Lime–fly ash binder composition controls freeze–thaw durability of stabilized clays

Abstract

This study examines the effects of freeze–thaw (F–T) cycles on the unconfined compressive strength (UCS) of untreated and stabilized soils using fly ash, lime, and lime–fly ash mixtures (30% FA, 5% L, and 30% FA + 5% L). Specimens were subjected to 2, 4, 6, 8, and 12 F–T cycles. Fly ash-only mixtures showed a continuous reduction in UCS with increasing cycles, whereas lime and lime–fly ash mixtures exhibited higher resistance to F–T deterioration. Strength remained relatively stable up to six cycles, followed by a noticeable decline; however, lime-treated samples maintained higher UCS than untreated soils even after 12 cycles. For S30FA5L, S5L, and B5L mixtures, UCS increased by approximately 27–35% during the 2nd to 4th cycles. Lime-treated soils exhibited higher strength and more brittle behavior, while lime–fly ash mixtures showed more ductile responses. Although fly ash addition did not significantly enhance strength compared to lime alone, it improved ductility. Furthermore, strength enhancement became more pronounced when CaO > 7.7%, CaO/Al₂O₃ > 0.44, CaO/SiO₂ > 0.13, and CaO/(Al₂O₃ + SiO₂) > 0.10. These findings highlight the importance of oxide composition and calcium availability in governing freeze–thaw durability of stabilized clays.

Introduction

Soil properties are highly influenced by environmental variations, including physical, chemical, and dynamic effects. These changes significantly affect the performance of engineering structures. When soil properties are inadequate from an engineering perspective, the subgrade may become susceptible to deformation, leading to structural cracking, settlement, and other forms of distress. Damages induced by swelling, freezing, or inherently weak soil characteristics may ultimately compromise structural safety. Improving soil properties through stabilization enhances durability and long-term performance, increasing resistance against unexpected environmental actions. Although soil improvement can be achieved using naturally available materials, excavation, transportation, and processing costs may render such approaches economically inefficient. In this context, the utilization of sustainable industrial by-products presents both economic and environmental advantages^1–4.

Among chemical stabilizers for clayey soils, lime is one of the most effective and widely used additives. Lime stabilization has been successfully applied in road embankments and subgrades for decades. Its advantages include cost-effectiveness, well-established field performance, and its ability to promote cementation and hydration reactions with silica- and alumina-rich clay minerals⁵. Lime-induced pozzolanic reactions generate cementitious products that enhance strength and reduce plasticity. Several studies have demonstrated that lime treatment improves freeze–thaw (F–T) resistance of fine-grained soils^6–9.

Fly ash is another conventional stabilizing agent frequently used to improve strength and durability characteristics of soils. Due to its reactive silica and alumina content, fly ash promotes secondary pozzolanic reactions and modifies the oxide balance within the stabilized matrix. According to¹⁰, fly ash is classified as Class C, Class F, or unclassified, primarily based on CaO content. Class C fly ash exhibits stronger self-cementing properties due to its higher calcium content, whereas Class F fly ash requires an external calcium source for effective pozzolanic activation. The effectiveness of fly ash in soil stabilization is fundamentally governed by its chemical composition, particularly its CaO content. Class C fly ash, characterized by a high calcium content, exhibits inherent self-cementing properties, thereby significantly enhancing early-age strength development¹¹. In contrast, Class F fly ash, which contains relatively low CaO, demonstrates limited reactivity in its untreated state; however, its reactivity can be substantially enhanced through lime addition or alkali activation, both of which promote pronounced pozzolanic reactions¹². These reactions result in the formation of cementitious products such as C–S–H and C–A–H gels, which effectively fill pore spaces and contribute to the development of a denser and more homogeneous soil microstructure^13,14. Moreover, under freeze–thaw cycles, these cementitious matrices play a critical role in restraining microcrack initiation and propagation, thereby mitigating strength degradation. Consequently, while Class C fly ash offers a clear advantage in terms of rapid early-age strength gain, the combined use of Class F fly ash with lime or alkali activation provides superior long-term performance and enhanced durability, particularly under adverse environmental conditions. When lime and fly ash are used in combination, the resulting soil exhibits both a more limited loss in strength and a more pronounced partial recovery (self-healing) behavior during cyclic loading, compared to soils stabilized with lime alone¹⁴.

In cold regions, repeated freeze–thaw cycles significantly deteriorate pavement subgrades and embankments^15,16. During freezing, cryogenic suction generates a hydraulic gradient that drives water migration toward the freezing front, leading to ice lens formation, increased void ratio, and frost heave. During thawing, the excess meltwater saturates the soil, reducing effective stress and strength¹⁷. As the number of cycles increases, progressive microstructural damage and crack propagation occur, resulting in cumulative strength degradation^18–20. Freeze–thaw cycling can also induce ice lens formation within lime-stabilized soils, contributing to structural deterioration⁸. The extent of strength reduction depends strongly on the chemical composition and mineralogical characteristics of the stabilized matrix¹⁶. Even after three F–T cycles, strength reductions of 36–48% have been reported²¹. Particularly when the number of cycles exceeds five, dramatic reductions in unconfined compressive strength (UCS) may occur⁹. Although some soils may exhibit relatively smaller strength losses under extended cycling²², freeze–thaw exposure generally accelerates deterioration, especially in soils that are already marginal from an engineering standpoint.

Numerous studies have investigated the influence of freeze–thaw cycles on the UCS and stress–strain behavior of chemically stabilized fine-grained soils^7,9,21–33. Lime contents as low as 4–6% have been shown to provide significant strength improvements. However, lime-treated soils often exhibit brittle failure behavior, characterized by peak stress occurring at axial strains of approximately 2–4%, followed by rapid post-peak stress reduction. The incorporation of fly ash alongside lime has been reported to mitigate brittleness and promote more ductile behavior. When optimized lime–fly ash mixtures are used, UCS values may remain stable even after high numbers (60) of freeze–thaw cycles³². Lime alone may also enhance freeze–thaw performance; for example, mixtures containing 8% lime exhibited a 39% increase in UCS after four cycles⁷. Conversely, Class F fly ash with low CaO content may exhibit progressive strength reduction under increasing freeze–thaw cycles due to insufficient cementation^7,22.

Oxide ratios are key parameters for interpreting the strength development of stabilized soils. In particular, CaO, CaO/Al₂O₃, CaO/SiO₂, and CaO/(Al₂O₃ + SiO₂) ratios play a significant role in assessing pozzolanic reactivity and the formation of cementitious products. Janz et al.³⁴ suggested that the CaO/SiO₂ ratio is an important indicator of the potential for pozzolanic reactions. It has been reported that when this ratio is approximately 0.7, its contribution to unconfined compressive strength (UCS) reaches a maximum³⁵. Furthermore, the CaO/(Al₂O₃ + SiO₂) ratio has been proposed as a useful parameter for evaluating the formation of cementitious bonds within the stabilized matrix³⁶.

In this study, the effect of freeze–thaw cycles on the unconfined compressive strength of soils stabilized with lime and/or fly ash was investigated. Although numerous studies have examined the mechanical behavior of stabilized soils under freeze–thaw exposure, limited research has simultaneously investigated the evolution of secant modulus together with oxide-based compositional thresholds for both low- and high-plasticity clays within a unified experimental framework. The main novelty of this study lies in integrating mechanical performance parameters with chemical composition indicators to establish quantitative relationships between oxide ratios and freeze–thaw durability. In particular, this study attempts to identify critical oxide ratio ranges that can be used as practical indicators for predicting the performance of stabilized soils. This approach provides a combined mechanistic and empirical understanding by linking macroscopic behavior (strength and stiffness degradation) with binder chemistry. Furthermore, by evaluating both low- and high-plasticity clays under identical conditions, the study offers insights into the generalizability of the proposed relationships across different soil types. Therefore, this work advances existing studies by moving beyond conventional strength-based evaluations and proposing a composition-driven framework for assessing the freeze–thaw performance of stabilized soils.

Materials

Soils

The low-plasticity clay used in this study (Fig. 1a) was obtained from Çaytaş Company, while the high-plasticity clay (Fig. 1b) was supplied by Çanbensan Company. Both soils were passed through a No. 200 sieve prior to testing to ensure uniform particle size distribution for laboratory evaluation. The Çaytaş material is classified as a bonding clay, whereas the Çanbensan material consists primarily of bentonite, known for its high swelling capacity and strong water affinity. The natural moisture contents of the low- and high-plasticity clays were determined as 3% and 9%, respectively. All samples were air-dried before testing to eliminate uncontrolled moisture effects and ensure reproducibility under freeze–thaw conditions Index properties were determined through hydrometer analysis, Atterberg limits testing, and specific gravity measurements. Specific gravity values, obtained in accordance with ASTM standard³⁷, were 2.63 for the CL soil and 2.65 for the CH soil. Liquid and plastic limits were measured following the standard³⁸, and soils were classified according to the Unified Soil Classification System³⁹ as CL (low-plasticity clay) and CH (high-plasticity clay). Hydrometer analysis conforming ASTM standard⁴⁰ revealed that the CL soil consisted of 48.67% clay and 51.33% silt, whereas the CH soil contained 81.17% clay and 18.83% silt. The significantly higher clay fraction in the CH soil indicates greater water retention capacity and lower permeability, both of which are critical parameters influencing freeze–thaw susceptibility. Fine-grained soils with high clay content are particularly prone to cryogenic suction and ice lens formation due to their ability to retain unfrozen water during sub-zero temperatures. Consequently, the CH soil is expected to exhibit higher freeze–thaw sensitivity compared to the CL soil. Compaction characteristics were determined using the Standard Proctor method⁴¹. The optimum moisture contents (OMC) were found to be 19.42% for the CL soil and 36.47% for the CH soil, while the corresponding maximum dry unit weights were 16.30 kN/m³ and 12.48 kN/m³, respectively. The substantially higher OMC of the CH soil reflects its elevated water demand and swelling potential, both of which are directly associated with increased freeze–thaw vulnerability in cold-region environments (Table 1; Fig. 2).

Fig. 1.

Soil used in the study a Çaytaş clay b Çanbensan clay⁴².

Table 1.

Index properties of soils used in the study⁴².

Index Property	Caytas clay	Canbensan Clay
Specific gravity	2.63	2.65
Clay (%)	48.67	81.17
Silt (%)	51.33	18.83
Liquid limit	34	230
Plastic limit	22	45
Plasticity index	12	185
Optimum moisture content (%)	19.42	36.47
Maximum dry unit weight (kN/m3)	16.30	12.48
USCS soil classification	CL	CH

Fig. 2.

Particle size distribution of soils and fly ash.

The chemical compositions of the materials used in the study were obtained from X-ray fluorescence (XRF) analysis. The XRF results indicate that both soils predominantly consist of SiO₂, Al₂O₃, and Fe₂O₃, which are typical oxide components of clay minerals. As expected, the lime contains a high proportion of CaO, reflecting its strong calcium-based reactivity. The fly ash used in this study was classified as low-calcium Class F fly ash in accordance with ASTM standard¹⁰, indicating limited self-cementing capability without external calcium activation.

Microstructural characteristics were examined using scanning electron microscopy (SEM) at 10,000× magnification. The SEM image of the Çaytaş clay (Fig. 3a) reveals an irregular and angular particle morphology with noticeable interparticle voids. The relatively coarse and angular structure of the low-plasticity clay suggests lower water retention capacity and a more open fabric, which may influence its response to freeze–thaw cycles by facilitating pore water migration. In contrast, the SEM image of the Çanbensan clay (Fig. 3b) shows comparatively smoother and more rounded particles with a denser and more aggregated structure. The closer particle arrangement and cohesive appearance are characteristic of bentonitic materials, which exhibit high surface activity and strong water affinity. Such microstructural features are typically associated with higher swelling potential and greater susceptibility to cryogenic suction under freezing conditions. The observed microstructural discontinuities in both soils confirm the absence of inherent cementation prior to stabilization. This indicates that any improvement in mechanical performance under freeze–thaw exposure can be directly attributed to the chemical stabilization mechanisms rather than pre-existing bonding within the natural soil matrix.

Fig. 3.

SEM images of soils with 10000x magnification a CL b CH⁴².

Lime

Hydrated lime was used as the chemical stabilizer in this study. The lime, produced by Eksioglu Company, was classified as CL 90-S type and is characterized by a high CaO content. The material is shown in Fig. 4. According to the XRF results presented in Table 2, the lime contains approximately 92.5% CaO, indicating its strong potential for calcium-driven pozzolanic reactions. To prevent premature carbonation caused by atmospheric CO₂ exposure, the lime was stored in airtight plastic containers prior to mixing. Proper storage is essential to preserve its reactivity, as carbonation reduces the availability of free CaO and may negatively affect stabilization efficiency. Scanning electron microscopy (SEM) analysis (Fig. 5) revealed irregularly shaped particles typical of hydrated lime. The microstructure suggests the presence of portlandite as the dominant phase, with minor calcite formations likely resulting from limited surface carbonation. The irregular morphology and high calcium content of the lime are expected to facilitate rapid cation exchange and subsequent formation of cementitious hydration products when mixed with clay minerals and/or fly ash.

Fig. 4.

Lime used in the study⁴².

Table 2.

Chemical properties of materials used in the study⁴².

Chemical properties	CL	CH	FA	Lime
Silicon dioxide (SiO2)	58.43	65.75	48.20	2.12
Aluminum oxide (Al2O3)	31.42	17.42	21.08	1.25
Ferric oxide (Fe2O3)	3.72	7.79	18.32	0.45
Calcium oxide (CaO)	0.00	0.00	2.49	92.50
Sulfur trioxide (SO3)	0.04	0.61	0.98	1.37
Magnesium oxide (MgO)	0.92	3.16	1.89	2.05
Potassium oxide (K2O)	3.23	1.49	3.88	0.16
Titanium dioxide (TiO2)	2.07	0.00	2.35	0.00
Loss on ignition (LOI)	NT	NT	2.14	NT

NT not tested, FA fly ash, all numerical values in percent.

Fig. 5.

SEM image of lime with 10000x magnification⁴².

Fly ash

The fly ash used in this study was obtained from the Eren Energy thermal power plant located in Zonguldak, Türkiye. The material is shown in Fig. 6. Fly ash is generally classified as Class C, Class F, or unclassified based on its chemical composition. In accordance with ASTM standard¹⁰, fly ash is classified as Class F when the sum of SiO₂, Al₂O₃, and Fe₂O₃ exceeds 50%, while the CaO content does not exceed 18%.Based on the chemical composition summarized in Table 2, the fly ash meets the requirements for Class F classification under ASTM standard¹⁰. Scanning electron microscopy (SEM) analysis (Fig. 7) reveals that the fly ash particles are predominantly spherical with a porous surface texture. The presence of hollow and irregular microspheres contributes to its relatively low density and influences pore structure modification when incorporated into soil matrices. The glassy (amorphous) phase observed in fly ash plays a critical role in its pozzolanic reactivity, as the dissolution of reactive silica and alumina under alkaline conditions enables the formation of secondary cementitious products⁴³. However, due to its relatively low CaO content, Class F fly ash does not exhibit strong self-cementing behavior. In freeze–thaw environments, the effectiveness of such fly ash in improving durability is therefore highly dependent on the availability of external calcium sources, such as lime, to activate pozzolanic reactions and enhance microstructural bonding.

Fig. 6.

Fly ash used in the study⁴².

Fig. 7.

SEM image of fly ash with 10000x magnification⁴².

Experimental program

Mixture codes

The mixture designations adopted in this study are summarized in Table 3.

Table 3.

Mixture codes for both soils.

Mixture Name	Code
Low plasticity clay	S
Low plasticity clay + 5% lime	S5L
Low plasticity clay + 30% fly ash	S30FA
Low plasticity clay + 30% fly ash + 5% lime	S30FA5L
High plasticity clay	B
High plasticity clay + 5% lime	B5L
High plasticity clay + 30% fly ash	B30FA
High plasticity clay + 30% fly ash + 5% lime	B30FA5L

The selected lime and fly ash contents were determined based on preliminary trials, commonly reported ranges in the literature, and practical considerations. The aim of this study is not to define an absolute optimum mixture but to investigate the effect of binder composition on freeze–thaw behavior. Results of unconfined compressive strength test for soils were obtained from doctoral thesis⁴² as shown in Fig. 8. The results indicate that increasing the fly ash (FA) content leads to a discernible trend in UCS values. Although the highest strength values are obtained within the 5–15% FA range, the UCS corresponding to 30% FA remains at an acceptable level for engineering applications. In particular, the gradual increase observed in the SFA series highlights the contribution of fly ash to binder formation and strength development. In this context, the selection of 30% FA is based on two primary considerations. First, although lower FA contents yield higher UCS values, the use of 30% FA does not result in a critical loss of strength, indicating that the structural integrity of the system is preserved even at high replacement levels. This provides a suitable balance between strength and material efficiency for soil stabilization applications. Second, fly ash is an industrial by-product, and its utilization at higher proportions contributes to waste valorization and promotes sustainable engineering practices. The use of 30% FA maximizes waste reuse while reducing the consumption of conventional binders and associated carbon emissions. Therefore, 30% FA represents an optimal balance between mechanical performance and environmental sustainability.

Fig. 8.

UCS results of mixtures⁴².

The selection of 5% lime was primarily guided by previous studies, which commonly report an optimum lime content within the range of 2–8%^44–46. The experimental results further support this choice, as UCS values show a significant increase when the lime content increases from 3% to 5%, whereas the improvement becomes limited or marginal beyond 5%. In addition, the optimum lime content was evaluated using ASTM D6276 standard⁴⁷. According to this standard, the minimum lime content that produces a pH value of 12.4 or higher should be selected, provided that the increase in pH with further lime addition is less than 0.04. As shown in Fig. 9, both S and B soils exceed the critical pH threshold at 5% lime content. The incremental pH changes beyond this level are 0.01 and 0.03 for S and B soils, respectively, satisfying the ASTM criterion. Therefore, 5% lime was also confirmed as the optimum content based on pH analysis. For mixtures containing 30% FA, the effect of lime becomes more pronounced. The combination of 30% FA and 5% lime results in a substantial increase in UCS, indicating that the limited binding capacity of fly ash alone is significantly enhanced through lime activation. Lime increases the system alkalinity, activating the amorphous phases of fly ash and accelerating pozzolanic reactions. The formation of additional cementitious products, such as C–S–H and C–A–H, leads to pore filling, improved particle bonding, and the development of a denser and more durable microstructure. The results further show that the 30% FA + 5% lime combination provides high strength performance, comparable to or more stable than mixtures with 7% lime, while requiring a lower amount of lime. This makes it a more sustainable and efficient alternative. Moreover, this combination enables higher utilization of industrial waste, reduces the demand for natural binders, and lowers the carbon footprint. Considering all these aspects, 30% fly ash, 5% lime, and their combined use (30% FA + 5% lime) were selected as the optimum mixture proportions in this study.

Fig. 9.

pH results of mixtures⁴².

Specimen preparation

All specimens were prepared using soil fractions passing through a No. 200 sieve. Following sieving, the materials were compacted using a Harvard Miniature Compactor with an energy level equivalent to the Standard Proctor effort, ensuring consistent compaction conditions across all mixtures. Both the natural soils and the stabilized blends were compacted at their corresponding optimum moisture contents (OMC).After mixing, the materials were sealed in plastic bags and stored for 24 h to allow uniform moisture distribution throughout the specimens. The prepared cylindrical specimens had an average height of 71 mm and a diameter of 33 mm, resulting in a height-to-diameter ratio slightly greater than 2, which satisfies the geometric requirements for unconfined compressive strength (UCS) testing. To prevent moisture loss and maintain constant water content during curing, the specimens were placed in a desiccator and cured for 28 days. To ensure reproducibility and consistency of the results, three replicate specimens were prepared for each mixture.

Freeze–thawing test

In cold-region environments, strength degradation of soils subjected to freeze–thaw (F–T) cycles may lead to severe structural distress. For soils containing a given moisture content, temperature reduction during the freezing phase induces volumetric expansion due to ice formation, whereas thawing results in softening associated with excess pore water generation. These processes reduce effective stress and shear strength, potentially causing settlement, cracking, and overall deterioration of structures founded on such soils. In this study, a closed-system freezing method was adopted for all mixtures subjected to F–T cycles. The closed-system approach was selected because fine-grained soils with low permeability require extended periods to re-establish moisture equilibrium. The limited duration between freezing and thawing phases in laboratory testing necessitates a controlled moisture condition to prevent artificial drainage or moisture redistribution that could alter the freeze–thaw response. Specimens were exposed to 2, 4, 6, 8, and 12 freeze–thaw cycles. The selected number of cycles was determined based on previous experimental studies^6,7,22,48,49, as well as the recommendations of ASTM standard⁵⁰. All specimens were cured for 28 days prior to F–T exposure. Freeze–thaw testing was conducted in accordance with ASTM standard⁵⁰. The number of cycles was selected based on previous studies in the literature^7,13,14, to simulate seasonal environmental effects. Each freeze–thaw cycle consisted of freezing the specimens at − 20 ± 2 °C for 24 h, followed by thawing at + 20 ± 2 °C for 24 h. During the freezing phase, specimens were placed in a freeze–thaw chamber at to ensure complete freezing of pore water. To ensure closed-system conditions, specimens were sealed with plastic film and stored in airtight containers to prevent moisture loss during the cycles. This procedure constituted one complete F–T cycle. Upon completion of each designated number of cycles (2, 4, 6, 8, and 12), specimens were subjected to unconfined compressive strength (UCS) testing to evaluate the evolution of mechanical performance under repeated freeze–thaw exposure.

Unconfined compression strength test

The compacted samples were tightly enclosed in plastic film to maintain their moisture content and stored in a desiccator for a 28-day curing period.Unconfined compressive strength (UCS) tests were performed according to ASTM standard⁵¹. The testing apparatus used in this study was a GDS triaxial testing system configured to perform UCS tests as shown in Fig. 10. One of the main advantages of this device is its ability to control displacement rate with five-digit precision and to automatically record data at 10-second intervals, ensuring high-resolution stress–strain measurements. The axial strain rate was selected as 1% per minute, which falls within the specified range of 0.5–2% per minute. Loading was continued until a sufficient number of data points, including the peak stress value, were obtained to clearly define the stress–strain response and failure behavior of the specimen.

Fig. 10.

Unconfined compressive strength test device and samples that were exposed to test.

Results and discussions

Compaction characteristics

The compaction curves obtained in accordance with ASTM standard⁴¹ for the Çaytaş (CL) and Çanbensan (CH) clays and their stabilized mixtures are presented in Figs. 11 and 12, respectively. For both soils, the addition of lime resulted in a decrease in maximum dry unit weight (MDUW) and an increase in optimum water content (OWC). This behavior is consistent with previous findings reported in the literature^29,52,53. The observed reduction in MDUW is attributed to flocculation–agglomeration reactions induced by calcium ion exchange, which lead to the formation of larger particle clusters and an open soil structure. The increase in OWC is associated with the additional water demand required for hydration and pozzolanic reactions. In contrast, the incorporation of 30% fly ash resulted in a decrease in OWC and a slight increase in MDUW. This trend is primarily related to the predominantly silt-sized and spherical morphology of fly ash particles, which improve packing efficiency and reduce water demand during compaction. In the S30FA mixture, the OWC and MDUW changed by 17.62% and 0.75%, respectively. For the B30FA mixture, the corresponding changes were 21.80% and 7.44%. While the MDUW of the low-plasticity clay remained nearly unchanged after fly ash addition, a more pronounced increase was observed in the high-plasticity clay. This difference can be attributed to the lower silt fraction in the Çanbensan clay compared to the Çaytaş clay, allowing fly ash particles to more effectively modify the particle size distribution and improve compaction characteristics. The trend observed in OWC variation was similar for both soils, reflecting the influence of fly ash on moisture demand and particle arrangement. From a cold-regions perspective, these compaction modifications are particularly relevant, as changes in density and moisture content directly affect freeze–thaw susceptibility by influencing pore structure, degree of saturation, and water migration potential.

Fig. 11.

Compaction characteristics of Çaytaş clay and mixtures.

Fig. 12.

Compaction characteristics of Çanbensan clay and mixtures.

Plasticity characteristics

The consistency limits of the soils and their respective mixtures were determined in accordance with ASTM standard³⁸. The resulting liquid limit, plastic limit, and plasticity index values are presented in Figs. 13 and 14.

Fig. 13.

Consistency characteristics of Çaytaş clay and mixtures.

Fig. 14.

Consistency characteristics of Çanbensan clay and mixtures.

For the low-plasticity Çaytaş clay (CL), the addition of 5% lime resulted in substantial increases in both liquid limit and plastic limit, by 47% and 63%, respectively, while causing only a minor change in the plasticity index (an increase of 16.67%). Although the magnitude of increase varied slightly, a similar trend was observed for the S30FA5L mixture. In contrast, the S30FA mixture exhibited a significant reduction in plasticity index, reaching approximately 0.42 times that of the untreated Çaytaş clay. For the high-plasticity Çanbensan clay (CH), the incorporation of fly ash and/or lime led to pronounced reductions in liquid limit. The liquid limits of B30FA, B5L, and B30FA5L mixtures decreased by 40.87%, 58.7%, and 47.39%, respectively, compared to the natural soil. While the plastic limit values of B30FA and B5L were comparable, the B30FA5L mixture exhibited a 31.91% increase in plastic limit. The plasticity index values of B30FA, B5L, and B30FA5L decreased to approximately 0.49, 0.26, and 0.32 times that of the untreated soil, respectively. Since the plasticity index is an indicator of swelling potential and strength characteristics, its reduction—although not directly proportional—generally suggests improved mechanical performance and reduced expansivity^29,52,54.

Effect of F-T cycles on stress-strain behavior of mixtures

Unconfined compressive strength (UCS) tests were conducted on all mixtures subjected to 0, 2, 4, 6, 8, and 12 freeze–thaw (F–T) cycles. The corresponding stress–strain responses are presented in Fig. 15. For the Çaytaş soil (S), the stress–strain curves shown in Fig. 15a indicate a comparatively more brittle behavior relative to other F–T conditions. The influence of F–T cycling on strain at failure (SAF) is particularly pronounced after two cycles. While the untreated S soil exhibited a failure strain of 3.33%, this value increased sharply to approximately 10% after 2 and 4 F–T cycles. With further increases in the number of cycles, the failure strain gradually decreased, stabilizing between 7.67% and 8.5% beyond six cycles. The volumetric expansion of pore water during the freezing phase is primarily responsible for this behavior. Transition from freezing to thawing leads to increased void ratio and microcrack development, making it difficult for the specimen to regain its original structural integrity⁸. Moreover, the irregular relationship observed between F–T cycle number and SAF may be attributed to microstructural instability within the soil fabric, as repeated freezing and thawing do not impose a strictly linear or uniform degradation pattern on the crystalline structure⁵⁵.

Fig. 15.

Stress–strain response of mixtures for all F-T cycles a S, b S30FA, c S30FA5L, d S5L, e B, f B30FA, g B30FA5L, h B5L.

The S30FA mixture exhibited a noticeable reduction in initial stiffness even after only two freeze–thaw (F–T) cycles. In particular, the specimen subjected to 12 cycles displayed more ductile behavior, as evidenced in Fig. 13b, where the stress level remained nearly constant beyond approximately 7% axial strain. Compared to the untreated S soil, the influence of F–T cycling on the strain at failure (SAF) of the S30FA mixture was less pronounced. While the SAF of the non-exposed S30FA specimen was 5.17%, the values for F–T-treated specimens ranged between 5.33% and 6.83% depending on the number of cycles. For the S30FA5L and S5L mixtures, the effect of freeze–thaw cycling on SAF was relatively insignificant, as clearly observed in Fig. 13c and d. The S30FA5L mixture demonstrated more ductile behavior compared to S5L, with stress levels tending to stabilize beyond approximately 5% axial strain for all F–T cycles (except the 0-cycle condition). The SAF of the untreated S30FA5L specimen was 2.50%, whereas values for the F–T-treated specimens remained approximately 2.67% (except for the 12-cycle case). In contrast, the S5L mixture exhibited comparatively more brittle behavior than the mixtures shown in Figs. 15a–c. The effect of freeze–thaw cycling on SAF for S5L was negligible, as values consistently ranged between 2.33% and 3.33% across all cycle conditions, including the untreated state.

The Çanbensan soil (B) exhibited a more ductile response compared to the Çaytaş soil (S). As shown in Fig. 15e, the brittleness observed after 2 and 8 freeze–thaw (F–T) cycles was comparable to the untreated B soil, whereas the remaining cycles displayed relatively more brittle behavior. The influence of F–T cycling on strain at failure (SAF) followed a trend similar to that observed for the S soil. While the untreated B soil exhibited a failure strain of 5.83%, this value increased to 7.67% and 10.17% after 2 and 4 cycles, respectively. With further increases in the number of cycles, SAF gradually decreased and stabilized between 7.33% and 8.33% beyond eight cycles. As illustrated in Fig. 15f, the initial stiffness of the B30FA mixture decreased markedly even after only two F–T cycles, similar to the trend observed for S30FA. However, the SAF behavior of B30FA differed from that of S30FA, as no pronounced variation was observed among different F–T cycles. While the untreated S30FA specimen exhibited an SAF value of 3.17%, the B30FA mixture showed SAF values ranging between 5.17% and 7.17%. For the B30FA5L and B5L mixtures, the effect of freeze–thaw cycling on SAF was also negligible, as clearly indicated in Fig. 15g and h, consistent with the behavior observed for S30FA5L and S5L mixtures. The B30FA5L mixture demonstrated slightly more ductile behavior than B5L. As shown in Fig. 13g, variations in stress response were observed for all F–T cycles except the 2-cycle condition. The SAF value of the untreated B30FA5L specimen was 1.83%, while values for the F–T-treated specimens ranged between 1.17% and 2.33%. Overall, the B5L and B30FA5L mixtures exhibited comparatively more brittle behavior than the mixtures presented in Fig. 13e and f. The influence of freeze–thaw cycling on SAF remained minimal, as values consistently ranged between 1.67% and 3.17% across all cycle conditions, including the untreated state.

Effect of F-T cycles on modulus of elasticity

Another parameter derived from the stress–strain curves is the secant modulus. In this study, the secant modulus (E_s) was determined by dividing 50% of the peak stress by the corresponding axial strain, which represents one of the most widely adopted approaches for evaluating stiffness in stabilized soils^56–58. In general, the secant modulus of both untreated soils and stabilized mixtures decreased with increasing freeze–thaw (F–T) cycles (Figs. 16, 17, 18, 19, 20, 21, 22 and 23). However, lime-containing mixtures exhibited a distinct trend during the early stages of F–T exposure, as ongoing pozzolanic reactions had not yet been completed. In these cases, the strengthening effect of cementation temporarily outweighed freeze-induced deterioration. As the number of F–T cycles increased further, microstructural damage became dominant, leading to concurrent reductions in both strength and stiffness. For the Çaytaş clay (S), the initial secant modulus under non-cyclic conditions was approximately 17,261 kPa (Fig. 16). Even two F–T cycles reduced this value to 2,633 kPa, corresponding to roughly 15% of the original stiffness. Beyond eight cycles, the modulus remained nearly constant at approximately 1,280 kPa. Although the S30FA mixture exhibited approximately 25% higher UCS than the untreated S soil, its secant modulus was about 18% lower (Fig. 17), primarily because the untreated soil reached peak stress at lower axial strain. With increasing F–T cycles, the secant modulus of S30FA progressively decreased; however, the rate of reduction was less severe than that observed for the untreated soil. Two cycles reduced stiffness by nearly half, while between 4 and 12 cycles, the modulus stabilized within the range of 3,594–4,922 kPa. The lowest modulus value for S30FA occurred after eight cycles, corresponding to approximately 25.35% of its initial value. The addition of 5% lime to fly ash (S30FA5L) nearly doubled the secant modulus under non-cyclic conditions (Fig. 18). For 2, 4, and 6 cycles, the modulus remained higher than the 0-cycle soil value, ranging between approximately 48,217 and 67,132 kPa (about 2.5–4 times that of untreated soil). After six cycles, reductions in secant modulus became consistent with the observed UCS decline. In the S5L mixture (Fig. 19), the secant modulus initially increased by approximately 3.5 times prior to F–T exposure, followed by a gradual decrease. Once the number of cycles reached eight, microstructural degradation became more influential than cementation effects, resulting in pronounced reductions in both strength and secant modulus.

Fig. 16.

Secant modulus variation with F-T cycle for S.

Fig. 17.

Secant modulus variation with F-T cycle for S30FA.

Fig. 18.

Secant modulus variation with F-T cycle for S30FA5L.

Fig. 19.

Secant modulus variation with F-T cycle for S5L.

Fig. 20.

Secant modulus variation with F-T cycle for B.

Fig. 21.

Secant modulus variation with F-T cycle for B30FA.

Fig. 22.

Secant modulus variation with F-T cycle for B30FA5L.

Fig. 23.

Secant modulus variation with F-T cycle for B5L.

For the Çanbensan clay (B), the initial secant modulus under non-cyclic conditions was approximately 12,143 kPa (Fig. 20). After four freeze–thaw (F–T) cycles, the modulus decreased to 3,519 kPa, corresponding to approximately 29% of its original value. No substantial difference was observed between four and eight cycles; however, after twelve cycles, the modulus declined to nearly 10% of the initial stiffness. These results clearly indicate that the untreated clay was more severely affected by F–T cycling compared to stabilized mixtures. Although the B30FA mixture exhibited approximately 25% higher UCS than the untreated soil, its secant modulus was about 18% lower (Fig. 21), reflecting differences in strain development at peak stress. With increasing F–T cycles, the secant modulus of B30FA progressively decreased, although the reduction trend differed from that observed in S30FA. Two cycles reduced stiffness to approximately 45% of its original value, while between four and twelve cycles, the modulus stabilized within the range of 5,019–6,493 kPa. The lowest modulus value occurred after eight cycles, corresponding to approximately 20% of its initial value. The addition of 5% lime to fly ash (B30FA5L) increased the secant modulus by approximately 3.7 times under non-cyclic conditions (Fig. 22). For two and four cycles, the modulus remained higher than the 0-cycle soil value; however, beyond six cycles, a decreasing trend was observed. In the B5L mixture (Fig. 23), the secant modulus initially increased nearly fivefold prior to F–T exposure. After a slight reduction at two cycles, a noticeable increase was observed up to six cycles. When the number of cycles reached eight, the destructive effects of freeze-induced chemical and microstructural degradation became dominant, resulting in a sharp decline in secant modulus.

Figure 24 illustrates the relationship between unconfined compressive strength (UCS) and secant modulus for the Çaytaş clay (S), Çanbensan clay (B), and their stabilized mixtures under 0, 2, 4, 6, 8, and 12 freeze–thaw (F–T) cycles. A clear linear correlation was observed between the secant modulus (E_s) and UCS, expressed as E_s = 55.898q_u ​– 5209.4, with a coefficient of determination R² = 0.882 indicating a strong stiffness–strength dependency consistent with findings reported in the literature⁵⁷.

Fig. 24.

Secant modulus – Unconfined compressive strength relation for all mixtures including soils.

Effect of CaO, CaAl₂O₃, CaO/SiO₂, Ca/(SiO₂ + Al₂O₃)

Silica (SiO₂) forms the primary basis of pozzolanic reactions by reacting with lime to promote the formation of calcium silicate hydrate (C–S–H) gels, while alumina (Al₂O₃) contributes to the formation of calcium aluminate hydrate (C–A–H) phases. Therefore, both oxides play a critical role in activating and sustaining pozzolanic reactions^59,60.

The oxide compositions obtained from XRF analysis are summarized in Table 4. The oxide ratios (CaO, CaO/Al₂O₃, CaO/SiO₂, and CaO/(SiO₂+Al₂O₃)) were determined directly from the XRF analysis of each stabilized mixture. The measured oxide contents (in wt%) were used to calculate the ratios for each blend. No theoretical estimation based on mixture proportions was employed; instead, all ratios were derived from the experimentally obtained chemical compositions of the corresponding specimens.

Table 4.

Chemical compositions of soils and mixtures.

Component	B	B30FA	B5L	B30FA5L	S	S30FA	S5L	S30FA5L
	(%)	(%)	(%)	(%)	(%)	(%)	(%)	(%)
SiO2	65.75	60.29	57.83	57.6	58.43	57.84	55.16	56.93
Al2O3	17.42	18.13	15.52	17.18	31.42	31.78	30.93	27.92
Fe2O3	7.79	8.44	8.08	8.64	3.72	4.76	3.80	5.08
K2O	1.49	1.77	1.30	1.73	3.23	2.90	2.72	2.68
CaO	–	4.47	9.21	7.59	–	0.71	6.12	3.24
TiO2	–	–	1.16	1.14	2.07	–	–	2.06
MgO	3.16	2.75	3.01	2.68	0.92	1.15	1.06	1.19
SO3	0.61	0.85	0.73	0.91	0.04	0.29	0.01	0.34
Na2O	3.59	2.77	2.86	2.36	–	–	–	–

The interactions among mineral constituents introduced by chemical additives significantly influence strength development, particularly CaO content and the oxide ratios CaO/Al₂O₃, CaO/SiO₂, and CaO/(SiO₂ + Al₂O₃), which are recognized as key controlling parameters^7,23. Figure 25 presents the UCS results of all soils and mixtures (S, B, S30FA, S5L, S30FA5L, B30FA, B5L, and B30FA5L), including the results of three replicates for consistency. The untreated soils correspond to uncured 0-day strengths, whereas stabilized mixtures represent 28-day cured specimens subjected to the specified F–T cycles. An increasing trend in strength with increasing CaO content is evident. For mixtures containing lime, the UCS values after 2 and 4 F–T cycles exceeded those of the 0-cycle condition, and even the 6-cycle strengths remained nearly comparable to the initial value, particularly for mixtures containing 3.24% and 4.47% CaO. This behavior suggests that cementation gains initially outweigh freeze-induced degradation; however, with further cycling, deterioration gradually becomes dominant. When CaO content was 0%, UCS ranged between approximately 200 and 253 kPa, whereas increasing CaO to 3.24% raised the strength range to 482–1452 kPa across all F–T conditions for the S30FA5L mixture. More pronounced strength gains were observed once CaO content exceeded approximately 6%, in agreement with⁷. Similar trends were identified for CaO/Al₂O₃, CaO/SiO₂, and CaO/(SiO₂ + Al₂O₃) ratios. As shown in Fig. 26, strength enhancement became evident when CaO/Al₂O₃ exceeded 0.44, whereas it has been suggested a threshold value of at least 1 for same situation⁷. For the CaO/SiO₂ ratio (Fig. 27), accelerated strength gains were observed when the ratio exceeded 0.1, compared to the 0.5 threshold proposed in the previous study. The discrepancy may be attributed to the inclusion of both low- and high-plasticity clays in the present investigation, whereas the earlier study focused solely on high-plasticity clay. Similarly, as shown in Fig. 28, the CaO/(SiO₂ + Al₂O₃) ratio exhibited a distinct strength-increasing trend once the value exceeded approximately 0.10. The UCS values exhibit an increasing trend with rising CaO, CaAl₂O₃, CaO/SiO₂, and Ca/(SiO₂ + Al₂O₃) ratios, attaining maximum values at approximately 7.7, 0.44, 0.13, and 0.10, respectively. Beyond these threshold values, a slight reduction in strength is observed, indicating the presence of an optimum chemical composition.

Fig. 25.

CaO effect on unconfined compressive strength relation for all mixtures including soils.

Fig. 26.

CaO/Al₂O₃ effect on unconfined compressive strength for all mixtures including soils.

Fig. 27.

CaO/SiO₂ effect on unconfined compressive strength for all mixtures including soils.

Fig. 28.

CaO/(Al₂O₃ + SiO₂) effect on unconfined compressive strength for all mixtures including soils.

The findings of this study are consistent with previous researches^61,62, which demonstrated that the behavior of lime-treated soils is primarily governed by pozzolanic reactions and chemical interactions within the soil matrix. These studies emphasized the role of pH evolution in promoting the dissolution of reactive oxides such as SiO₂ and Al₂O₃, leading to the formation of cementitious compounds responsible for strength development. Furthermore, it has been reported that chemical composition, particularly the presence of sulphates, can significantly influence stabilization performance and durability⁶². These findings are in agreement with the results of the present study, where the combined use of fly ash and lime improved the mechanical performance and freeze–thaw resistance of the stabilized soil.

Effect of F-T cycles on unconfined compression strength of mixtures

The unconfined compressive strength (UCS) values obtained for all mixtures under different freeze–thaw (F–T) cycles are presented in Fig. 29. The plotted curves represent the average values of three replicate specimens for each mixture.

Fig. 29.

UCS versus freeze-thaw cycles for all mixtures of (a) S (b) B F–T cycles.

As shown in Fig. 27a and b, the UCS values of the untreated soils (S and B) exhibit a decreasing trend with increasing F–T cycles. Microstructural degradation, changes in chemical composition, and volumetric expansion–contraction associated with freezing and thawing collectively contribute to strength reduction^{7,21,23,49,63}. Since the Class F fly ash used in this study does not possess strong self-cementing properties, strength reduction began as early as the second F–T cycle in fly ash-only mixtures 7. During the freezing phase, pozzolanic reactions tend to slow down due to reduced temperature, whereas partial reactivation may occur during thawing^23,63–65,. Furthermore, although specimens were cured for 28 days, cementation reactions may not have been fully completed. In mixtures containing high CaO content and lime stabilization, the early cementation gains appear to temporarily outweigh freeze-induced deterioration, which explains why the first four F–T cycles did not significantly reduce UCS values. However, as cyclic temperature fluctuations continued, strength degradation became evident after six cycles. Nevertheless, even after twelve F–T cycles, the UCS values of lime- and lime–fly ash-stabilized mixtures remained higher than those of untreated S and B soils not subjected to F–T exposure. This contrast is particularly notable when compared to fly ash-only mixtures, whose UCS values continuously decreased with increasing cycle number. The overall findings are consistent with previous studies^6,7. In the absence of a calcium-based cementitious binder, both untreated soils and fly ash-only mixtures exhibit progressive strength deterioration under repeated F–T cycling, a trend widely reported in the literature^{7,22,24,25,66}.

XRD analysis

XRD analysis was conducted on samples to learn about the minerals on them. This analysis is conducted with a PANalytical Empyrean X-Ray Diffractometer with Cu-Kα radiation at a voltage of 45 kV and wavelength of 1.54 Å. Scanning speed is 2°/min, and continuous scan mode is selected. Electrical current used in this analysis is 40 mA. To prepare the samples for analysis, they were dried at 60 °C in an oven for 24 h and soil to make them finer. Afterwards, specimens were stored for 24 h in a sealed plastic bag in a condition of 100% relative humidity at a temperature of 22 ± 2 °C in an air conditioning cabinet. All samples were stored in the same air conditioning cabinet at room temperature (22 ± 2 °C) with a lower humidity (approximately 30%) for about 90 min before XRD analysis.

Results of expansive soil and the mixtures were given in Fig. 30. Montmorillonite (M) peaks at 2θ = 8.82°. This value is approximately equal to 10 Å. This is 12–15 Å for the air-cured sample. Drying in the oven makes the interlayer water disappear. This makes the peak values 10 Å⁶⁷. Peak intensity values of montmorillonite decrease for B30FA, B30FA5L, and B5L compared to B. Reduction rates for B30FA, B30FA5L, and B5L mixtures are 12.88%, 26.91%, and 24.85%, respectively. This indicates the decomposition and breakage of the montmorillonite mineral structure⁶⁸. The peak broadness of B30FA seems to decrease since silty particles of fly ash are replaced with clay particles of B soil⁶⁹. Calcium silicate hydrate (C-S-H) peaks were not observed in the B30FA and S30FA mixture since fly ash used in this study has low CaO content (2.49%). There may be some fly ash-treated mixtures having C-S-H peaks even if they are F-type, as known⁷⁰. S30FA5L, S5L, B30FA5L and B5L mixtures contain C-S-H peaks since they contain cementing agents, and their CaO content is higher than S30FA and B30FA. Remarkable results obtained here are that no ettringite peak was observed. Detecting ettringite peaks is challenging since the peak intensities of minerals except ettringite are high, and XRD is a rapid analysis⁷¹. Peak intensity values for S5L, S30FA5L, B5L and B30FA5L mixtures are lower than their respective soils. The main reason for this situation can be attributed to the reduction in clay minerals in stabilized soil because of the pozzolanic reaction^72,73.

Fig. 30.

XRD of soil and mixtures.

Practical implications and limitations

In field applications, the binder system consisting of 30% fly ash and 5% lime can be implemented using conventional soil stabilization techniques. The in-situ soil is first loosened to the desired depth and prepared by removing oversized particles and organic matter. Subsequently, lime and fly ash are uniformly spread over the soil surface and mixed using a grader to achieve a homogeneous mixture. Water is then added to reach the optimum moisture content. The resulting mixture is compacted using appropriate compaction equipment to achieve the target density, typically at least 95% relative compaction. Finally, the stabilized layer is cured under controlled moisture conditions to ensure adequate strength development. This procedure enables the effective transfer of laboratory findings to field-scale applications.

Despite these promising results, several limitations should be carefully considered when applying laboratory findings to real field conditions. While parameters such as moisture content, compaction energy, and curing conditions can be well controlled in laboratory environments, achieving uniform mixing and consistent compaction in the field may be challenging. In addition, field conditions involve more complex factors, including temperature fluctuations, repeated traffic loading, drainage conditions, and dynamic effects, all of which may influence the strength and durability of the stabilized soil. Therefore, appropriate precautions should be taken when extrapolating laboratory-scale results to field applications. The selected number of freeze–thaw cycles is considered representative of seasonal environmental conditions commonly encountered in cold regions. Previous studies suggest that 5–15 cycles can reasonably simulate short- to medium-term field exposure. However, it should be noted that actual field conditions may involve a larger number of cycles, and therefore the results should be interpreted within this experimental framework.

Pozzolanic reactions in fly ash–stabilized systems may continue over extended curing periods, typically beyond 28 days and up to 56–90 days or longer, contributing to ongoing strength development^74,75. Experimental studies have also shown that significant increases in UCS occur with extended curing durations, particularly between 28 and 56 days^76–78. However, in systems containing low-calcium fly ash, the limited CaO content may restrict the extent of these reactions, potentially reducing their efficiency. Therefore, further studies incorporating longer curing durations are recommended to better evaluate long-term strength evolution.

From an economic and environmental perspective, the binder system containing 30% fly ash and 5% lime can generally be considered feasible. Fly ash, as an industrial by-product, is a low-cost and widely available material, making it more economical compared to conventional stabilizers such as cement. Moreover, the beneficial use of large quantities of industrial waste accumulated in disposal sites contributes positively to environmental sustainability. Compared to cement-based stabilization, the use of fly ash results in lower carbon emissions, supporting more sustainable construction practices. The addition of lime also enhances early strength development. However, the economic feasibility of fly ash depends on the distance to the project site and associated transportation costs.

The mechanical behavior of stabilized soils is highly sensitive to initial water content and compaction conditions. In the present study, all specimens were prepared at optimum moisture content (OMC) and maximum dry density (MDD) to ensure uniformity across mixtures. This approach minimizes variability and allows for a consistent comparison of binder effects. However, it should be noted that deviations from OMC or changes in compaction energy may significantly affect strength and durability. Therefore, the results presented herein are representative of optimum compaction conditions, and further studies are required to evaluate the sensitivity of the mixtures under varying field conditions.

Conclusions and summary

This study investigated the deformation and strength behavior of soft (S) and expansive (B) soils stabilized with fly ash alone, lime alone, and lime–fly ash mixtures under freeze–thaw (F–T) exposure. The stress–strain characteristics of the mixtures and the influence of F–T cycles on unconfined compressive strength (UCS) were systematically evaluated. The principal findings are summarized as follows:

1. The stress–strain responses of the S and B soils were generally similar. The strain at failure (SAF) increased after the first two F–T cycles and subsequently stabilized at comparable levels. Strain-softening behavior was clearly observed in S, B, S30FA, and B30FA mixtures. This behavior was evident in the S5L mixture only after 2 and 4 cycles, whereas it was consistently observed in the S30FA5L mixture across all cycle conditions. In the B30FA5L and B5L mixtures, strain-softening behavior was less pronounced compared to S30FA5L and S5L; however, B30FA5L exhibited slightly greater softening relative to B5L.

2. The variation in UCS with increasing F–T cycles differed markedly between lime-containing mixtures and fly ash-only mixtures. The cementitious effect of CaO provided resistance against freeze–thaw deterioration up to six cycles, as evidenced by the nearly unchanged UCS values of S30FA5L, S5L, B30FA5L, and B5L mixtures compared to their respective 0-cycle conditions. Notably, even after 12 F–T cycles, the UCS values of lime- and lime–fly ash-stabilized mixtures remained higher than those of the untreated soils. In contrast, fly ash-only mixtures (S30FA and B30FA), lacking self-cementing capability, exhibited progressive strength degradation with increasing F–T cycles, following trends similar to those of the natural soils.

3. Considering the combined UCS performance and brittleness characteristics derived from the stress–strain curves, the S30FA5L and B5L mixtures may be regarded as optimal stabilization alternatives for their respective soils. The B30FA5L mixture also demonstrated performance comparable to B5L. The Class F fly ash used in this study is widely available and cost-effective; however, its disposal presents environmental challenges. From a practical standpoint, mixtures containing 30% fly ash and 5% lime offer a balanced solution in terms of mechanical performance, economic feasibility, and environmental sustainability, particularly for cold-region applications where freeze–thaw durability is critical.

4. The observed improvements in strength and stiffness of the stabilized soils can be attributed to the combined effects of calcium availability and pozzolanic reactions. The addition of lime increases the concentration of calcium ions in the system, leading to the formation of calcium hydroxide (Ca(OH)₂), which subsequently reacts with the amorphous silica (SiO₂) and alumina (Al₂O₃) present in both the soil and fly ash. These reactions result in the formation of cementitious compounds such as calcium silicate hydrate (C–S–H) and calcium aluminate hydrate (C–A–H), which bind soil particles together and enhance the overall structural integrity of the matrix. As a result, higher CaO content and favorable oxide ratios contribute to improved mechanical performance.

5. The influence of freeze–thaw cycles can be interpreted through changes in the pore structure of the stabilized soil. During freezing, the expansion of pore water generates internal stresses that induce microcracking, while subsequent thawing leads to an increase in pore volume and connectivity. However, in stabilized soils, the formation of cementitious products reduces pore size and connectivity, thereby limiting water migration and mitigating freeze–thaw damage. This explains the relatively lower degradation observed in specimens with higher binder content and more effective pozzolanic reactions.

6. The transition from brittle to more ductile behavior with increasing binder content can be explained by the evolution of soil fabric and bonding mechanisms. In untreated or lightly treated soils, failure tends to be brittle due to weak interparticle contacts. In contrast, the formation of a continuous cementitious matrix in fly ash–lime mixtures promotes stress redistribution and energy dissipation, resulting in a more ductile response under loading. This behavior is particularly beneficial under cyclic environmental conditions such as freeze–thaw exposure.

7. The results indicate that the mechanical performance and durability of stabilized soils are governed not only by the amount of binder but also by the chemical composition and resulting microstructural development. The integration of oxide-based ratios with mechanical parameters provides a more comprehensive understanding of the stabilization mechanism and supports the development of composition-based design approaches. Further micro-level analysis can be performed in order to determine the micro-behavior better.

8. While the results provide valuable insight into the short-term durability of stabilized soils, direct extrapolation to long-term performance (i.e., higher numbers of F–T cycles) should be made with caution. Progressive degradation mechanisms may become more pronounced at higher cycle numbers, and further studies are required to evaluate long-term behavior. Although specific threshold values for oxide composition cannot be definitively established based on the present dataset, the results indicate that sufficient availability of reactive silica, alumina, and calcium is essential for effective pozzolanic reactions.

9. Compared to traditional stabilizers such as cement and slag, lime–fly ash mixtures offer a more sustainable alternative with lower carbon emissions, while still providing adequate mechanical performance.

Author contributions

Conceptualization, Ö.F.Ç and E.D.; methodology, Ö.F.Ç.; E.D.; validation, E.D., Ö.F.Ç. and S.S.; resources, Ö.F.Ç. and E.D.; writing—original draft preparation, E.D.; writing—review and editing, E.D., Ö.F.Ç, SS. All authors have read and agreed to the published version of the manuscript.”

Funding

This research was funded by Zonguldak Bülent Ecevit University, grant number 2017-37891158-02.

Data availability

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.