nuclear energy. The deep geological repository (DGR), which employ a multi-barrier system to isolate HLW from the biosphere over geological timescales, are internationally recognized as the most technically feasible solution [1,2,3]. Granite, one of the main host rock types for repositories, possesses inherent advantages such as high strength, low permeability, and low porosity. However, natural fractures within granite continuously evolve under complex geological conditions [4,5,6,7,8,9,10,11], forming potential pathways for groundwater flow and radionuclide migration that compromise the long-term safety of the repository [12,13]. These fractures are commonly filled with natural minerals including calcite, quartz, and clay minerals [14,15,16]. Under the coupled thermal–hydraulic–mechanical–chemical (THMC) conditions prevailing in the near-field of repository, these fracture-filling minerals may undergo compaction, dissolution, and precipitation, thereby altering the sealing state of the fractures [8,17,18,19,20]. Therefore, elucidating the sealing processes and mechanisms of mineral-filled fractures under coupled THMC conditions is a fundamental issue for both repository safety assessment and the development of natural sealing materials.
Under non-high-temperature and non-high-pressure conditions, substantial densification of crystal-mineral-filled natural fractures requires not only physical changes but, more importantly, free-face dissolution–precipitation or pressure solution [21]. Previous studies have extensively investigated the kinetic behavior of compaction, dissolution, and precipitation in monomineralic systems of calcite or quartz [22,23,24,25,26,27]. Weyl [17] and Tada et al. [18] demonstrated that stress-induced mineral dissolution and compaction mechanisms can lead to progressive rock deformation. Ellis [28] found that increasing pore fluid salinity significantly enhances calcite solubility. Hilgers et al. [29] revealed the accelerating effect of temperature on quartz dissolution–precipitation. Zhang et al. systematically explored the influences of temperature, effective stress, solution chemistry, and grain size on the pressure solution of calcite, demonstrating that the pressure solution strain rate increases with rising temperature and decreasing grain size, whereas specific ions such as Mg2+ and HPO42− strongly inhibit the pressure solution process [24,26,30]. At the engineering scale of DGR, Rimstidt et al. [31], Dobson et al. [32], and Zhang [33] performed fracture sealing experiments on tuff and clay rock, confirming the potential for mineral precipitation to induce long-term fracture closure. Recent studies indicate that contrasting mechanical and chemical properties of mineral phases in polymineralic systems generate coupled inter-mineral effects. These effects—including stress concentration at rigid–soft contacts, differential dissolution, and clay lubrication—collectively control compaction and sealing behavior [34,35,36].
Nevertheless, existing studies exhibit notable limitations. First, experimental investigations have largely focused on tuff or clay rock [37]; systematic studies on granite and its typical fracture-filling minerals remain scarce, and direct microstructural evidence accessible to visual observation is lacking. Second, the filling materials employed in experiments have predominantly been monomineralic (e.g., pure calcite or pure quartz). In reality, fracture fillings in granite are typically heterogeneous assemblages of mixed minerals with a broad grain size distribution [2]. Previous studies have not accounted for the mechanical–chemical coupling effects among different mineral phases. These shortcomings, to some extent, constrain the reliability of predictions for the long-term evolution of fracture sealing in the repository near-field.
This study focuses on the critical issue of granite fracture sealing in the near-field of DGR. Taking the Beishan granite and its common fracture-filling minerals as the test samples, a specially designed apparatus capable of simultaneous monitoring of temperature, fluid pressure, stress, and strain was developed. A series of fracture sealing tests were conducted under varying conditions of grain size, mineral composition, and fluid chemistry. The objective is to reveal the sealing processes and mechanisms of mixed-mineral-filled fractures under repository near-field conditions, to provide a critical basis for repository safety assessment, and to lay a theoretical foundation for the development of fracture sealing materials.

2. Sample Collection and Preparation

All rock and mineral materials used in this study were collected from deep boreholes at the Beishan site, the pre-selected candidate area for China’s HLW repository. The rock samples consist of fresh granodiorite—the predominant lithology at the site—taken from the 465.6–601.5 m depth of borehole BS49 (core diameter 63 mm). Fracture-filling materials were recovered from 11 boreholes (BS35–BS42 and BSQ29–BSQ31) distributed across the site. These materials are predominantly polymineralic mixtures, dominated by calcite, the most abundant fracture-filling mineral in Beishan granite, together with quartz and clay minerals (mainly smectite) [2,15].
All samples were prepared following specific procedures to meet experimental requirements after collection. The granite core was first cut into 10 cm long columns (Figure 1a), and the outer surfaces were polished flat and smooth to fit the uniaxial pressure cell. A circumferential notch with a cutting depth of approximately 15 mm was then machined at the mid-height of each column. The sample was fractured along this notch by applying a force, forming a natural rough cross-section with a diameter of approximately 30 mm as the experimental fracture surface (Figure 1b). Due to the fact that almost all fracture-filling materials collected comprised mixed mineral assemblages, they were hand-picked under a microscope, ground using a mortar, and screened out pure mineral particles or powders of calcite and quartz with average grain sizes of 75 μm, 150 μm, and 250 μm, respectively, using a nickel wire mesh. Clay minerals were separated and purified by gravity settling and other purification techniques.
The pore fluid in the majority of the tests was a saturated CaCO3 solution. This solution was selected to provide a stable Ca–CO3 chemical environment and to facilitate identification of calcite dissolution–precipitation within the limited laboratory timescale; it should therefore be regarded as an idealized chemical boundary condition rather than a direct analogue of natural Beishan groundwater. The solution was prepared at room temperature by adding excess CaCO3 powder to deionized water and stirring vigorously for approximately 40 h. One set of tests used deionized water to isolate the contribution of pore fluid chemistry.

3. Test Methodology

3.1. Test Facility

The experimental apparatus, as shown in Figure 2, is built around a custom-designed uniaxial pressure cell comprising a base, a cylindrical chamber, and an upper piston. Fluid inlet and outlet ports are integrated into the base and the piston, respectively. The internal chamber has a cross-sectional diameter of 65 mm to accommodate granite columns of 63 mm diameter. Axial stress is applied by a geotechnical consolidation frame with a maximum loading capacity of 100 kN, transmitted to the sample through a circular loading head equipped with a load cell. Pore fluid pressure is generated and maintained at a constant level by an osmotic pressure–volume controller and delivered into the pressure cell through a high-pressure isolation vessel filled with the prepared solution. Sealing rings are fitted around the piston and the base adjacent to the sample to prevent solution leakage. Grooved metal spacers with a cross-shaped recession were placed at both the top and bottom surfaces of the sample to ensure unimpeded fluid flow. The temperature control system employed an immersion circulator (accuracy ±0.5 °C) operating under closed-loop PID control, with dimethyl silicone oil as the circulating medium. The exterior of the pressure cell was wrapped with insulating wool to minimize heat loss. For data acquisition, a high-precision displacement transducer (resolution ±1 μm) was mounted between the upper piston and the cell body to continuously record the axial compaction strain of the specimen. Pore pressure transducers (accuracy 0.25%) were installed at the upper and lower fluid ports to monitor the pore fluid pressure. A thermocouple embedded in the base of the pressure cell provided real-time monitoring of the internal temperature. All sensor data were acquired and stored in real time by a data logger running GDSLAB software. The entire apparatus was calibrated against certified reference materials to verify measurement accuracy prior to the tests.

3.2. Test Procedure

The designated fracture-filling minerals were homogeneously mixed and spread evenly over the fracture surface, after which the assembled sample was placed into the pressure cell. Because the mineral particles were manually placed on rough granite fracture surfaces, the initial packing state could vary significantly among samples. To reduce the uncertainty caused by loose packing, particle bridging, and unstable initial contacts, and to improve the comparability among different tests, the fracture-filling material was first pre-compacted under dry conditions by applying an axial stress (σa) of 30 MPa for approximately 0.5 h. This step served as an experimental standardization procedure to establish a reproducible initial microstructure, rather than as a direct analogue of natural fracture evolution.
Subsequently, the axial stress was reduced to 6–8 MPa, and the pore fluid was injected. A constant pore fluid pressure (Pf) of 5 MPa was maintained while the temperature was raised to 90 °C. The temperature of 90 °C was adopted because it is close to the maximum thermal constraint commonly considered in HLW repository design and represents the early thermal stage after waste emplacement [38]. Once the temperature stabilized, the axial stress was gradually increased to the target value for each group such that the effective stress (σe = σa − Pf) equaled 15 MPa. This effective stress was selected to approximate the upper range of the in situ horizontal stress at the investigated depth interval of the Beishan site [39]. The imposed conditions of 90 °C and 15 MPa effective stress were therefore used to examine the chemo-mechanical response of fracture-filling minerals under a representative upper-bound near-field condition.
Throughout the experiment, the normal displacement across the fracture and other parameters were continuously recorded at 10 min intervals to capture the mineral deformation and fracture sealing progress. The test was terminated once the rate of displacement change became negligible. After each test, the system was cooled and depressurized, and the post-experiment pore solution was collected through the ball valve. The pressure cell was then disassembled, and the granite column was retrieved, dried, weighed, and preserved for SEM analysis. A total of 12 such tests were conducted in this study, each continuously monitored for 9–12 days, yielding a cumulative experimental duration of 138 days.

3.3. Data Analysis

To characterize the degree of fracture sealing, the data analysis focused on the axial compaction strain of the sample, the porosity of the filling material, and its microstructure. The compaction strain (εv) directly quantifies the degree of compaction deformation of the filling material under uniaxial stress and is calculated from the displacement transducer data as ε V = Δ L L 0 (Figure 3). The porosity of the mineral filling material indirectly reflects the degree of fracture sealing and was determined from sample measurements using the following formula:
Φ = V P V t o t = 1 − w s l π r 2 ρ s
where Vtot is the total volume of the filling layer, VP is the total pore volume within the layer, l is the layer thickness, r is the radius of the fracture surface, Ws is the mass of fracture filling minerals, and ρs is the mean density of the minerals, calculated as the weighted average of the mass fractions of the constituent mineral phases.
SEM images provided direct evidence of mineral deformation and dissolution–precipitation [40]. Specifically, the porosity of the filling material was derived from representative BSE images through pixel analysis and thresholding using ImageJ 1.54g software [41]. Additionally, mineral grain sizes were measured and statistically analyzed using the Analyze Particles function of ImageJ.

4. Test Results

Overall, the tests were completed successfully. Compaction strain and porosity virtually exhibited clear systematic variations over time, and microstructural images also provided evidence of mineral compaction and dissolution–precipitation. Despite the limited duration (~300 h per test), the systematic variations in strain and porosity, along with microstructural evidence, suffice to elucidate the governing fracture sealing mechanisms. Detailed test conditions and results are presented in Table 1.

4.1. Compaction Behavior and Multi-Stage Evolution

The evolution of compaction strain for the fracture filling composed of fine-grained calcite particles (mean grain size 75 μm) is presented in Figure 4, where the axial strain (ΔL/L0) and calculated porosity (ϕ) are compared under three conditions of dry, deionized water, and saturated CaCO3 solution. Based on the change rates of strain and porosity, the mineral deformation process can be divided into three distinct stages of pre-compaction, early stage of solution involvement, and equilibrium-approaching.

4.1.1. Pre-Compaction Stage (0–0.5 h)

During the first 0.5 h of the test, the filling material underwent purely rapid mechanical compaction under an axial stress of 30 MPa. This stage was dominated by instantaneous grain rearrangement, accompanied by brittle fracturing of grains at highly stressed contacts. The resulting fragments filled adjacent intergranular voids, triggering further sliding and re-rearrangement. For the fine-grained calcite, this mechanical deformation essentially reached a quasi-static equilibrium within 0.5 h, as indicated by the flattening of the strain curve (Figure 4b). Notably, no pore fluid had yet been injected and the temperature remained at ambient conditions, so chemical processes such as dissolution–precipitation were negligible during this stage.

4.1.2. Early Stage of Solution Involvement (0.5 h to Several Tens of Hours)

Upon injection of the pore fluid and heating to 90 °C, the strain curve displayed a pronounced acceleration in its early phase (Figure 4a). Under saturated CaCO3 solution, the cumulative strain increment from 1 h to 60 h reached 6.8%, corresponding to an average strain rate of 3.2 × 10−7 s−1, while the porosity decreased by 7.4%. This enhanced deformation is mainly attributable to two concurrent mechanisms. Firstly, the liquid phase effectively reduced the friction coefficient at grain contacts that facilitated physical rearrangement. Secondly, the elevated temperature likely enhanced the reactivity of the minerals [42], creating favorable conditions for the onset of chemical processes. In contrast, the dry sample exhibited a considerably slower strain increase, with an average rate of only 2.4 × 10−7 s−1 from 1 h to 60 h, and its strain increment decayed earlier. This is because its deformation relied entirely on mechanical grain fracturing and rearrangement.

4.1.3. Equilibrium-Approaching Stage (After Several Tens of Hours)

After several tens of hours, the strain entered a phase of slow growth accompanied by a gradual decline in porosity, exhibiting a stepwise pattern (Figure 4a). For the sample tested under saturated CaCO3 solution, the strain increases between 221 h and 280 h was merely 0.7%, corresponding to an average strain rate of only 3.3 × 10−8 s−1. This rate is roughly one order of magnitude lower than that in the early stage of solution involvement. The intermittent small “jumps” visible on the strain curve suggest localized restructuring of the grain assembly, likely associated with dissolution at stress concentration positions. Toward the end of the test, the strain curve flattened, indicating that the filling had attained a new quasi-equilibrium granular structure. A comparison of the microstructures after pre-compaction and post-test further reveals marked differences in grain morphology and pore architecture between two stages (Figure 5).
Although each test was monitored for nearly 300 h, the slowly increasing strain trend at the end of the tests and the observed microstructures (Figure 5b) indicate that complete compaction and even mutual healing of the mineral grains were not achieved. This observation suggests that natural fracture sealing is a protracted and complex geological process requiring a sufficiently long duration, which is difficult to fully reproduce within the time span accessible to laboratory tests.

4.2. Role of Pore Fluid

A comparison of the results under the three conditions demonstrates that the dry sample entered the quasi-equilibrium state earliest, with a final strain of 14.8%, substantially lower than the 24.6% achieved under saturated CaCO3 solution. The final strain under deionized water (20.6%) fell between these two values. Clearly, purely mechanical compaction can realize only a limited volume reduction. However, the presence of a pore fluid (particularly saturated CaCO3 solution) substantially amplifies the volume loss, likely indicating a fundamental change in the compaction behavior of the filling minerals. SEM images provided direct microscopic evidence for this inference: only grain fracturing and dense packing were observed under dry conditions, whereas mineral dissolution and precipitation were identified under saturated CaCO3 solution (Figure 6). The neoformed precipitates not only filled residual pores but also developed intergranular cementation between mineral grains, indicating a sealing efficacy that exceeds mere mechanical compaction. It should be noted, however, that the saturated CaCO3 solution represents a near-optimal chemical condition for calcite precipitation. Consequently, the maximum compaction strain of 24.6% and final porosity of ~4.8% observed under this condition should be interpreted as values reflecting favorable-case scenarios. Under the actual groundwater chemistry of the Beishan site, the presence of additional ionic species and potential inhibitors (e.g., Mg2+, HPO42−) may suppress the efficiency of calcite pressure solution–precipitation. This implies that the fracture sealing efficiency in the repository near-field could be less pronounced than the experimental results suggest, an important consideration for the long-term performance assessment discussed in Section 5.3.

4.3. Grain-Size Effect

Parallel tests were conducted using calcite particles with mean grain sizes of 75 μm, 150 μm, and 250 μm, together with a bimodal mixture composed of 50 wt% 75 μm and 50 wt% 250 μm particles, totaling four experimental groups. The results (Figure 7a) show that the fine-grained sample of 75 μm ultimately attained the lowest porosity of only 4.8%, attributable to more efficient particle rearrangement and chemical processes. The samples with different grain sizes exhibited distinct responses at different stages of the tests. The coarse-grained sample of 250 μm displayed a markedly higher strain rate in the early stage, owing to the rapid closure of larger pores between grains during initial compaction. As compaction progressed, the fine-grained sample actually showed a relatively greater porosity reduction, which is closely related to its larger specific surface area and more frequent grain contact. More grain contacts provide more positions of stress concentration, which favor the initiation of pressure solution between minerals.
Post-test statistical analysis of grain sizes from SEM images further corroborates these findings (Figure 7b,d). The coarser calcite particles exhibited a broader grain-size distribution after testing, reflecting pronounced fragmentation of calcite particles. Their mean grain size decreased from an initial 150 μm to a final 26.8 μm, with a standard deviation (σ) of 9.13 μm. In contrast, the mean grain size of the fine-grained sample decreased from 75 μm to 12.1 μm with σ = 7.54 μm. A smaller mean grain size implies less intergranular porosity, which benefits the overall sealing performance of the filling material. Additionally, the mineral grains and the intergranular porosity observed in BSE images (Figure 7c,e) further confirm the above interpretation.

4.4. Effects of Mineral Composition

Three groups of samples with identical mean grain size but different calcite-to-quartz mass ratios (4:1, 1:1, and 1:4) were prepared for comparison. The strain curves in Figure 8 show that increasing quartz content resulted in smaller compaction strain and higher final porosity, and vice versa. This pattern indicates that calcite exhibits significantly greater compressibility and brittle–plastic deformability compared to quartz, making it more susceptible to grain deformation and denser packing under identical stress conditions. The presence of quartz significantly restricted the overall compaction of the mixture, owing to its high hardness and low reactivity, which rendered it resistant to mechano-chemical deformation under the temperature and pressure conditions imposed in these tests.
Notably, in tests using calcite–quartz mixtures as the fill, SEM imaging captured the stress-coupling effect between the two mineral phases (Figure 6g–i). It is virtually certain that quartz grains remained undissolved under the experimental conditions [43]. However, stress concentrations developed significantly at their contacts with calcite, causing preferential fracturing and dissolution of the adjacent calcite surfaces, with calcite re-precipitation subsequently observed near these high-stress positions. The newly precipitated calcite microcrystals effectively filled narrow intergranular voids, which is of critical importance for substantial fracture sealing.
Given that clay minerals (e.g., illite and smectite) are common constituents of fracture fillings in Beishan granite [15], smectite was further incorporated into the mixed mineral assemblages at mass fractions of 10% and 20%. Figure 9b–d visually present the mesoscale structural evolution of the same clay-bearing sample at different test stages: under constant axial stress, the mineral grains became progressively more tightly packed over time, and visible fine pores were markedly reduced. As shown in the porosity evolution curves (Figure 9a), the addition of clay minerals accelerated the porosity reduction, primarily attributable to the intrinsic plasticity and interlayer sliding capability of clay minerals, which facilitated relative sliding and rearrangement of rigid grains and thereby enhanced the overall compaction of the mixture. Within the formally tested range, the 20 wt% clay-bearing assemblage exhibited the lowest final porosity (2.1%), indicating that clay minerals can effectively promote micropore filling and grain rearrangement when present in an appropriate proportion. This result, however, should not be interpreted as a quantitatively determined optimum or threshold clay content. An exploratory test with 30 wt% clay showed apparent post-test redistribution of clay particles outside the fracture surface, suggesting that excessive clay may compromise the structural stability of the fill under solution-saturated conditions [44]. As this test was not part of the complete controlled experimental series, this observation is treated here as qualitative evidence only, and the potential mobilization of excessive clay minerals remains a hypothesis requiring further systematic verification.

5. Discussion

5.1. A Multi-Stage Process: From Mechanical Compaction to Chemo-Mechanical Sealing

The experimental results reveal that the sealing of fractures filled with mixed mineral assemblages is a multi-stage process that transitions progressively from purely mechanical compaction to coupled chemo-mechanical densification. During the initial pre-compaction stage, mineral grains undergo instantaneous brittle fracturing and rearrangement under axial stress, leading to a rapid reduction in porosity. However, this stage produces only dense packing of grains without intergranular cementation, and thus true sealing is not achieved. With the involvement of pore fluid and the increase in temperature, fluid lubrication and thermal activation jointly shift the deformation behavior from purely mechanical to coupled mechanical–chemical, as evidenced by the contrasting strain evolutions under dry and solution-saturated conditions (Section 4.1). The strain evolutions under different solution conditions clearly demonstrate that purely mechanical compaction can achieve only limited volume reduction, and the involvement of a chemically active fluid is the key factor driving effective fracture sealing [45,46]. Especially under saturated CaCO3 solution, numerous minute dissolution pits develop on calcite surfaces, and fine clusters or needle-shaped neoformed crystals precipitate preferentially near grain contacts. These neoformed crystals form cemented structures with cohesive strength between grains, marking the transition from mere volume reduction to enhanced closure of the fracture.
It must be noted that the term “sealing” in this study refers to porosity reduction, microstructural densification, and intergranular cementation, rather than direct hydraulic isolation. In heterogeneous fracture fillings, a decrease in total porosity does not necessarily eliminate connected pore pathways; accordingly, the present results should not be used to quantitatively infer hydraulic sealing capacity without further flow-through and permeability measurements. Nevertheless, the evidence collectively indicates that the driving force of the fracture sealing process undergoes a transition from effective-stress-dominated mechanical rearrangement to stress-chemistry-coupled pressure solution–precipitation (Figure 10). This latter process not only further reduces porosity but also establishes chemical bonding between grains, providing a chemo-mechanical mechanism that is expected to drive long-term fracture densification and reduce hydraulic connectivity, although direct hydraulic verification remains necessary.
The identification of pressure-solution-related processes is partly constrained by the available evidence. SEM observations revealed dissolution pits, blurred grain contacts, and local cementation, which are consistent with stress-assisted dissolution–precipitation at grain contacts. However, these features alone cannot fully distinguish contact-stress-controlled pressure solution from mechanically assisted free-face dissolution followed by precipitation, because dissolved Ca and Si concentrations were not continuously monitored and a complete element mass balance was not performed. Calcite pressure solution is therefore interpreted here as a plausible and important contributor to chemo-mechanical densification, rather than as a quantitatively isolated mechanism. Future experiments incorporating time-resolved solution chemistry and mass-balance constraints are needed to quantify the relative contributions of pressure solution, free-face dissolution, and precipitation. Since the temperature and hydro-chemical conditions in the repository near-field cannot be maintained at these experimental levels over geological timescales, this coupled chemo-mechanical process in natural settings will span a considerably longer duration and is likely more complex.

5.2. Contrasting Mechanical and Chemical Behaviors of Mineral Phases in Mixed Assemblages

The experimental results highlight a fundamental characteristic of mixed mineral assemblages: different mineral phases exhibit markedly contrasting mechanical responses and chemical reactivities under identical thermal–hydraulic–mechanical–chemical conditions [47], generating complex coupling effects through inter-mineral interactions.
Calcite displays pronounced dual mechanical–chemical behavior under the experimental conditions. Mechanically, its low hardness imparts considerable brittle–plastic deformability, enabling it to accommodate volume reduction through grain fracturing, sliding, and rearrangement [48,49]. Chemically, the enhanced solubility of calcite at stress concentration sites is the dominant factor driving pressure solution [50]. These tests have confirmed that pressure solution–precipitation of calcite is the key process driving chemo-mechanical densification of the fracture filling.
In striking contrast, quartz exhibits high rigidity and chemical inertness under the experimental conditions (90 °C, σe ≤ 30 MPa), resisting deformation, dissolution, and precipitation [51]. However, the high stiffness of quartz facilitates stress concentrations at quartz–calcite contacts, thereby elevating the local chemical potential and promoting calcite pressure solution [35,52]. This demonstrates that rigid quartz does not merely play a passive role in limiting compaction; rather, it functions as a local stress-redistribution component. From the perspective of Hertzian contact theory, quartz-bearing contacts are expected to sustain higher local contact pressure than calcite–calcite contacts under the same normal load, owing to the higher reduced elastic modulus of the quartz–calcite pair [53]. This provides a mechanical basis for the preferential calcite dissolution observed at quartz–calcite contacts in the SEM images. Importantly, this local effect does not imply that higher quartz fractions improve bulk sealing performance; the experiments show that increasing quartz content reduces the overall compaction strain and increases final porosity. Quartz should therefore be interpreted as a phase that may locally enhance pressure solution at specific contacts, rather than as a component that generally enhances the bulk sealing performance of the assemblage. No direct contact-stress calculation or finite-element simulation was performed in this study; future work should incorporate Hertzian contact analysis or numerical contact modeling to quantify stress concentration factors at hetero-phase contacts and evaluate their influence on local dissolution kinetics and macroscopic strain evolution.
Clay minerals play a distinctive role in the mixed filling system. Their layered structure and low shear strength enable them to act as a lubricant for crystalline grain rearrangement, accelerating volume reduction by filling intergranular voids and promoting grain sliding [34]. Additionally, given that the clay fraction is dominated by smectite, swelling-assisted micropore filling may serve as a plausible auxiliary mechanism under solution-saturated conditions. However, the present experimental design cannot distinguish swelling-induced pore filling from mechanically driven clay redistribution, and therefore the contribution of smectite swelling is discussed as a complementary hypothesis rather than a directly verified process. Within the tested range (10–20 wt% clay), the data demonstrate that 20 wt% clay addition yields the lowest final porosity; this result should not be interpreted as a quantitatively determined optimum or critical clay threshold. An exploratory 30 wt% clay test suggested possible redistribution or instability of excessive clay under solution-saturated conditions, but this observation should be treated only as qualitative evidence. The threshold at which clay mobilization becomes detrimental remains to be determined by future experiments incorporating higher clay fractions and direct monitoring of clay redistribution [23].
In summary, the chemo-mechanical densification of mixed-mineral-filled fractures in Beishan granite is primarily governed by the mechanical compaction and pressure solution–precipitation of calcite. Notably, stress transfer involving quartz and lubrication and pore-filling by clay minerals also play positive roles. The dynamic coupling of these mechanisms in time and space collectively determines the sealing behavior and its long-term evolution.

5.3. Implications for Long-Term Fracture Sealing in HLW Repositories

The experimental results and mechanistic insights gained from this study provide several implications for evaluating the long-term sealing of granite fractures in the near-field of deep geological repositories. An important insight is that stress-assisted pressure solution–precipitation of calcite in saturated CaCO3 solution is a key process contributing to fracture-filling densification under favorable conditions. However, the sealing process in granite fractures is likely to be considerably slower than that in claystone fractures. Previous studies have shown that fractures in claystone can achieve a substantial reduction in permeability within months to years under THM conditions [33], benefiting from the inherent plasticity and swelling capacity of clay minerals that enable efficient fracture filling [54,55]. In contrast, granite fracture fillings are dominated by crystalline minerals with limited plasticity; their densification relies on grain fracturing, rearrangement, and localized pressure solution–precipitation, with the rate governed by temperature, pressure, and solution chemistry. This implies that once the groundwater chemistry in the repository near-field deviates from conditions favorable for pressure solution (e.g., CaCO3 saturation), the fracture sealing efficiency is likely to be substantially reduced. Therefore, constraining the long-term evolution of fluid chemistry may become a key factor in predicting fracture sealing performance during the safety assessment of HLW disposal.
The influence of mineral heterogeneity on fracture sealing is not determined by a single factor but is instead governed by mechanical–chemical coupling. This study demonstrates that the relative proportions of different mineral phases are a key factor controlling the chemo-mechanical densification behavior in polymineralic systems. An excessive rigid component reduces overall compaction, an insufficient reactive component limits the contribution of pressure solution–precipitation, and the lubrication and pore-filling effects of clay minerals likewise exhibit an optimal range. This balance of proportions also regulates local stress transfer and solute transport pathways, which is crucial for determining the effectiveness of long-term fracture sealing. This insight suggests that the mineral assemblage of natural fracture fillings should be incorporated into repository safety assessment frameworks. Predictive models of fracture sealing should also incorporate mesoscale parameters such as the mass ratio and spatial distribution of filling minerals to improve the reliability of long-term fracture evolution predictions.
At the engineering application level, these findings offer guidance for the design of fracture sealing materials. An ideal natural sealing material should possess the following characteristics: (1) the mineral constituents are those naturally occurring in host rock fractures, thereby avoiding drastic alterations to the hydro-chemical environment caused by foreign materials, which is critical for radionuclide migration and waste container corrosion; (2) compressible and chemically reactive crystalline minerals (e.g., calcite) serve as the main body to ensure the dominant contributions of mechanical compaction and mineral precipitation, with a moderate addition of rigid components (e.g., quartz) to locally accelerate pressure solution through stress transfer; and (3) clay minerals are added in controlled proportions to exploit their lubrication and void-filling functions while avoiding the risk of loss under fluid flow.
Several limitations of the present study must be explicitly recognized when extending the results to repository-relevant conditions. First, the experiments were conducted at fixed temperature (90 °C) and effective stress (15 MPa), whereas the repository near-field will experience transient thermal decay, evolving stress redistribution, and spatially heterogeneous groundwater chemistry. The uniaxial loading geometry, adopted for compatibility with the custom-designed pressure cell, does not reproduce the true triaxial in situ stress state, and the absence of lateral confinement may overestimate compaction efficiency relative to in situ conditions. Second, the 30 MPa dry pre-compaction step was used as a standardization procedure rather than a geological analogue; although this enabled controlled comparison among different mineral assemblages, it may have produced a denser initial fabric than natural fracture fillings. Third, the use of saturated CaCO3 solution represents a favorable chemical boundary condition, selected to enhance the observability of calcite dissolution–precipitation within a laboratory timescale. Actual Beishan groundwater contains multiple dissolved ions, and species such as Mg2+ and HPO42− may inhibit calcite pressure solution or modify precipitation pathways [24,26,30]; the compaction strain and porosity reduction obtained in this study should therefore be interpreted as favorable-case values.
Overall, the present experiments should be regarded as a controlled laboratory investigation of short-term compaction and stress-assisted dissolution–precipitation in mixed mineral assemblages. They provide mechanistic constraints for future long-term evaluation, but direct prediction of repository-scale hydraulic sealing requires permeability monitoring, longer-duration experiments, realistic groundwater chemistry, triaxial or laterally confined loading, in situ validation at the Beishan Underground Research Laboratory (URL), and coupled THMC/reactive-transport modeling [56,57,58].

6. Conclusions

This study systematically elucidates the chemo-mechanical densification processes of mixed-mineral-filled granite fractures under repository near-field conditions through a series of controlled laboratory tests. The results demonstrate that fracture-filling densification is a multi-stage process transitioning from mechanical compaction to chemo-mechanical sealing. Under the favorable chemical condition of saturated CaCO3 solution, calcite undergoes stress-assisted dissolution–precipitation, contributing to porosity reduction and intergranular cementation. This process provides a key chemo-mechanical mechanism for the progressive densification of mineral-filled fractures. More importantly, mineral heterogeneity does not necessarily inhibit fracture-filling densification; rather, interactions among mixed mineral phases may enhance porosity reduction and local cementation under favorable conditions. The high compressibility and dissolution–precipitation reactivity of calcite play a dominant role. Rigid quartz acts as a local stress-redistribution component that promotes preferential deformation or dissolution of adjacent calcite at quartz–calcite contacts, although excessive quartz reduces the overall compaction efficiency. Within the tested range, moderate clay addition promotes grain-interface lubrication and micropore filling, with the 20 wt% clay-bearing assemblage exhibiting the lowest final porosity. However, the optimum clay content and the possible contribution of smectite swelling require further systematic verification.
These findings provide experimental constraints and mechanistic insights for the long-term evaluation of mineral-filled fracture evolution, but direct prediction of repository-scale hydraulic sealing requires further permeability measurements, in situ validation, and coupled THMC modeling. The compaction strain and porosity reduction obtained under saturated CaCO3 solution should be interpreted as favorable-case values; systematic verification under natural Beishan groundwater chemistry is needed. Future work will include in situ experiments at the Beishan URL, combined with numerical modeling of multiple mineral phase interactions, to reliably extrapolate laboratory-derived mechanistic understanding to the assessment of long-term sealing performance at the engineering scale.

Author Contributions

X.T.: Supervision, Writing—Review & Editing, Resources, Conceptualization, Methodology. J.W.: Project administration, Supervision, Resources, Conceptualization, Methodology. J.-W.W.: Methodology, Validation, Formal analysis. J.-L.X.: Data Curation, Investigation. Z.-C.Z.: Data Curation, Resources. K.L.: Formal analysis, Investigation. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the China Atomic Energy Authority (CAEA) through the Geological Disposal Program (FZ2101), CAEA Innovation Center for Geological Disposal of High-Level Radioactive Waste (CXJJ2110).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors have reviewed and edited the output and take full responsibility for the content of this publication.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
HLWHigh-level Radioactive Waste
THMCThermal–Hydraulic–Mechanical–Chemical
URLUnderground Research Laboratory
DGRDeep Geological Repository

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Figure 1. (a) Granite column sample with a fracture, and (b) the rough fracture surface.
Figure 1. (a) Granite column sample with a fracture, and (b) the rough fracture surface.
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Figure 2. Schematic illustration of the experimental apparatus. (a) pressure loading head, (b) high-pressure isolation vessel, (c) osmotic pressure–volume controller, (d) constant-temperature immersion circulator, (e) axial load cell, (f) pore pressure transducer, (g) displacement transducer, (h) thermocouple, (i) grooved metal spacer, (j) sealing ring, (k) ball valve, (l) granite column, (m) fracture-filling minerals, (n) pore solution, (o) dimethyl silicone oil, (p) uniaxial pressure cell.
Figure 2. Schematic illustration of the experimental apparatus. (a) pressure loading head, (b) high-pressure isolation vessel, (c) osmotic pressure–volume controller, (d) constant-temperature immersion circulator, (e) axial load cell, (f) pore pressure transducer, (g) displacement transducer, (h) thermocouple, (i) grooved metal spacer, (j) sealing ring, (k) ball valve, (l) granite column, (m) fracture-filling minerals, (n) pore solution, (o) dimethyl silicone oil, (p) uniaxial pressure cell.
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Figure 3. Evolution of compaction strain of the fracture-filling layer. L0 represents the initial fracture thickness before testing (a). ΔL1/L0 represents the vertical strain after pre-compaction (b). ΔL2/L0 represents the vertical strain at the end of the test (c). Axial stress (σa) is maintained constant throughout the test.
Figure 3. Evolution of compaction strain of the fracture-filling layer. L0 represents the initial fracture thickness before testing (a). ΔL1/L0 represents the vertical strain after pre-compaction (b). ΔL2/L0 represents the vertical strain at the end of the test (c). Axial stress (σa) is maintained constant throughout the test.
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Figure 4. Temporal evolution of compaction strain and calculated porosity of fine-grained calcite samples under saturated CaCO3 solution, deionized water, and dry conditions. (a) The entire test duration; (b) the early stage (0–4 h). Pre-compaction under dry conditions begins at point 1; axial stress is reduced to 6–8 MPa and pore solution is injected at point 2; axial stress is increased to the target value at point 3. Φprec denotes the porosity after pre-compaction; Φet denotes the final porosity at the end of the test.
Figure 4. Temporal evolution of compaction strain and calculated porosity of fine-grained calcite samples under saturated CaCO3 solution, deionized water, and dry conditions. (a) The entire test duration; (b) the early stage (0–4 h). Pre-compaction under dry conditions begins at point 1; axial stress is reduced to 6–8 MPa and pore solution is injected at point 2; axial stress is increased to the target value at point 3. Φprec denotes the porosity after pre-compaction; Φet denotes the final porosity at the end of the test.
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Figure 5. Pore distribution in fracture-filling minerals analyzed from BSE images. The porosity of the fill in sample LXFD08 after pre-compaction (a) and at the end of the test (b) is 15.5% and 4.8%, respectively. Red pixels represent pores; the unified threshold range in ImageJ is 0–100.
Figure 5. Pore distribution in fracture-filling minerals analyzed from BSE images. The porosity of the fill in sample LXFD08 after pre-compaction (a) and at the end of the test (b) is 15.5% and 4.8%, respectively. Red pixels represent pores; the unified threshold range in ImageJ is 0–100.
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Figure 6. Microstructural evidence of mechanical deformation, dissolution, and re-precipitation of minerals. (a) Dissolution pits and microcracks on a calcite crystal surface, sample LXFD08, BSE image. (b,c) Clusters of fine-grained (1–3 μm) neoformed calcite crystallites precipitated on the sample surface, BSE images. (d) Mechanical fragmentation of calcite particles under dry conditions, sample LXFD14, BSE image. (e) Pressure solution at calcite grain contacts resulting in blurred contact boundaries and local cementation (white arrows), BSE image. (f) A fracture in a calcite crystal with neoformed acicular crystals (white arrows), SEI image. (g,h) Fragmentation and pressure solution at calcite–quartz contacts due to stress concentration (white arrows), sample LXFD12, BSE images. (i) Serrated calcite–quartz contact morphology, with calcite partially crushed into microcrystals filling the contact gap, BSE image.
Figure 6. Microstructural evidence of mechanical deformation, dissolution, and re-precipitation of minerals. (a) Dissolution pits and microcracks on a calcite crystal surface, sample LXFD08, BSE image. (b,c) Clusters of fine-grained (1–3 μm) neoformed calcite crystallites precipitated on the sample surface, BSE images. (d) Mechanical fragmentation of calcite particles under dry conditions, sample LXFD14, BSE image. (e) Pressure solution at calcite grain contacts resulting in blurred contact boundaries and local cementation (white arrows), BSE image. (f) A fracture in a calcite crystal with neoformed acicular crystals (white arrows), SEI image. (g,h) Fragmentation and pressure solution at calcite–quartz contacts due to stress concentration (white arrows), sample LXFD12, BSE images. (i) Serrated calcite–quartz contact morphology, with calcite partially crushed into microcrystals filling the contact gap, BSE image.
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Figure 7. Porosity evolution and post-test grain size distribution of calcite samples with different initial grain sizes. (a) Porosity evolution over time calculated from compaction strain. (b,d) Grain size distribution of the fill after testing in sample LXFD09 and LXFD08. N—number of mineral particles; dmax and daver—maximum and average grain size within the statistical range; σ—standard deviation reflecting the degree of size dispersion. (c,e) Pore distribution analyzed from BSE images, with porosities of 7.3% and 4.8%, respectively.
Figure 7. Porosity evolution and post-test grain size distribution of calcite samples with different initial grain sizes. (a) Porosity evolution over time calculated from compaction strain. (b,d) Grain size distribution of the fill after testing in sample LXFD09 and LXFD08. N—number of mineral particles; dmax and daver—maximum and average grain size within the statistical range; σ—standard deviation reflecting the degree of size dispersion. (c,e) Pore distribution analyzed from BSE images, with porosities of 7.3% and 4.8%, respectively.
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Figure 8. (a) Temporal evolution of compaction strain and porosity for mixed calcite–quartz samples; (b) variation of final strain and porosity as a function of quartz mass fraction.
Figure 8. (a) Temporal evolution of compaction strain and porosity for mixed calcite–quartz samples; (b) variation of final strain and porosity as a function of quartz mass fraction.
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Figure 9. (a) Porosity evolution over time for mixed mineral assemblages containing clay minerals; (b–d) microstructural evolution of sample LXFD22 at different test stages.
Figure 9. (a) Porosity evolution over time for mixed mineral assemblages containing clay minerals; (b–d) microstructural evolution of sample LXFD22 at different test stages.
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Figure 10. Conceptual model of the multi-stage deformation process of mixed mineral assemblages in granite fractures under DGR near-field conditions. The horizontal axis represents relative process progression rather than quantitative geological time.
Figure 10. Conceptual model of the multi-stage deformation process of mixed mineral assemblages in granite fractures under DGR near-field conditions. The horizontal axis represents relative process progression rather than quantitative geological time.
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Table 1. Summary of test conditions and results.
Table 1. Summary of test conditions and results.
Test No.Duration
(h)		Axial Pressure
(kN)		Fracture Surface Area
(mm2)		Pore SolutionInitially Added Fracture-Filling MineralsCompaction Strain (ΔL/L0)Porosity (Φ)
Thickness
(mm)		Mass
(g)		Mineral Components:
Average Grain Size, Mass Ratio		ApcPtΔSApcPtΔΦ
LXFD08294.613.2660.2CaCO30.70.90CAL: 75 μm, 100%15.1%24.6%+9.5%15.5%4.8%−10.7%
LXFD05270.614.3716.0CaCO30.70.98CAL: 75 μm, 80%
QTZ: 75 μm, 10%
CLAY: 10%		13.5%23.3%+9.8%14.0%3.0%−11.0%
LXFD09286.311.4572.3CaCO30.50.49CAL: 150 μm, 100%21.5%31.9%+10.4%19.6%7.3%−12.3%
LXFD11269.413.2660.2CaCO30.70.93CAL: 75 μm, 80%
CLAY: 20%		13.1%22.8%+9.7%13.0%2.1%−10.9%
LXFD12284.313.2660.2CaCO30.30.89CAL: 75 μm, 78%
QTZ: 75 μm, 22%		12.5%21.0%+8.5%17.4%8.5%−8.9%
LXFD15277.716.6829.2CaCO30.30.48CAL: 75 μm, 49%
QTZ: 75 μm, 51%		10.1%15.5%+5.4%19.9%14.8%−5.1%
LXFD21264.018.2907.5CaCO30.71.02CAL: 250 μm, 100%23.8%34.1%+10.3%22.3%10.2%−12.1%
LXFD22255.817.6881.0CaCO30.81.04CAL: 75 μm, 60%
QTZ: 75 μm, 20%
CLAY: 20%		10.6%16.5%+5.9%15.7%9.8%−5.9%
LXFD23268.914.4720.7CaCO30.40.92CAL: 250 μm, 50%
CAL: 75 μm, 50%		16.9%29.1%+12.2%20.0%6.2%−13.8%
LXFD24269.213.2660.2CaCO30.81.03CAL: 75 μm, 20%
QTZ: 75 μm, 80%		7.2%10.4%+3.2%22.1%19.4%−2.7%
LXFD26289.412.9646.6D-water0.80.97CAL: 75 μm, 100%14.3%20.6%+6.3%19.4%13.0%−6.4%
LXFD14285.09.9660.2/0.40.90CAL: 75 μm, 100%11.4%14.8%+3.4%14.9%11.5%−3.4%
Note: The initial effective stress (σe) of all tests is 15 MPa, the temperature is 90 °C. CaCO3—artificially prepared saturated calcium carbonate solution; D-water—deionized water; CAL—calcite; QTZ—quartz; CLAY: clay mineral. Apc—after pre-compaction; Pt—post-test; ΔS—compaction strain variation; ΔΦ—porosity variation. Fracture surface area was measured individually from each fractured cross-section, and the axial load was adjusted accordingly to maintain a uniform stress condition.
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MDPI and ACS Style

Tian, X.; Wang, J.; Wang, J.-W.; Xie, J.-L.; Zhou, Z.-C.; Li, K. Compaction and Pressure Solution of Mixed Mineral Assemblages: Implications for Granite Fracture Sealing in the Near-Field of High-Level Radioactive Waste Repository. Minerals 2026, 16, 603. https://doi.org/10.3390/min16060603

AMA Style

Tian X, Wang J, Wang J-W, Xie J-L, Zhou Z-C, Li K. Compaction and Pressure Solution of Mixed Mineral Assemblages: Implications for Granite Fracture Sealing in the Near-Field of High-Level Radioactive Waste Repository. Minerals. 2026; 16(6):603. https://doi.org/10.3390/min16060603

Chicago/Turabian Style

Tian, Xiao, Ju Wang, Jia-Wei Wang, Jing-Li Xie, Zhi-Chao Zhou, and Ke Li. 2026. "Compaction and Pressure Solution of Mixed Mineral Assemblages: Implications for Granite Fracture Sealing in the Near-Field of High-Level Radioactive Waste Repository" Minerals 16, no. 6: 603. https://doi.org/10.3390/min16060603

APA Style

Tian, X., Wang, J., Wang, J.-W., Xie, J.-L., Zhou, Z.-C., & Li, K. (2026). Compaction and Pressure Solution of Mixed Mineral Assemblages: Implications for Granite Fracture Sealing in the Near-Field of High-Level Radioactive Waste Repository. Minerals, 16(6), 603. https://doi.org/10.3390/min16060603

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