A Leading Thermodynamic Dissipation Framework for Crystallization-Induced Deformation in Unsaturated Sulfate-Saline Soils

Sulfate-saline soils are typical of arid and semi-arid regions, and their behaviour is closely tied to the local climate rather than to any single soil property. Low rainfall limits leaching, and evaporation steadily concentrates dissolved salts in the pore water. For long periods, this accumulation can remain mechanically inconsequential. Problems tend to emerge when temperature begins to vary, especially during cooling. As sulfate solubility decreases, crystallization becomes unavoidable and once that happens, the response of the soil is no longer dictated by fluid flow or thermal diffusion. Crystals begin to occupy pore space, interfere with grain contacts, and introduce stresses that were not present before. The surface manifestations—uplift, cracking, or gradual distortion of structures are usually delayed, which makes the underlying cause easy to underestimate. The challenge is that this process does not follow a simple sequence. Temperature changes influence water movement, but water redistribution also determines where salts concentrate and where crystals form first. Crystallization pressure develops locally, but its effects depend on how constrained the pore network happens to be at that moment. In unsaturated soils, the situation is even less straightforward. Liquid water, vapor, solid particles, and salt crystals all coexist, and none of them can be treated as passive. Their interactions evolve continuously, often far from equilibrium, especially during transient cooling.

Most models addressed this complexity by assembling Darcy-type flow for water, Fourier-based heat transfer, and conventional stress–strain relations which are limited because they tend to smooth out irreversible effects and assume that coupling between fields remains mild. However, in practice, crystallization is dissipative. Latent heat is released, pore geometry changes irreversibly, and energy is lost in ways that cannot be recovered by reversing temperature alone. When these aspects are simplified or embedded indirectly into parameters, predictive accuracy suffers. This becomes most apparent under strong thermal gradients or during rapid crystallization, precisely when salt-induced damage accelerates and engineering risk increases. To this end, new research paper published in Computers and Geotechnics and led by Professor Bing Bai from the Beijing Jiaotong University, the researchers inaugurated a thermodynamically consistent multi-field model that couples heat transfer, fluid migration, mechanical deformation, and sulfate crystallization in unsaturated soils. The new model quantitatively links microscale phase transitions to macroscopic strain and pressure evolution by embedding crystallization kinetics and latent heat effects within a dissipation-based framework. Their experimental validation confirmed its ability to reproduce cooling-induced salt expansion with high fidelity. This work is part of the significant innovative idea developed by the first author Bing Bai in recent years to establish a generalized constitutive model for granular materials.

The research team conducted large-scale one-dimensional salt expansion tests on unsaturated sulfate-saline soil specimens subjected to staged cooling. They embedded temperature sensors along the sample height captured spatial and temporal thermal evolution, while displacement gauges continuously recorded deformation throughout the cooling process. They applied cooling incrementally with each temperature step maintained long enough to allow thermal redistribution and crystallization to progress toward quasi-steady conditions. The authors found as temperature declined, sulfate solubility decreased, and induced progressive precipitation of sodium sulfate hydrates within the pore space. This crystallization was accompanied by measurable soil expansion, which unfolded in distinct stages corresponding to changes in the imposed temperature gradient. The team performed numerical simulations which reproduced experimental trends and the model captured the delayed cooling response within the soil interior, which highlighted the role of heat conduction and the moderating influence of latent heat released during crystallization. They observed in regions closer to the cooling boundary to have faster temperature decline and earlier onset of crystallization, whereas the interior experienced a more gradual transition. This spatial variability translated directly into differences in crystallization rate and final crystal content, revealing that slower cooling can, paradoxically, lead to more complete crystallization due to sustained supersaturation over longer periods. Moreover, simulated phase volume fractions showed a systematic decrease in liquid content and saturation as crystallization proceeded, accompanied by an increase in gas phase volume driven by evaporation and gas–liquid exchange. The crystalline phase exhibited the most pronounced growth, progressively occupying pore space and altering porosity distribution along the sample height.  Furthermore, they found the evolution of pore water pressure reflected a balance between crystallization-induced compaction and subsequent soil expansion and near the cooling boundary, pore pressure initially increased as pore connectivity diminished, then gradually decreased as volumetric expansion relieved internal stresses. Additionally, they observed in deeper regions, slower crystallization allowed capillary effects and matrix suction to dominate, producing distinct pressure trajectories. Throughout these processes, the simulated volumetric strain closely matched experimental measurements, which demonstrate that the model successfully linked microscale phase transitions to macroscopic deformation.

In conclusion, the work of Professor Bing Bai and colleagues a physically grounded pathway for predicting and mitigating crystallization-driven damage in sulfate-saline ground systems. The study by Professor Bing Bai matters because it delivers a thermodynamically consistent THMC coupling framework. The new proposed model moves beyond traditional equilibrium-based descriptions and provides a coherent mechanism for linking energy transfer, phase transition kinetics, and mechanical response. It addresses a long-standing gap between observed salt-induced deformation and the assumptions embedded in many existing models. We believe one of the most important implications is the ability to quantify sulfate crystallization amount directly as a function of temperature and moisture conditions which can enable better estimation of crystallization pressure and its contribution to volumetric expansion. The inclusion of latent heat feedback also demonstrate how crystallization can locally buffer temperature changes and modify subsequent transport processes and these feedbacks are important and relevant in natural settings where cooling occurs gradually and spatially non-uniformly. It is noteworthy to mention, the new findings highlight the sensitivity of salt expansion to cooling paths, thermal conductivity, and initial salt content. Professor Bing Bai and colleagues showed stepwise cooling to moderate crystallization pressure fluctuations and reduce overall deformation, which point to practical strategies for construction or thermal management in saline soil regions. Similarly, the strong influence of initial salt content relative to porosity highlights the importance of chemical characterization during site investigation, as small differences in sulfate concentration can lead to markedly different deformation outcomes. The new model also provides a framework to evaluate mitigation measures. Adjustments to soil composition, compaction level, or thermal boundary conditions can be assessed within the unified THMC model which can allow engineers to explore trade-offs between stability, construction feasibility, and long-term performance. Beyond sulfate-saline soils, the thermodynamic approach adopted by the authors has broader relevance for other geomaterials where phase transitions, crystallization, or chemical reactions interact with mechanical behavior. In a nutshell, this scientific contribution to generalized constitutive model for granular materials is world-class work. This new leading study contributes to the ongoing effort to integrate micro-scale processes into continuum descriptions of soil behavior and can be considered a foundation for future extensions to freeze–thaw cycles, multi-salt systems, and coupled environmental loading scenarios.