Mitigation of Expansive Soil Through Controlled Thermal Treatment: Geotechnical and Microstructural Assessment

Abstract

Expansive soils present a significant geotechnical challenge due to their pronounced volume changes with moisture variations, leading to substantial infrastructure damage. This study investigates the efficacy of thermal stabilization in mitigating the swell potential and compressibility of a high-plasticity, kaolinite-rich clay from Al Ghat, Saudi Arabia. As well, the changes in basic properties including consistency limits, specific gravity, and compaction characteristics were studied and highlighted. Microstructural studies using X-ray diffraction (XRD), Scanning electron microscopy (SEM), and Energy-dispersive X-ray spectroscopic (EDX) were performed to trace the structural changes and interpret the achieved improvement. Soil specimens were subjected to heat treatment at levels of 200 °C, 400 °C, and 600 °C for two hours, after which their geotechnical and microstructural properties were comprehensively evaluated. The results demonstrate a direct correlation between increasing temperature and the reduction in expansive behavior. Treatment at 600 °C caused a substantial decrease in the plasticity index from 27.00 to 2.94. Correspondingly, oedometer tests showed that the free swell was reduced from 6% to nearly zero, and the swelling pressure was eliminated, dropping from 250 kPa to 0 kPa. XRD analysis confirmed kaolinite decomposition through dehydroxylation, producing metakaolin with diminished water absorption capacity. SEM further revealed significant particle aggregation and the formation of a coarser soil fabric. The findings confirm that heat treatment at temperatures of 400 °C and above is a highly effective method for permanently stabilizing kaolinitic expansive soils, rendering them suitable for construction applications.

1. Introduction

Expansive clay soils represent a significant challenge in geotechnical engineering due to their characteristic ability to undergo pronounced volume changes when subjected to moisture variations [1,2,3]. These volume fluctuations result in considerable structural damage, such as cracks in buildings, roads, and other infrastructure, ultimately compromising their functionality and durability [4,5]. Expansive soils, prevalent across many regions worldwide, are particularly problematic in arid and semi-arid climates, where frequent wetting and drying cycles exacerbate their instability [6,7].

The problematic behavior of expansive soils is closely associated with their mineralogical composition, particularly the presence of clay minerals such as kaolinite, illite, and montmorillonite, which all contribute to soil swelling [8,9]. Kaolinite’s structure comprises alternating layers of silica tetrahedra and alumina octahedra, which facilitate limited water adsorption between layers, resulting in measurable swelling and shrinkage [10]. Although typically less expansive compared to montmorillonite, the swelling and shrinkage cycles of kaolinite-rich soils can still impose substantial stresses on structures, leading to premature deterioration and considerable economic impacts [11]. Estimates indicate that damage from expansive soils accounts for billions of dollars annually on a global scale, emphasizing the critical need for effective stabilization strategies [12].

Various stabilization techniques have traditionally been employed to mitigate the adverse effects of expansive soils, including chemical additives such as lime, cement, and industrial by-products like fly ash and slag [3,7,13,14]. Although these methods can be effective, they often involve environmental concerns, high costs, and long-term durability issues [15]. Thus, researchers have continually sought more sustainable and efficient alternatives, leading to increased interest in thermal stabilization methods. Thermal stabilization involves the application of heat to soil to alter its physical and chemical properties significantly [16,17,18,19,20]. By subjecting expansive soils, including kaolinite-rich soils, to elevated temperatures, critical changes occur in their mineral structure, such as dehydration, partial dehydroxylation, and structural reorganization of clay minerals [21,22,23]. These changes substantially decrease the soil’s swelling potential, enhance mechanical strength, and reduce plasticity. Previous studies have indicated that specific temperature thresholds can effectively transform expansive soils into non-expansive or significantly less expansive materials, suitable for construction and infrastructure applications [16,17].

Previous research has demonstrated the effectiveness of heat treatment in altering the behavior of expansive soils. Wang et al. [16] showed that heating kaolinite and bentonite soils at temperatures up to 600 °C significantly reduced their swelling potential, with kaolinite becoming non-expansive at around 400 °C. Kabubo et al. [21] found that heating black cotton soil to 600 °C improved its maximum dry density (MDD) by 15% and decreased its plasticity index and swelling potential by over 30%. Tamiru et al. [24], investigating an Ethiopian expansive clay, noted that significant improvements began at 400 °C, with the soil becoming non-plastic at 600 °C, which in turn eliminated its swelling pressure. Li et al. [19] reported that heating expansive soil up to 140 °C led to notable microstructural changes, including increased particle size and density, which enhanced strength and reduced hydraulic conductivity. Yao et al. [25] utilized microwave heating to treat montmorillonite-rich soil, observing substantial reductions in liquid and plastic limits and a transition to a non-cohesive structure. These studies highlight that both conventional and microwave heating methods can lead to permanent changes in soil behavior, although the extent of improvement depends on factors such as mineral type, temperature, and duration of heating.

While previous research has established that thermal treatment can mitigate the swelling potential of various expansive clays, many studies have focused on highly expansive montmorillonite-rich soils. However, a systematic link between specific temperature thresholds, the resulting mineralogical and microstructural transformations, and the comprehensive geotechnical response for high-plasticity, kaolinite-rich clays remains less documented. Therefore, the primary contribution of this research is to bridge this gap by conducting a multi-scale investigation on a kaolinitic expansive soil from the Al Ghat region. This study aims to establish a direct causal relationship between the observed macroscopic improvements; such as the reduction in plasticity and the elimination of swelling pressure; and the underlying microscopic changes revealed through XRD, SEM, and EDX analyses. By doing so, this work seeks to define critical temperature thresholds for the effective stabilization of this regionally significant soil type and provide a more complete understanding of its thermal treatment mechanisms.

2. Experimental Work

2.1. Collection of Expansive Soil Sample

The soil samples investigated in this study were obtained from Al Ghat city, a region situated roughly 250 km northwest of Riyadh, Saudi Arabia. Several disturbed samples were obtained from different points within the same area and homogenized to prepare representative material. The collected materials were air-dried, pulverized, and then passed through the ASTM No. 40 sieve (425 µm) to ensure particle uniformity before being stored in airtight plastic containers. Laboratory analysis classified the soil as CH (high-plasticity clay) under the Unified Soil Classification System (USCS) and as A-7-6 in the AASHTO system, reflecting its significant clay content and expansive potential. X-ray diffraction (XRD) analysis revealed kaolinite as the dominant clay mineral, with trace amounts of illite. Notably, montmorillonite; a mineral often associated with high swell potential; was absent, suggesting a comparatively stable mineralogical composition.

2.2. Specimen Preparation and Experimental Methodology

Figure 1 shows the experimental methodology conducted in this study. The clay specimens underwent thermal treatment in an electric laboratory furnace (Matest), equipped with digital temperature regulation capable of reaching approximately 1000 °C. Designed for high-temperature applications, the furnace ensures uniform heat distribution via an internal heating mechanism, while its thermally resistant chamber maintains stability during prolonged operation.

In this study, expansive clay samples were subjected to controlled heating at target temperatures of 200 °C, 400 °C, and 600 °C, with an exposure duration of 120 min at each temperature. The selection of these temperature gradients was based on established findings from the geotechnical literature regarding the thermal decomposition of the dominant clay minerals identified in the soil [2,16,18,26,27]. The 200 °C level was chosen to investigate the effects of removing physically adsorbed water, where only minimal structural change is expected. The 400 °C threshold is a critical temperature recognized as the approximate onset of dehydroxylation in kaolinite, a process where the crystalline structure begins to break down and the mineral can become non-expansive. Finally, the 600 °C level was selected to ensure the near-complete and irreversible transformation of kaolinite into its non-expansive, amorphous metakaolin phase, a temperature at which other expansive soils have also shown significant and permanent improvement. This stepped approach, grounded in previous research, allows for a systematic evaluation of the soil’s properties at distinct stages of its thermal transformation.

As shown in Figure 1, visual comparisons between untreated and heat-treated specimens revealed significant physical changes, particularly in color and texture, suggesting modifications to the clay’s structural integrity. These observations provided preliminary insight into the impact of thermal conditioning on the material’s properties.

Both untreated and treated soil samples were subjected to compaction testing according to the ASTM standard using the mini-compaction method established by Sridharan and Sivapullaiah [28], from which the MDD and optimum moisture content (OMC) were determined for each heating condition. This method was selected due to the limited quantity of soil available after thermal treatment. The original research by Sridharan and Sivapullaiah demonstrated that for fine-grained soils, applying 36 blows per layer with a 1.0 kg hammer in the mini-apparatus yields MDD and OMC values that correlate very closely with the Standard Proctor test (ASTM D698 [29]), with differences typically within ±0.02 kN/m³ and 0.25%, respectively. Therefore, the compaction parameters determined using this method are considered directly scalable and relevant for controlling standard field compaction efforts. For free swell testing, specimens were prepared at their respective MDD and OMC values. Deionized water was added incrementally to the soil at the predetermined OMC, followed by thorough hand mixing to eliminate particle agglomeration and ensure homogeneity. The mixtures were then sealed in plastic bags and cured at room temperature for 24 h to promote uniform moisture equilibration.

Swelling and compression behavior were evaluated using oedometer tests (ASTM D4546 [30]). The cell provided with an LVDT transducer, this in turn connected to the computer through an appropriate software to trace the attained deformations. Specimens were molded in a split-ring consolidometer with an internal diameter of 70 mm and a height of 20 mm, compacted to 15 mm at MDD to accommodate a 5 mm swell allowance. Under a nominal seating load of 7 kPa, specimens were permitted to swell freely until equilibrium was achieved. The axial deformations recorded by transducer were monitored via software, equilibrium assumed when the change on deformation tends to vanished. Subsequent loading increments (12.5–800 kPa) were applied to measure consolidation curves, with the swelling pressure defined as the stress required to restore the specimen to its initial void ratio.

3. Results and Discussion

3.1. Particle Size Distribution of Soil

Figure 2 illustrates the particle size distribution obtained from laser diffraction particle size analyzer conducted on untreated expansive clay and clay heated to 600 °C. It is evident from the curves that thermal treatment significantly altered the particle size characteristics of the soil. The untreated clay displays a relatively finer gradation with approximately 50% of particles smaller than about 12 µm, while the expansive clay heated to 600 °C exhibits coarser particles, where approximately 50% of the particles are smaller than around 200 µm. The observed alterations in particle size distribution following thermal treatment at 600 °C are consistent with findings from previous studies on expansive soils [19,25]. This shift towards coarser particle sizes following heating may be attributed to the aggregation of soil particles and structural changes, including mineralogical transformations and dehydration, induced by exposure to elevated temperature.

3.2. Compaction Characterestics

The results of compaction tests on untreated soil and heated soil under temperatures of 600 °C, 400 °C, and 200 °C are shown in Figure 3. The results of the compaction test on Al Ghat expansive soil indicate that thermal treatment significantly alters the compaction behavior of the soil. The untreated soil exhibited the highest MDD (approximately 1.63 g/cm³) at an OMC of about 26%. When the soil was heated at 200 °C for 120 min, a slight reduction in MDD was observed, although the compaction curve remained close to that of the untreated soil. However, at 400 °C, the MDD decreased further, and the curve flattened, indicating a loss in compaction efficiency. At 600 °C, the soil showed the lowest MDD values across all water contents, with a nearly horizontal curve, suggesting that excessive heating degraded the soil structure. This reduction in MDD with increasing temperature can be attributed to thermal decomposition of clay minerals and disruption of the soil fabric. These changes reduce the soil’s ability to pack densely under compaction energy. Overall, the results demonstrate that while moderate heating (200 °C) causes limited change, higher temperatures (400 °C and 600 °C) significantly reduce the compactability of expansive soil, which may compromise its engineering performance.

3.3. Effect of Thermal Treatment on Consistency Limits and Specific Gravity

The experimental results reveal significant alterations in the consistency limits and physical properties of the expansive soil when subjected to elevated temperatures as demonstrated in

Table 1. Effect of Thermal Treatment on Consistency Limits and Specific Gravity.

Properties	Temperature, °C
	Untreated	200	400	600
Liquid Limit, LL, %	60.00	57.64	45.61	34.96
Plastic Limit, PL, %	33.00	39.00	38.78	32.02
Plasticity Index, PI	27.00	18.64	6.83	2.94
Shrinkage Limit, SL, %	15.00	23.39	32.74	30.24
Specific Gravity, Gs	2.800	2.899	2.868	2.804

. The liquid limit (LL) exhibited a consistent downward trend with increasing temperature, decreasing from 60.00% in the untreated specimen to 34.96% at 600 °C, representing a reduction of approximately 42%. This reduction can be attributed to the dehydration of clay minerals and the progressive breakdown of the crystalline structure of kaolinite, which is the predominant clay mineral identified through XRD analysis, along with trace amounts of illite [31,32].

The plastic limit (PL) demonstrated a complex, non-linear behavior pattern with increasing temperature. Initially, there was an increase from 33.00% in the untreated sample to 39.00% at 200 °C, followed by a subsequent decrease to 38.78% at 400 °C and a significant drop to 32.02% at 600 °C. This two-stage phenomenon can be explained by distinct changes occurring at different temperature thresholds. The initial increase at 200 °C is attributed to the removal of loosely bound adsorbed water, which causes the fine, platy clay particles to form larger, more stable aggregates. This aggregation results in a coarser soil fabric that, in turn, requires more water to lubricate the particles and achieve a plastic state, hence the rise in the PL. In contrast, the subsequent decrease at higher temperatures (400 °C and above) is driven by a more profound and dominant change: the irreversible destruction of the kaolinite mineral structure through dihydroxylation [31,33,34]. As the clay transforms into a non-plastic, amorphous material, its inherent ability to hold water and exhibit plasticity is permanently lost, leading to a significant reduction in the Plastic Limit.

The plasticity index (PI), calculated as the difference between LL and PL, showed a dramatic reduction from 27.00 in the untreated soil to merely 2.94 at 600 °C. This substantial decrease indicates a fundamental change in the soil’s plasticity characteristics, suggesting that the heated soil would exhibit significantly reduced swelling potential and improved dimensional stability [31,32]. According to established classification systems, clays with plasticity indices greater than 20 are generally considered highly expansive [35]. while the reduced values observed after thermal treatment indicate a transition to less problematic soil behavior.

The shrinkage limit displayed an inverse relationship with temperature compared to other consistency limits. The value increased from 15.00% in the untreated specimen to 32.74% at 400 °C, with a slight decrease to 30.24% at 600 °C. This behavior reflects the structural modifications occurring within the clay matrix, where the removal of interlayer water and the collapse of the clay structure result in a denser arrangement of particles [32]. Specific gravity measurements revealed relatively minor fluctuations, ranging from 2.800 in the untreated soil to a maximum of 2.899 at 200 °C, followed by values of 2.868 and 2.804 at 400 °C and 600 °C, respectively. The initial increase at 200 °C likely corresponds to the removal of water molecules from the clay structure, while the subsequent variations may be attributed to phase transformations and the formation of new mineral phases at elevated temperatures [31,32].

The observed changes in soil properties can be explained by the thermal decomposition mechanisms affecting clay minerals. At temperatures between 200 and 400 °C, dehydroxylation of clay minerals begins, leading to the breakdown of octahedral sheets and the formation of amorphous phases. At 600 °C, the original clay structure is largely destroyed, resulting in the formation of new crystalline phases with markedly different properties compared to the parent material [33].

3.4. Swelling and Consolidation Characteristics

Figure 4 illustrates the axial strain (%) development with time (minutes) during the free swell oedometer test for untreated expansive soil and soils subjected to thermal treatments at temperatures of 200 °C, 400 °C, and 600 °C for 2 h. Initially, all specimens were subjected to a minimal seating stress (7 kPa) before water was introduced to permit free swelling, and the axial strains were recorded over an extended duration. The untreated soil exhibited significant swelling behavior, progressively increasing in axial strain and reaching approximately 6% strain over time. This response highlights its high expansive nature, characterized by continuous volume increase upon moisture exposure. For soil heated at 200 °C (G200), the axial strain notably decreased compared to untreated soil, reaching approximately 3% after prolonged exposure to water. This reduced swelling indicates that moderate heating partially mitigates soil expansiveness due to modest changes in soil structure and mineralogy.

At 400 °C (G400), further reductions in axial strain were observed, stabilizing around 2%. This indicates a substantial decrease in swelling potential attributed to more pronounced thermal-induced transformations within the soil particles, enhancing structural stability. Most notably, soil heated at 600 °C (G600) showed virtually no swelling, with axial strain values remaining around zero throughout the entire testing duration. This behavior suggests almost complete elimination of soil expansiveness, demonstrating significant mineralogical and structural transformations due to high-temperature thermal treatment, substantially enhancing the soil’s stability against swelling. These observations confirm that thermal stabilization significantly reduces the expansive potential of soils. Higher treatment temperatures lead to greater mitigation of swelling behavior, as clearly demonstrated by the progression from untreated to highly stabilized conditions at 600 °C.

Figure 5 illustrates the results from free swell oedometer tests, presenting the relationship between axial strain (%) and applied stress (kPa) for untreated expansive soil and soil heated at temperatures of 200 °C, 400 °C, and 600 °C, each for a duration of 2 h. Initially, the samples were subjected to a small seating load of 7 kPa, and then water was introduced to permit the soil to swell freely. Subsequently, the stress was incrementally increased to determine the swell pressure, swelling index (C_s), and compression index (C_c). The untreated soil exhibited significant swelling behavior under the initial minimal load, reaching approximately 6% axial strain. As stress increased, axial strain progressively reduced, ultimately transitioning from expansion to compression, reflecting the typical behavior of untreated expansive soils under incremental loading.

Soil treated thermally displayed notably different swelling behaviors. At 200 °C (G200), soil swelling was reduced to approximately half the magnitude observed in untreated soil, indicating partial mitigation of the swelling potential. Further reduction in swelling was observed at 400 °C (G400), suggesting increased stiffness and decreased expansiveness due to mineralogical transformations caused by heating.

The most substantial reduction in swelling occurred at 600 °C (G600), where initial swelling strains were markedly minimized, approaching zero under the low initial stress and eventually showing slight compression even at relatively low stresses. This indicates a near-complete elimination of expansiveness, reflecting significant structural and mineralogical alterations from high-temperature exposure.

These findings clearly demonstrate that thermal treatments significantly reduce the expansive potential of soils by decreasing their swelling strain and altering their response to incremental loading. Consequently, treated soils exhibit lower swell pressures and reduced swelling (C_s) and compression indices (C_c), highlighting the effectiveness of thermal stabilization as a method for enhancing the engineering behavior of expansive soils.

Figure 6 and Figure 7 illustrate the effect of thermal treatment on the swelling and compression characteristics of the expansive soil under investigation. Figure 6 presents the relationship between the heating temperature applied to the soil samples and the resulting compression index (C_c) and swelling index (C_s). As observed, both indices exhibit a general decreasing trend with increasing temperature. The C_c shows a relatively modest decrease between 100 °C and 400 °C, dropping from approximately 0.16 to 0.14. However, a much more significant reduction occurs between 400 °C and 600 °C, where C_c falls sharply to about 0.045. Similarly, the C_s decreases steadily from approximately 0.06 at 100 °C to about 0.01 at 600 °C. This indicates that heating the soil reduces both its compressibility under load and its tendency to rebound or swell upon unloading, with the effect becoming more pronounced at higher treatment temperatures, particularly above 400 °C.

Figure 7 displays the influence of the heating temperature on the swelling pressure (S_p)and free swell percentage. The free swell percentage shows a consistent and marked decrease as the temperature rises, declining from 6% at 100 °C to nearly 0% at 600 °C. This strongly shows that thermal treatment efficiently reduces the soil’s inherent propensity to expand when exposed to moisture without restriction. The S_p also generally decreases with increasing temperature, starting at 250 kPa at 100 °C and reducing to 0 kPa at 600 °C. Notably, there is a slight increase in S_p observed between 200 °C and 400 °C, rising from roughly 180 kPa to 240 kPa, before the sharp decline continues. Despite this intermediate behavior, the overall trend clearly indicates that higher heating temperatures lead to a substantial reduction in the pressure the soil exerts upon wetting under confined conditions.

Collectively, these figures demonstrate that thermal treatment significantly mitigates the expansive nature of the soil. The reduction in compression index, swelling index, free swell, and S_p indicate a fundamental alteration in the soil’s structure and/or mineralogy due to heating, rendering it less susceptible to volume changes upon variations in moisture content. The results suggest that heating, especially at temperatures exceeding 400 °C, could be an effective ground improvement technique for controlling the problematic behavior of expansive soils.

3.5. XRD Analyses

Figure 8 compares the XRD patterns of the soil in its untreated state and after being heated at 600 °C for 2 h. The heat treatment resulted in a fundamental alteration of the soil’s mineralogy. The diffraction peaks observed between 10° and 12° 2θ in the untreated sample, corresponding to illite and kaolinite, are absent in the pattern of the heated soil. The loss of these peaks signifies the thermal decomposition of these clay minerals.

While the clay mineral structures were altered, the primary peaks for quartz at 26.7° 2θ and calcite at 29.5° 2θ persisted, confirming the thermal stability of these non-clay minerals at the tested temperature. The diffractogram for the heated sample displays fewer peaks of lower intensity, which reflects the transformation of crystalline clay structures into more amorphous material. The disappearance of the kaolinite peaks, specifically, is evidence of its dehydroxylation and conversion to metakaolin. These mineralogical changes are consistent with the established thermal decomposition pathways for clay minerals involving dehydration and dehydroxylation.

The transformation from well-crystallized kaolinite to amorphous metakaolinite occurs through the removal of structural hydroxyl groups, leading to the breakdown of the octahedral aluminum layer while preserving the tetrahedral silica framework in a distorted form. This phase transformation is irreversible and represents a fundamental alteration in the mineral structure that directly influences the physical and mechanical properties of the soil.

The XRD results provide crystallographic evidence supporting the observed changes in Atterberg limits and microstructural modifications discussed previously. The loss of the layered structure of kaolinite eliminates the mineral’s capacity for interlayer water adsorption, which explains the significant reduction in liquid limit values. Furthermore, the formation of metakaolinite creates a more stable mineral phase with reduced susceptibility to moisture-induced volume changes, correlating with the decreased plasticity index and improved dimensional stability observed in the thermally treated soil. The complete disappearance of kaolinite peaks and the formation of amorphous phases also explain the microstructural changes observed in the scanning electron microscopy (SEM) analysis, where particle fusion and densification occurred as a result of the thermal breakdown of the original clay mineral structure.

3.6. SEM Images

Figure 9 shows the SEM images of the soil in its untreated state and after heating at 600 °C for 2 h. SEM images reveal distinct microstructural changes occurring in the clay soil following thermal treatment at 600 °C. The untreated clay soil (Figure 9a) exhibits a relatively smooth and cohesive surface texture with fine-grained particles arranged in a compact matrix. The microstructure shows the characteristic platy morphology of clay minerals with overlapping sheets and minimal visible porosity at the observed magnification.

In contrast, the heated clay soil (Figure 9b) displays a markedly different microstructural arrangement. The thermal treatment has induced substantial textural modifications, with the formation of a more heterogeneous surface characterized by increased roughness and the development of distinct particle aggregations. The heated specimen shows evidence of particle fusion and sintering effects, where individual clay particles have coalesced into larger aggregates with clearly defined boundaries. This transformation is consistent with the thermal decomposition of kaolinite at elevated temperatures, where the breakdown of the crystalline structure leads to the formation of amorphous phases and subsequent recrystallization processes.

The microstructural changes observed through SEM analysis provide direct evidence for the mechanisms underlying the alterations in Atterberg limits discussed earlier. The particle reorganization and densification evident in the heated specimen correlate with the reduced plasticity index and increased shrinkage limit values. The formation of more stable particle arrangements through thermal treatment explains the improved dimensional stability characteristics observed in the heated clay soil.

The development of a coarser particle texture in the thermally treated specimen suggests enhanced drainage characteristics and reduced water retention capacity, which directly contributes to the observed reduction in liquid limit values. Furthermore, the formation of particle aggregates creates a more stable soil structure that is less susceptible to volume changes upon moisture variation, thereby reducing the expansive potential of the original clay soil.

3.7. EDX Analysis

Energy-dispersive X-ray spectroscopic (EDX) analysis of untreated and heated expansive soil at 600 °C; as shown in Figure 10; reveals notable changes in the elemental composition due to thermal treatment. In the untreated soil, the predominant elements are oxygen (53.63 wt%), silicon (17.60 wt%), and aluminum (10 wt%), which are characteristic of clay minerals. Additional elements include iron (6.7 wt%), sulfur (3.2 wt%), calcium (5.3 wt%), and magnesium (1.44 wt%). After heating to 600 °C, the oxygen content decreased significantly to 49.66 wt%, indicating the loss of hydroxyl groups and water molecules during dehydroxylation. The aluminum and silicon contents remained relatively stable, with slight increases to 10.2 wt% and 17.7 wt%, respectively, suggesting minimal structural changes in the aluminosilicate framework. These compositional modifications reflect the thermal stability of the fundamental clay mineral structure while demonstrating the elimination of hydration-related components.

The thermal treatment resulted in substantial changes in several elements, with calcium content exhibiting the most pronounced increase from 5.31 wt% to 7.1 wt%, representing a 44% increase from its initial concentration. Magnesium similarly increased from 1.4 wt% to 1.6 wt%. Conversely, iron content decreased from 6.7 wt% to 6.20 wt%, while sulfur content increased from 3.2 wt% to 4.6 wt%. These changes are attributed to the thermal decomposition processes and the formation of new oxide and hydroxide mineral phases during heating. The observed elemental modifications highlight the impact of thermal treatment on the soil’s composition, particularly the reduction in oxygen content and the redistribution of cationic elements, which contribute to the altered physical and chemical properties of the heated soil. The enrichment of calcium and formation of cementitious phases enhance mechanical properties and dimensional stability, while the reduced oxygen content correlates with decreased water affinity and reduced swelling potential, collectively improving the engineering performance of the treated expansive soil.

3.8. Linking Microstructural Changes to Geotechnical Improvements

The experimental results demonstrate a clear causal link between the microstructural changes induced by thermal treatment and the dramatic improvements in the soil’s macroscopic geotechnical properties. The most significant of these improvements was the complete elimination of expansive behavior at 600 °C, where the free swell was reduced from 6% to nearly zero and the swelling pressure was eradicated, dropping from 250 kPa to 0 kPa. This transformation is not merely a physical change but a fundamental and irreversible alteration of the soil’s mineralogy and fabric.

The underlying cause for this improvement is revealed by the XRD analysis, which shows that heating to 600 °C destroys the original crystalline structure of the kaolinite mineral. The characteristic diffraction peaks for kaolinite, present in the untreated soil, are entirely absent after treatment, signifying the thermal decomposition of the mineral. This process, known as dehydroxylation, transforms the expansive kaolinite into an amorphous, non-expansive phase called metakaolin. The destruction of kaolinite’s layered crystalline structure permanently eliminates the primary mechanism for interlayer water adsorption, which is the root cause of swelling.

This mineralogical transformation is visually confirmed by the SEM images, which show a profound change in the soil’s fabric. The untreated clay exhibits a smooth, cohesive matrix of fine, platy particles with a high surface area. In contrast, the soil heated to 600 °C displays a coarser, more stable structure characterized by distinct particle fusion and the formation of larger, sintered aggregates. This aggregation significantly reduces the effective surface area available for water interaction and creates a more rigid soil skeleton. This irreversible change in both the soil’s mineralogical composition and its physical fabric fully explains the elimination of its ability to swell, thereby rendering it a stable material suitable for engineering applications.

3.9. Considerations for Field Application and Associated Risks

While our laboratory investigation demonstrates that controlled thermal treatment at 600 °C effectively transforms the expansive Al Ghat clay into a stable, non-expansive material, translating these findings to in situ field applications requires careful consideration of several practical challenges. The uniform heating achieved in our laboratory furnace is difficult to replicate in the field, where soil heterogeneity can lead to non-uniform heating patterns. This differential heating can cause uneven shrinkage, creating tensile stresses that may result in the formation of thermal cracks [2]. Furthermore, the high energy intensity required to heat large soil volumes raises significant concerns regarding both economic feasibility and environmental impact from potential greenhouse gas emissions.

The process of heating soil also involves managing significant moisture loss and the resulting vapor pressure [36]. As shown in this study, thermal treatment fundamentally alters the soil’s relationship with water. In a field setting, this generates substantial vapor pressure. In a low-permeability clay like the one studied, this pressure can build up, potentially causing localized fracturing. Energy transfer itself is a major problem; for instance, innovative methods like microwave heating are often limited to a shallow penetration depth of around 20 cm, making the treatment of deeper soil layers challenging [37].

Given these significant in situ challenges, a more immediately feasible engineering application would be the ex situ treatment of excavated expansive soils. In this approach, problematic soil could be thermally treated at a dedicated facility and then used as a high-quality, non-expansive engineered fill for embankments, backfills, or pavement subgrades. This method would allow for greater quality control and efficiency. Therefore, while our findings confirm the high potential of thermal stabilization, these field-related risks underscore the need for further research, framing our work as a vital foundational step toward developing practical ground improvement solutions.

4. Conclusions

This study comprehensively investigated the effect of heat treatment at 200 °C, 400 °C, and 600 °C on the geotechnical properties of a kaolinite-rich expansive clay. The experimental results demonstrate that thermal stabilization is a highly effective method for mitigating the soil’s expansive behavior, with the degree of improvement directly correlating with the treatment temperature. The key findings are summarized as follows:

• Thermal treatment induced significant and favorable changes in the soil’s physical characteristics. The liquid limit decreased by approximately 42% when heated to 600 °C, while the plasticity index shows a dramatic reduction from 27.00 in the untreated state to just 2.94 at 600 °C. This transformed the soil from a high-plasticity clay to a material with significantly reduced swelling potential. Conversely, the MDD decreased as temperatures rose, particularly at 400 °C and 600 °C, indicating a degradation of the soil’s compactability due to structural changes.
• Heating the soil to 600 °C led to a significant coarsening of its texture, with the median particle size increasing from approximately 12 µm to 200 µm. SEM analysis confirmed this, revealing that the smooth, platy structure of the untreated clay was transformed into a more heterogeneous matrix characterized by particle fusion and aggregation.
• Oedometer tests provided conclusive evidence of the efficacy of the heat treatment. The free swell of the soil was progressively reduced with increasing temperature, decreasing from 6% in its natural state to being virtually eliminated at 600 °C. Similarly, the swelling pressure was reduced from 250 kPa to 0 kPa at 600 °C. Both the C_c and C_s also decreased, with the most significant reductions occurring above 400 °C.
• XRD analysis revealed the underlying cause of these behavioral changes. Treatment at 600 °C resulted in the thermal decomposition of the crystalline clay minerals, evidenced by the disappearance of kaolinite and illite peaks. This process, identified as the conversion of kaolinite to metakaolin, irreversibly destroys the layered mineral structure responsible for water adsorption and swelling. EDX analysis supported this by showing a decrease in oxygen content, consistent with dehydroxylation.

In summary, the research confirms that heating kaolinitic expansive soil to temperatures of 400 °C and above fundamentally alters its mineralogy and microstructure, thereby rendering it a stable, non-expansive material suitable for engineering applications. These findings highlight the potential of thermal stabilization as a reliable and permanent ground improvement technique.