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Review

The Effect of Clay Plasticity on Thermally Induced Volume Change of Saturated Clay: A State-of-the-Art Review

by
Dinitha Vidurapriya
* and
Hossam Abuel-Naga
School of Computing, Engineering and Mathematical Sciences, La Trobe University, Melbourne, VIC 3086, Australia
*
Author to whom correspondence should be addressed.
Minerals 2026, 16(3), 303; https://doi.org/10.3390/min16030303
Submission received: 3 February 2026 / Revised: 3 March 2026 / Accepted: 11 March 2026 / Published: 13 March 2026
(This article belongs to the Section Clays and Engineered Mineral Materials)
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Abstract

This review comprehensively examines the influence of clay plasticity on thermally induced volume changes in saturated clays, which is a critical factor in the design and performance of energy geostructures, nuclear waste repositories, and thermal ground improvement systems. This study synthesises experimental and theoretical findings, demonstrating that the plasticity index and mineralogical composition significantly govern the magnitude and nature of volume change during heating and cooling cycles, with stress history playing a pivotal role. Unlike previous review papers that primarily discuss general thermo-mechanical behaviour or constitutive modelling frameworks, this review explicitly focuses on plasticity as the central unifying parameter influencing thermally induced volume change. It further provides a structured synthesis that integrates plasticity, stress history, and microstructural mechanisms. Normally consolidated clays exhibit irreversible thermal contraction, which intensifies with plasticity, whereas highly overconsolidated clays typically exhibit reversible expansion. Lightly overconsolidated clays exhibit transitional behaviour characterised by initial expansion followed by collapse. This review links these macroscopic responses to microstructural mechanisms, including interparticle physicochemical forces, diffuse double-layer dynamics, and bound water behaviour, highlighting the limitations of idealised electrochemical models and emphasising the importance of micromechanical processes. It further explores how plasticity modulates temperature-dependent reductions in preconsolidation pressure, thermal softening, cyclic thermal deformation, and time-dependent thermal creep, with higher plasticity clays showing greater sensitivity and longer stabilisation periods. The findings underscore the necessity of incorporating plasticity and stress history into constitutive models to accurately predict the thermo-mechanical behaviour of clays under service conditions, with significant implications for the long-term reliability of thermal geotechnical applications.

1. Introduction

Thermal geomechanics has become a focus of geotechnical research owing to the dual need for sustainable energy systems and safe underground disposal of hazardous waste. Applications such as geothermal energy extraction, underground thermal energy storage, thermal consolidation for ground improvement, and nuclear waste repositories expose fine-grained soils to significant temperature variations [1,2,3,4]. Because clay plays a key role in these applications, especially as an engineered barrier, it is essential to understand and accurately model its thermomechanical behaviour [2,5,6].
Foundational studies have established that temperature alters the consolidation and deformation characteristics of clays [7,8]. Subsequent temperature-controlled oedometer and triaxial investigations confirmed that thermally induced volume changes depend strongly on stress history, particularly the overconsolidation ratio (OCR) [1,3,9,10,11,12,13,14]. Beyond the stress history, intrinsic material properties, most notably clay plasticity, govern the magnitude of the thermal volume change and remain underrepresented in thermally induced volumetric responses [15,16,17,18].
Several recent review papers have summarised aspects of thermally induced volumetric behaviour in fine-grained soils, but they do not isolate plasticity as a primary factor governing thermally induced volume change. Hoseinimighani and Szendefy [19] provide a critical review of mechanisms for thermally induced volume changes and discuss roles of microstructure and physicochemical interactions, yet plasticity is not treated as a central explanatory variable. Similarly, Saggu, Chakraborty, Roy and Basu [20] review the broader thermos-mechanical behaviour of saturated soils at elevated temperatures, emphasising stress–strain response and irreversible contraction, but without systematically evaluating how plasticity index controls thermal contraction, expansion, or reversibility across clay types. Reviews in related fields discuss thermal effects in clay-rich materials for various uses, such as clay minerals in fired brick manufacturing [21] and thermal treatment for stabilising expansive soils [22]. In these works, changes in plasticity are primarily treated as processing effects rather than as key factors driving fundamental thermo-mechanical volume changes in saturated clays.
Other reviews address plasticity indirectly through mineralogical variability, shrink-swell behaviour, or stabilisation of low- and high-plasticity clays [23,24], but the emphasis is on expansive behaviour under environmental actions or on improvement methods rather than on a consolidated synthesis of plasticity-controlled thermally induced volume change under controlled thermal–mechanical testing conditions. Advances in experimental apparatus, such as the development of temperature-controlled oedometers, have been reviewed or documented [25,26]. However, these contributions mainly emphasise instrumentation and testing capabilities rather than incorporating plasticity into a mechanistic understanding of thermally induced volume changes. Therefore, despite many reviews available, no recent synthesis has systematically examined how clay plasticity influences thermally induced volume changes across different consolidation states and thermal loading paths. This highlights the necessity for the current review. This state-of-the-art review examines how clay plasticity influences thermally induced volume changes by linking micro-scale mechanisms, such as diffuse double-layer effects, adsorbed-water dynamics, and fabric rearrangement, to macro-scale responses across consolidation states and under cyclic thermal loading. The reviewed experimental evidence spans temperatures from 20 to 90 °C and overconsolidation ratios up to 12. While earlier reviews have primarily synthesised thermal behaviour in terms of stress-history effects or constitutive modelling approaches, the present review systematically evaluates plasticity as the governing material descriptor and develops a consolidated interpretation of thermal volume-change behaviour across different clay types and consolidation states. The discussion then considers temperature-dependent preconsolidation pressure and thermal softening and examines how clay plasticity regulates thermal creep, with an emphasis on long-term, time-dependent deformations under monotonic and cyclic thermal loading.
A structured literature search was conducted in Scopus and Web of Science, supplemented by Google Scholar and manual screening of reference lists, to identify studies on thermally induced volume change of clayey soils. Searches covered 1990–2025 using combinations of terms related to temperature loading, clays, plasticity index, and volume change, together with test keywords. Studies were included if they reported temperature-controlled mechanical testing with relevant volume change outcomes and provided PI; studies without clay-dominated materials or relevant outcomes were excluded. Records were screened by title and abstract, followed by full text review, with additional papers identified via citation tracking.

2. Background on Thermally Induced Volume Change in Clays

The thermo-mechanical behaviour of clay has received special attention from geotechnical specialists over the last two decades, mainly due to the potential application of clayey soils, such as bentonite, as barrier materials in nuclear waste disposal facilities [2,5,6,13,27,28]. A thorough understanding of clay behaviour at elevated temperatures, up to approximately 100 °C and sometimes up to 200 °C, is therefore essential [1,2,29,30].
Early pioneering theories by Campanella and Mitchell [7] and Paaswell [8] showcased the influence of temperature variations on soil thermal behaviour [31,32]. Subsequent studies have consistently demonstrated that increased temperatures of saturated clays affect their volume change, shear strength, stiffness, and hydraulic behaviour [6,27,28]. Experimental investigations typically involve modified odometers or temperature-controlled triaxial apparatuses [1,3,7,9,10,11,12,13,14].
The thermally induced volumetric strain (εV) is widely accepted to comprise both elastic (εVe) and plastic (εVp) components [28,33]. While thermo-elastic strains are generally reversible, plastic strains are irreversible owing to thermally induced changes in the fabric [28,33]. The behaviour of these volumetric strains is mainly influenced by the stress history of the soil [13,28].
  • Normally Consolidated (NC) Clays: Upon drained heating, these soils demonstrate irreversible volumetric contraction, often referred to as “Thermal collapse” [1,3,16,28,33,34,35]. This thermal collapse is shown in Figure 1, which summarises the experimental results reported by Di Donna and Laloui [36] using a natural silty clay, together with those reported by Vega and McCartney [37] using Bonny Silt.
  • Highly Overconsolidated (OC) Clays: In contrast, as shown by the experimental results of Abuel-Naga, Bergado, Bouazza and Ramana [5] for natural soft Bangkok clay in Figure 1, highly overconsolidated clays exhibit predominantly reversible thermal expansion. [1,3,16,28,33,34,35,38].
  • Lightly Overconsolidated (OC) Clays: These clays exhibit more complex behaviour, as perfectly demonstrated in Figure 1 by the results of Vega and McCartney [37], often showing an initial thermal expansion followed by a thermal contraction (or collapse) [1,3,35]. Various studies have further noticed that the intrinsic characteristics of the clay heavily affect the overconsolidation ratio (OCR) at which the volumetric strain behaviour transitions from contraction to dilation [39].

3. Impact of Clay Plasticity on Thermally Induced Volume Change

Clay plasticity, commonly expressed by the plasticity index (PI), is a convenient macroscopic indicator of mineralogy and fabric, with high PI generally associated with smectite or illite clays that have high surface area, cation exchange capacity, and stronger bound water effects, and low PI more often linked to kaolinitic or silt-rich materials [17,40]. These mineralogical and surface chemistry differences govern water adsorption and interparticle forces, and therefore strongly influence the magnitude of thermally induced volume change [41]. Across many studies, higher PI clays consistently show larger thermally induced contraction, greater reductions in preconsolidation pressure, and stronger thermal creep under comparable testing conditions [42]. However, PI is an indirect index and cannot uniquely capture differences in mineral species, pore fabric, macropore structure, depositional history, or cementation, meaning clays with similar PI can still behave differently thermally [43]. Accordingly, this review uses PI as a generalised descriptor but explicitly acknowledges its limits and, where possible, ties PI temperature trends back to measured mineralogy, specific surface area, and microstructural evidence to avoid overgeneralisation. Given the variability in materials, Table 1 consolidates the key studies within the scope of this review, summarising the clay type, reported PI range, test method, applied temperature range, and other important experimental details. The studies discussed in the text are not limited to those listed in Table 1; rather, the table highlights the most noteworthy and representative literature used to frame the synthesis.

3.1. Direct Relationship Between Plasticity and Thermal Volume Change

The thermally induced volumetric strain (εV) is strongly dependent on soil plasticity. Early studies have indicated that the plasticity index influences thermal consolidation in normally consolidated clays [44,48]. This behaviour was further quantified by Abuel-Naga, Bergado and Bouazza [16], who reported that soft Bangkok clay (PI ≈ 60) contracted by ~6% for a temperature rise of 65–70 °C, reinforcing the trend of greater thermal contraction with higher plasticity and aligning with the data compiled in Figure 2. More recent results have corroborated this dependence. Abu-Farsakh, Idries and Chen [15] measured εV values of 0.0065, 0.0093, and 0.0298 for low-, medium-, and high-PI clays, respectively, at 70 °C, and noted that these magnitudes were apparently independent of applied normal stress. Similar observations were made for low-plasticity materials by Kirkham, Tsiampousi and Potts [17]. These experimental findings indicate that εV is predominantly controlled by soil plasticity and the mineralogical characteristics that define clay behaviour [15,18,45].

3.2. Microstructural Mechanisms Influenced by Plasticity

The thermal response of clayey soils results from intricate interactions between elastic and plastic mechanisms, which are controlled by mineralogy, pore structure, and physicochemical interactions at multiple scales. Macroscopic observations provided clear evidence of two distinct behavioural regimes. NC clays exhibit irreversible thermal contraction, commonly termed thermal collapse, whereas highly OC clays exhibit reversible thermoelastic expansion [16,31,36]. These contrasting responses suggest that heating activates both reversible and irreversible processes of energy storage and dissipation, facilitated by microstructural and physicochemical pathways [4,33]. The extent and nature of these processes depend strongly on clay plasticity, which is intrinsically linked to the specific surface area, mineral composition, and water retention capacity [31,51].
Thus, plasticity functions as a unifying descriptor connecting mineral-scale interparticle forces to observable macroscopic deformation. It defines the degree of coupling between the thermal, hydraulic, and mechanical effects, determining the sensitivity of the material to thermal variations and the extent to which plastic rearrangement may occur. Understanding how plasticity mediates these mechanisms requires a detailed examination of both idealised physicochemical hypotheses, most notably the Diffused Double Layer (DDL) theory, and micromechanical models that account for bound water and interparticle contact behaviour.

3.2.1. The Physicochemical Debate: Diffused Double Layer (DDL) Theories

Early micromechanical approaches to explain thermally induced volumetric changes in saturated clays drew heavily from the physicochemical interpretation of interparticle forces governed by DDL interactions [7,16]. The DDL describes the electrostatic field surrounding a charged clay particle, in which counterions in the pore fluid balance the surface charges, creating a repulsive force that influences interparticle spacing and, consequently, the bulk volume of the soil. Temperature variations can theoretically alter this force balance by modifying the dielectric permittivity (ε) of water, ionic activity, or thermal energy of the ions [52].
This framework generated two competing hypotheses in the literature regarding the direction of the thermal volumetric change.
  • Contraction Hypothesis (Thermal Softening Model): Several studies have argued that increasing temperature decreases the relative dielectric permittivity of water, directly weakening electrostatic repulsion and reducing the DDL thickness [48,51]. The consequent reduction in interparticle spacing was suggested to result in volumetric contraction, which is consistent with the experimentally observed decreases in preconsolidation pressure upon heating [27,53]. This model has been widely used to explain the thermal softening phenomena observed in NC clays, in which heating induces plastic contraction at a constant stress.
  • Expansion Hypothesis (Thermo-Elastic Dilation Model): In contrast, other theoretical and early experimental contributions proposed that increased thermal energy enhances ionic mobility within the double layer, potentially increasing repulsive forces and leading to DDL expansion [7,16]. This mechanism causes volumetric dilation, consistent with the reversible expansion often observed in highly OC clays.
While both hypotheses sought to rationalise thermally induced volume changes through electrochemical forces, their opposing predictions reveal the difficulty of applying the idealised DDL framework to complex clay–water systems.

3.2.2. Limitations of an Idealised DDL Model

The persistence of contradictory findings when applying the DDL theory stems from its inherent simplifications and failure to capture the mechanical and structural constraints present in real soils [54]. Two fundamental shortcomings were identified.
1.
Nonbulk Behaviour of Confined Water: The DDL model assumes that the thermodynamic and dielectric properties of water confined in nanopores are equivalent to those of bulk water [4]. However, this assumption has been disproved by experimental investigations on low-porosity, highly compacted clays. Baldi, Hueckel and Pellegrini [9] demonstrated that the thermal expansion of water confined within clay interlayers deviates significantly from the bulk values. Under high confinement, the hydrogen-bond network and molecular dynamics of water are strongly modified, resulting in reduced mobility and altered dielectric responses [55,56]. Consequently, the conceptual use of bulk-water-based permittivity changes to explain volumetric deformation is untenable.
2.
Inability to Explain Irreversibility: DDL interactions are non-contact and inherently reversible, producing elastic responses upon loading or temperature changes [51]. They cannot independently cause the irreversible volumetric contraction and energy loss characteristics of plastic deformation. The thermal collapse observed in NC clays thus requires an additional mechanism that permits permanent microstructural reorganisation and particle rearrangement. The absence of such a mechanism renders the DDL-based explanation incomplete in cases where heating induces significant irreversible strain [13,57].
These limitations indicate that a purely electrochemical perspective is inadequate for capturing the full scope of thermally induced behaviour in saturated clays. Realistic modelling requires coupling the physicochemical effects of temperature with the mechanical and structural evolution of the aggregation fabric.

3.2.3. Beyond the DDL: Microscale Particle Contacts, Bound Water and Irreversible Processes in Thermal Volume Change

The recognition of the limitations of idealised DDL theories has led to the development of more comprehensive micromechanical models that incorporate the coupled roles of effective stress, interparticle contacts, and bound water dynamics [35,55]. These models propose that macroscopic thermal behaviour arises from the competition between reversible elastic effects and irreversible structural reconfiguration processes. The governing parameters controlling the dominance of each mechanism are the stress history, expressed as the overconsolidation ratio (OCR), and the intrinsic plasticity of the material.
Thermal Softening Through Bound Water Alteration
The phase change of bound pore water is considered a fundamental mechanism for the irreversible thermal deformation of clays [4,33,56]. Pore water in clay exists in two forms: solid-like adsorbed water (or bound water), which is tightly held to the particle surfaces, and mobile free water in larger pore spaces. Adsorbed water has a higher viscosity and shear modulus, contributing significantly to the stiffness of the interparticle contacts [19]. Adsorbed water exists in three primary forms of hydration [58,59]: (i) cation hydration, where water interacts with cations attached to the surface of the clay particle; (ii) surface hydration, where water molecules interact with the hydroxyl groups on the clay face; and (iii) dipolar water molecules attracted to the clay particle by Van der Waals forces. In addition to the forms of hydration, water molecules are also present in the DDL in a higher concentration compared to the pore water.
The first monolayer of bound water exhibits semi-solid behaviour with high viscosity and an ordered molecular structure. Upon heating, increased molecular motion destabilises this structure, converting some bound water into more mobile, loosely held water [4,55]. This alters the particle contacts from a rigid, load-bearing system to a more flexible one, thereby reducing the interparticle shear resistance and triggering local instability [51]. As the strength of the contacts weakened, the effective yield stress of the soil decreased. Under sustained confining pressure, the system transitions to a new stable configuration with a higher density. This transformation corresponds to a thermoplastic strain, which may progress gradually as a time-dependent deformation or thermal creep [13]. Microstructural investigations of compacted clays using SEM, TEM, and mercury intrusion porosimetry have revealed that heating primarily alters the larger pores in compacted clays [35,60]. These studies consistently report that volumetric deformation results from macropore collapse and particle rearrangement rather than DDL modification, supporting the dominance of micromechanical processes over purely electrochemical mechanisms.
OCR Controlled Reversible and Irreversible Thermal Deformation
In highly OC clays, particle arrangements are mechanically stable, and interparticle distances are largely constrained by prior overloading. Under heating, the structure resists rearrangement, resulting in reversible changes dominated by mineral and water thermal expansion and modified DDL repulsion [1,38,54]. The material exhibited elastic dilation without significant mechanical energy dissipation.
Heating weakens the physicochemical bonds by altering the bound water layers at the interparticle contacts [7]. In NC clay, this weakening allows particles and particle aggregates to slip and rearrange into a denser and more stable arrangement under a constant overburden pressure [51,60]. The combined effect of reducing the bound water layer and particle rearrangement is irreversible and leads to a net plastic contraction of the soil skeleton [61], which can be observed macroscopically as thermal softening or thermal creep [4].
Energy Dissipation and Physicochemical Coupling
The dual macroscopic behaviour of clays reflects conflicting and coexisting energy pathways. Reversible thermoelastic behaviour arises from interparticle repulsion and lattice expansion, whereas irreversible thermoplastic contraction results from reduced contact forces facilitated by thermally activated transformations of adsorbed water.
The energy supplied by heating is divided into (i) elastic strain energy stored in the mineral lattice and DDL, which is recovered upon cooling, and (ii) irreversible dissipation due to microstructural rearrangement and collapse of the clay network, expressed as plastic strain. The relative dominance of these components is controlled by the OCR and plasticity [62]: highly overconsolidated, low-PI clays tend to store energy elastically, whereas normally consolidated, high-PI clays preferentially dissipate energy through structural breakdown and reconfiguration.

3.2.4. Role of Plasticity Index in Thermal Behaviour

The PI provides a quantitative link between mineralogy, surface chemistry and microstructural response. High-PI clays generally contain greater proportions of smectite and illite minerals, which possess large specific surface areas, high cation exchange capacities, and thick layers of bound water [31,51]. These features enhance the sensitivity of the material to temperature-induced water dissipation and weakening of interparticle contacts.
The greater specific surface area of high-PI clays increases the number of active adsorption sites for bound water, magnifying the impact of thermal energy on interparticle stability. Consequently, such clays undergo greater volumetric contractions at equivalent temperature increments than low-PI materials [55].
The coefficient of thermal softening is related to the temperature derivative of the interparticle repulsive force. Because particle size and surface chemistry influence this relationship, materials with higher PI, typically finer-grained and richer in smectitic minerals, tend to exhibit higher thermal softening [51]. This analytical connection supports the empirical observations of the increased sensitivity of high-PI soils to heating [15,16,31,33]. Consequently, the PI serves as both a fundamental indicator of sensitivity to bound water alteration and a macroscopic parameter that controls the magnitude of thermal softening and plastic contraction.

3.3. Temperature Effects on Preconsolidation Pressure

Temperature reduces the preconsolidation pressure, PC, of clays, thereby initiating plastic deformation and causing thermal softening of the yield stress. Laloui and Cekerevac [27] introduced Equation (1) to characterise this reduction in preconsolidation pressure, represented on a semilogarithmic scale, observed in oedometric paths under thermomechanical loading conditions.
P C ( T ) = P C ( 0 ) ( T 0 ) { 1 − γ log ( T T o ) }
The preconsolidation pressure at time T is denoted as PC(T), and PC(0)(T0) is the preconsolidation pressure at the reference time T0. Here, γ is the gradient of the equation in the PC/PC(0) vs. log (T/T0) plane, and represents the material parameter. Equation (1) is graphically represented in Figure 3 as a solid black line for a sandy clay sample for reference [63]. Tidfors and Sällfors [53] observed reduced PC in heated samples relative to unheated controls, and later Favero, Ferrari and Laloui [1] reported a reduced yield threshold under high-temperature compression in Opalinus Clay. Results from other experimental studies with similar findings are shown in Figure 3. Comparative studies indicate that the magnitude of this thermally induced reduction is strongly soil-dependent and increases with plasticity, with high-plasticity clays exhibiting larger temperature-driven volumetric and structural changes than low-plasticity clays [17,64]. In addition, recent work has shown that the decrease in preconsolidation pressure in high-plasticity clays is also strain-dependent [65], indicating a coupled influence of temperature and strain history on yield behaviour. This relationship is further confirmed when γ is plotted against LL, as shown in Figure 4. Although there are claims of increased plasticity leading to larger thermally induced volumetric change, the plot of γ versus LL shows considerable scatter, with some higher values of γ occurring at higher values of LL; the opposite can also be observed. This observation further supports claims that, although the reduction of PC with heating is consistent, the magnitude of γ is moderated not only by clay plasticity, but also by testing conditions, including drainage and stress paths [1,18,45,53,66,67,68]. This thermal softening of saturated clay is a crucial component in many constitutive models used to describe the thermo-elastoplastic behaviour of clays [6,27,33]. Therefore, comprehensive and standardised testing across various mineralogies, drainage conditions, and stress paths is essential to develop reliable predictive models for γ and the thermal behaviour of the preconsolidation pressure.

4. Thermal Cyclic Behaviour and Plasticity

Soils in engineering are rarely exposed to a single heating and cooling cycle. Instead, they undergo repeated heating and cooling cycles, resulting in complex cumulative deformations over time. However, similar to the monotonic thermal response, the NC and OC clays exhibited significantly different reactions to the thermal cycles.
NC and lightly overconsolidated OC clays commonly accumulate irreversible contractions during cyclic heating and cooling. Experimental results by Campanella and Mitchell [7], Di Donna and Laloui [36], Vega and McCartney [37] showing the accumulation of volumetric contractions are graphically illustrated in Figure 5. As shown in Figure 5 and Figure 6, the axial strain accumulation usually proceeds at a decreasing rate and reaches a thermally stabilised state in which additional plastic deformation is small [33,35,36,46,74,75,76].
Clay plasticity significantly affects the accumulation of plastic strain under thermal cycles. Figure 7 shows the axial strain versus temperature for the cyclic heating and cooling tests on three clays with different plasticities [47]. The clay with the highest plasticity exhibited the most significant accumulation of axial strain. At steady state, according to Mu, Ng, Zhou and Zhou [47], the accumulated axial strain of clay with PI = 21% was 264% larger than that with PI = 12% and 52% larger than that with PI = 16%. These experimental results for cyclic loading agree well with the observations for monotonic thermal responses and can be explained by the theories suggested in Section 3.2.4. Figure 7 further confirms the statement by Abuel-Naga, Bergado, Bouazza and Ramana [5] and Di Donna and Laloui [36] that high-PI clays require more heating and cooling cycles to reach a stabilised state. However, it should be noted that the available experimental evidence linking PI to cyclic thermal strain accumulation is limited to a small number of laboratory studies, and broader datasets covering diverse clay types and stress conditions are still required to establish generalised conclusions.
In contrast to NC and lightly OC clays, highly OC clays tend to respond in a largely reversible and thermoelastic manner during thermal cycling, with minimal accumulation of plastic strain [33,77], emphasising the significance of stress history in understanding the cyclic thermal response. Reports of volume recovery during cooling are variable, ranging from considerable recovery to minimal or no volume change [3,7,13,35,57]. As shown in Figure 6, the experimental findings of Vega and McCartney [37] revealed that for highly overconsolidated clays (OCR = 7.36 and 30.29), the initial volume was not recovered during cooling, contradicting the commonly assumed reversible thermal response. Further evidence against a purely thermoelastic response in stiff clays was provided by Favero, Ferrari and Laloui [1], who observed that highly overconsolidated Opalinus Clay accumulated irreversible thermal contraction. The observed dilative thermoplastic behaviour indicates that the volumetric response of heavily overconsolidated clays during thermal cycling cannot always be described as being purely thermoelastic. Tiwari and Basu [3] linked these differences to variations in the properties of the tested materials, emphasising that fundamental clay characteristics, such as plasticity, govern thermal behaviour. Together, these observations underscore that plasticity and stress history must be considered when interpreting cyclic thermally induced deformation in clays.

5. Temperature Effect on Consolidation Creep Behaviour

The influence of temperature on the creep behaviour of clay is a significant factor, often referred to as thermal creep or thermal secondary consolidation [78]. In general, the literature indicates a strong relationship in which an increase in temperature tends to increase the creep rate or creep coefficient of clayey soils [43,78,79]. Experimental results confirmed that the creep index (Cα) and thermal creep coefficient (ψT) increased with heating [45]. However, while many studies have reported an increase in the creep rate with temperature, the results for specific soils can vary. For example, Chen, Feng, Chen and Yin [80] and Li, Chen, Feng, and Yin [81] reported that the creep coefficient (Cαe), the nonlinear creep parameter (ψ0), and the creep strain limit (εlc) decreased with increasing vertical stress and temperature. Such observations further strengthen the claims that the creep is both temperature- and stress-dependent [65].
Creep behaviour may also cease after the soil is subjected to thermal cycling, possibly because thermal cycling induces particle rearrangement and microstructural alteration, making further compaction or pore-water expulsion difficult [31]. A slower heating rate may result in a higher creep rate. This behaviour is attributed to differences in thermally induced pore-water pressure, which lead to inconsistent soil microstructures after thermal primary consolidation [43].

5.1. Mechanisms Driving Temperature Effects on Creep

The increase in the creep rate or creep coefficient with temperature is primarily attributed to changes in the physical and chemical interactions within the clay structure and pore water [43,78]. These changes facilitate particle rearrangement and viscous flows.
  • Viscosity and permeability changes: Elevated temperatures cause a decrease in the apparent viscosity of the pore water and at the contacts between particles by converting bound water into free water [43,78]. This reduction in viscosity enhances hydraulic conductivity [81].
  • Microstructural changes and particle interactions: Temperature variations cause changes in the internal microstructure of the soil. Higher temperatures can lead to denser soil microstructures and promote the rearrangement of soil particles [54]. The degradation of the adsorbed water layer causes the particles to come closer together, which alters the contact force between them. Temperature changes influence the intensity of interparticle forces and reduce repulsion, making the rearrangement and compaction of soil particles easier and accelerating the process [60,82].

5.2. Influence of Clay Plasticity Parameters

The characteristics of clay, including its mineralogy and plasticity, regulate the magnitude of the temperature effect on the creep behaviour. The variation in the creep behaviour due to temperature depends on the inherent properties of the clay.
In an experimental comparison between two clays, as illustrated in Figure 8, a clay with a plasticity index (PI) of 31% showed a greater increase in the creep index (Cα) with increasing temperature than a clay with a lower PI of 23.8% [45]. This observation is further supported by the experimental creep index reported by Houston, Houston and Williams [83] for clay with PI = 47%, which showed even greater sensitivity to temperature.
These results indicate that the creep behaviour of the more plastic clay was more affected by temperature than that of the less plastic clay [45]. This behaviour closely follows the trend observed for other thermal responses, with clays of higher plasticity being more sensitive to temperature changes. It is important to note that the current experimental basis for this interpretation remains limited to a small number of studies and clay types, and further systematic investigations across a wider range of materials and stress conditions are needed to clearly establish how plasticity governs the temperature dependence of the creep index.

6. Conclusions

This review synthesises evidence that clay plasticity is not a secondary descriptor but a governing control on thermo-mechanical behaviour in saturated clays, alongside stress history. Across the reviewed data, higher plasticity (reflecting mineralogy, specific surface area, and bound water content) is consistently associated with larger thermally induced volumetric strains in normally consolidated states, stronger thermal softening through a greater temperature-dependent reduction in preconsolidation pressure, increased accumulation of irreversible strain during thermal cycling, and a more pronounced temperature sensitivity of creep. In contrast, highly overconsolidated clays generally exhibit mainly reversible thermal expansion, although the literature also reports irreversible strains even at high OCR, indicating that plasticity and fabric constraints must be considered together. The main limitation in the current literature is the lack of standardised, comparable thermo-mechanical datasets. Reported trends show notable scatter due to differences in mineralogy, pore water chemistry, stress path, drainage conditions, heating rate, and incomplete reporting of microstructural state. Future work should prioritise coordinated testing programmes across well-characterised clays to derive plasticity-dependent thermal parameters, integrate microstructural observations with macroscopic response, and develop constitutive formulations that explicitly incorporate plasticity and stress history to enable reliable long-term predictions in thermal geotechnical applications.

Author Contributions

Conceptualization, H.A.-N. and D.V.; methodology, H.A.-N.; investigation, H.A.-N. and D.V.; writing—original draft preparation, D.V.; writing—review and editing, H.A.-N. and D.V.; supervision, H.A.-N. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

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

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Thermally induced volumetric strain of clays at different OCRs [28,36,37].
Figure 1. Thermally induced volumetric strain of clays at different OCRs [28,36,37].
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Figure 2. Relationship between thermally induced volumetric strain and plasticity index [13,15,17,27,29,44,48,49,50] (after Abuel-Naga, Bergado, Ramana, Grino, Rujivipat and Thet [44]).
Figure 2. Relationship between thermally induced volumetric strain and plasticity index [13,15,17,27,29,44,48,49,50] (after Abuel-Naga, Bergado, Ramana, Grino, Rujivipat and Thet [44]).
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Figure 3. Temperature-dependent variation of preconsolidation pressure in clay soils [14,39,44,63,69].
Figure 3. Temperature-dependent variation of preconsolidation pressure in clay soils [14,39,44,63,69].
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Figure 4. Material parameter γ vs. liquid limit [14,39,44,53,63,69,70,71,72,73].
Figure 4. Material parameter γ vs. liquid limit [14,39,44,53,63,69,70,71,72,73].
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Figure 5. Axial strain variation with cyclic thermal loading [7,36,37].
Figure 5. Axial strain variation with cyclic thermal loading [7,36,37].
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Figure 6. Accumulated axial strain across the thermal cycles [7,37].
Figure 6. Accumulated axial strain across the thermal cycles [7,37].
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Figure 7. Variation in axial strain with clay plasticity index under cyclic loading (After Mu et al. [47]).
Figure 7. Variation in axial strain with clay plasticity index under cyclic loading (After Mu et al. [47]).
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Figure 8. Change in creep index of clay with temperature [45,83].
Figure 8. Change in creep index of clay with temperature [45,83].
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Table 1. Summary of clay types, plasticity ranges, test methods and temperatures used in thermo-mechanical studies.
Table 1. Summary of clay types, plasticity ranges, test methods and temperatures used in thermo-mechanical studies.
Clay TypeCitationsPlasticity Range (PI)Test Types UsedTemperature Ranges (Cycles)Other Important Information
Boom Clay[13]25–50Isotropic compression
Drained heating/cooling
Suction-controlled oedometer testing		20–100 °C.
Includes cycles such as 22–100–27 °C and 20–95–20 °C.		A stiff clay from Mol, Belgium, with a mineralogy of illite, kaolinite, smectite, and quartz
Soft Bangkok Clay[16,28,44]60Drained/undrained triaxial compression
Isotropic/anisotropic consolidation
Modified oedometer		22–90 °C. Cycles include 25–50/70/90–25 °C and 22–90–22 °CDeltaic-marine soft clay with 54% to 71% smectite group minerals
Kaolin/MC Clay/EPK[35]19–35Oedometer
Triaxial shear		5–90 °C. Cycles include 8 °C to 45 °C and paths up to 68 °CIncludes >96% kaolinite
Illite Clay[45]24–112Triaxial testing
Oedometer		10–100 °C. Staged heating is used up to 100 °C.A clay from Germany and includes 12 reconstituted samples
Lateritic Clay[43,46]20.6–21Isotropic consolidation
Thermal oedometer		5–70 °C. Cycles such as 20–50–5–20 °C are used for cyclic stabilisationContains goethite and hematite, which enhance aggregates and stiffen the soil skeleton
Opalinus Clay[1]15High-pressure oedometer with thermal volume change monitoring20–80 °CAn overconsolidated shale/claystone from Switzerland used for nuclear waste storage studies
Loess[47]13–24Thermal invar oedometer15–70 °C. Subjected to 3–5 heating-cooling cycles to reach stable statesClay component (kaolinite, chlorite, illite) is more thermally sensitive than the silt/quartz skeleton
Bonny Silt[37]4Temperature-regulated oedometer
Thermal consolidation under cyclic loading		3–91 °C. Tested over 4 heating/cooling cyclesClassified as low activity, indicating minimal clay minerals
Barmer Bentonite[30]208–398Temperature-controlled oedometer60–200 °C. Involves sustained heating for up to 28 daysFrom Rajasthan, India, features a large specific surface area and high ion-exchange capacity
KSS (Kaolin-based)[17]18.5Temperature-controlled oedometer tests at various vertical pressures5–70 °C. Includes repeat thermal cycling to observe accumulation of strainArtificial clay (50% kaolin, 25% quartz silt, 25% fine quartz sand)
FEBEX Bentonite[38]HighTemperature-controlled oedometer
Axial deformation under mechanically unconfined conditions		25–80 °CA Spanish bentonite used for buffer experiments has high swelling capacity
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Vidurapriya, D.; Abuel-Naga, H. The Effect of Clay Plasticity on Thermally Induced Volume Change of Saturated Clay: A State-of-the-Art Review. Minerals 2026, 16, 303. https://doi.org/10.3390/min16030303

AMA Style

Vidurapriya D, Abuel-Naga H. The Effect of Clay Plasticity on Thermally Induced Volume Change of Saturated Clay: A State-of-the-Art Review. Minerals. 2026; 16(3):303. https://doi.org/10.3390/min16030303

Chicago/Turabian Style

Vidurapriya, Dinitha, and Hossam Abuel-Naga. 2026. "The Effect of Clay Plasticity on Thermally Induced Volume Change of Saturated Clay: A State-of-the-Art Review" Minerals 16, no. 3: 303. https://doi.org/10.3390/min16030303

APA Style

Vidurapriya, D., & Abuel-Naga, H. (2026). The Effect of Clay Plasticity on Thermally Induced Volume Change of Saturated Clay: A State-of-the-Art Review. Minerals, 16(3), 303. https://doi.org/10.3390/min16030303

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