在核废料深地处置和垃圾填埋场工程中,作为缓冲和防渗层的压实膨润土与地下水盐溶液及垃圾渗滤液相互作用可诱发膨润土层间阳离子发生改变,进而导致其膨胀性和密封性能演变。为了分析层间阳离子对压实膨润土宏观膨胀特性的影响规律和机制,对不同阳离子基膨润土开展膨胀变形和膨胀力试验,并进行X射线衍射和扫描电镜观测。膨胀试验结果表明:不同阳离子基膨润土的膨胀力呈现出Mg基土>Ca基土>Na基土>K基土的规律,而膨胀变形则表现为Na基土>K基土>Ca基土≈Mg基土;不同阳离子基膨润土的膨胀力与膨胀应变的发展速率与浸水时间之间呈现良好的线性变化规律。在体积受限的膨胀力试验中,Ca基与Mg基膨润土的层间膨胀(层间距)更大,而在无约束的膨胀变形过程,Na基膨润土的渗透膨胀更强烈。层间阳离子的水化能及其与晶层间的相互作用控制着压实膨润土吸水膨胀过程中微观结构演变和宏观膨胀势的显现。试验研究结果可为相关工程设计及安全评估提供参考。

[Objective] Compacted bentonite is a critical buffer material for the deep geological disposal of nuclear waste and landfill engineering. Its swelling characteristics are directly affected by cation exchange with groundwater or landfill leachate. While existing research has clarified cation-related hydration and swelling mechanisms in bentonite powder or via molecular dynamics simulations, studies on the influence of interlayer cations on compacted samples remain limited. This study aims to fill this gap by investigating the influence of interlayer cations on the macroscopic swelling behavior of compacted bentonite, thereby providing technical support for engineering design and safety assessment. [Methods] The test material was bentonite from Shouguang, Shandong Province, containing a montmorillonite content exceeding 95% and exhibiting excellent swelling and adsorption properties. Four types of homoionic bentonite (Na-based, K-based, Ca-based, Mg-based) were prepared via cation exchange by mixing bentonite with 1 mol/L solutions of NaCl, KCl, CaCl₂, or MgCl₂. The samples were compacted to a dry density of 1.1 g/cm³ with an initial moisture content of 20%. A self-developed integrated consolidation-swelling device was used for volume-constrained swelling pressure tests and unconfined swelling strain tests. Complementary characterizations included X-ray diffraction (XRD) to measure interlayer spacing (d001) and scanning electron microscopy (SEM) to observe microstructural changes. [Results] The final swelling pressure of the four types of homoionic bentonite followed a distinct hierarchy: Mg-based (5.98 MPa) > Ca-based (5.79 MPa) > Na-based (4.49 MPa) > K-based (3.61 MPa). Swelling pressure curves exhibited a “rise-decline-stabilization” trend: rapid growth occurred within 0-2 h, with Mg/Ca-based samples peaking at 6.21 MPa and 5.18 MPa, respectively, followed by a slight decline due to aggregate splitting and pore filling, and finally stabilizing after 18 h. Na/K-based samples stabilized after 22 h without an early decline. The ratio of time to swelling pressure showed a strong linear correlation with time. This correlation enabled the prediction of the maximum swelling pressure, and the predicted values were highly consistent with the measured values. For swelling strain, the order was Na-based (72.7%) > K-based (51.5%) > Ca-based (47.0%) ≈ Mg-based (48.7%). Swelling strain curves exhibited three stages: slow growth (0-0.06 h, clay particle hydration), rapid growth (0.06-10 h, dominant interlayer swelling), and stabilization. The swelling strain of Mg/Ca-based samples stabilized faster than that of Na/K-based ones. The ratio of time to swelling strain also showed excellent linearity with time, and the reciprocal of slope could be used to predict the maximum strain. XRD results showed that after swelling pressure tests, Mg/Ca-based samples had larger interlayer spacings (18.62 Å/18.10 Å, 3 layers of interlayer water) than Na/K-based ones (15.63 Å/15.39 Å, 2 layers). After swelling strain tests, Na/K-based samples adsorbed over 4 layers of water, resulting in larger spacings (23.68 Å/20.98 Å). SEM images revealed that Na/K-based samples had thin, curled lamellar aggregates with uniform pores, while Ca/Mg-based samples had thicker aggregates and more macropores. [Conclusion] Interlayer cations regulate the macroscopic swelling behavior of compacted bentonite through their hydration energy and interaction with the crystal layers. Divalent cations (Ca²⁺ and Mg²⁺) have higher hydration energy than monovalent cations (Na⁺ and K⁺), enabling stronger interlayer swelling under volume-constrained conditions and consequently resulting in higher swelling pressure. The minimum swelling pressure recorded for K-based bentonite stems from the matching of its ionic radius with the montmorillonite crystal cavity, which promotes the formation of stable interlayer structures. Under unconfined conditions, monovalent cations (Na⁺ and K⁺) have weaker interaction with the crystal layers, allowing more interlayer water adsorption and intense osmotic expansion. This accounts for the larger swelling strain of Na- and K- based samples. The development rates of swelling pressure and strain have a good linear relationship with immersion time, and the proposed empirical formulas provide a reliable tool for predicting maximum swelling characteristics. Microstructural evolution (aggregate splitting and pore distribution) is closely linked to interlayer cation type, which further determines macroscopic performance. The findings of this study highlight the significant effect of interlayer cations and emphasize the necessity of considering interlayer cation composition of bentonite-based barriers. The differences in the macroscopic swelling pressure and swelling strain of different cationic bentonites should be given high priority in relevant engineering design and evaluation.