Introduction

Construction on expansive clay soils often leads to engineering problems such as differential and nonsymmetric settlements. Soil stabilization is widely used to improve the mechanical properties of problematic soils. Lime is widely used as a stabilizer to improve the mechanical properties of fine-grained parts. It should be noted that the soil provides the active materials for lime hydration reactions1,2,3,4,5. Therefore, adding alternative materials containing high amounts of active particles, such as silicon dioxide (SiO₂) or aluminum oxide (Al₂O₃), can improve hydration reactions and reduce the amount of lime used for soil stabilization6,7 (Kulkarni et al., 2023). On the other hand, nanomaterials that contain active particles have high chemical reactivity due to their microscopic dimensions, which improves the formation of hydration reaction products. Nano silica and nano-aluminum oxide (nano-Al₂O₃) are among these materials8,9,10,11,12.

In recent years, increasing attention has been directed toward the synergistic use of lime and nanomaterials for soil stabilization. Several studies have shown that incorporating reactive nanomaterials into lime-treated soils can significantly enhance their mechanical performance. Ghorbani et al.13 investigated the combined use of lime with nano-SiO₂ and nano-ZnO in high-plasticity clay and reported substantial improvements in unconfined compressive strength (UCS) and California Bearing Ratio (CBR) compared with lime-treated soil alone. These strength gains were associated with a denser microstructure and improved interparticle bonding, as observed in SEM images. Akbari et al.14 demonstrated that partial replacement of lime with nano-zeolite, a natural pozzolanic nanomaterial, improves the mechanical behavior of lime-stabilized kaolin clay. Their results indicated that nano-zeolite enhances pozzolanic reactions by promoting the consumption of Ca(OH)₂ and increasing the formation of cementitious hydration products. These findings were supported by both SEM and XRD analyses. Similarly, Khodaparast et al.15 reported that adding nano-zinc oxide (nano-ZnO) to lime-treated silty clay leads to notable increases in strength and shear parameters. At an optimum nano-ZnO content of approximately 1.5%, the internal friction angle and cohesion increased by about 25% and 37%, respectively. These improvements were attributed to accelerated pozzolanic reactions and refinement of the soil microstructure. Karimiazar et al.16 evaluated the effect of adding nano-silica and lime on the mechanical properties of clayey sand by performing UCS, direct shear tests, and X-ray diffraction (XRD) tests. This research showed that adding 1.2% nano-silica to clayey sand stabilized with 8% lime increased the effective internal friction angle and cohesion by 17 and 32% at the 28-day curing time. Joju and Chandrakaran17 reported that the incorporation of nano-silica into lime-stabilized marine clay increased UCS by up to three times compared with untreated soil, while also reducing the optimum lime content.

In this study, the mechanical behavior of high-plasticity clay (CH) stabilized with a combination of lime and nano-aluminum oxide is systematically investigated. The combined effects of different lime and nano-Al₂O₃ contents, along with curing time, on compaction characteristics, unconfined compressive strength, indirect tensile strength, ultrasonic pulse velocity, and microstructural features are evaluated using UCS, ITS, UPV, SEM, and XRD analyses. In addition, for the first time, meaningful empirical relationships between ultrasonic pulse velocity (UPV) and strength parameters (UCS and ITS) are proposed for CH clay stabilized with lime and nano-Al₂O₃, enabling nondestructive assessment of the mechanical behavior of the treated soil. Furthermore, the obtained mechanical results are interpreted through detailed SEM and XRD analyses to elucidate the microstructural mechanisms responsible for the observed strength improvement.

Experimental program

Materials

This research uses clay with Gs = 2.65, PL = 22%, LL = 53% and PI = 31%. According to the Unified Soil Classification System, this soil is in the CH category. The grain size distribution curve is shown in Fig. 1. Also, lime with Gs = 2.3 was used. The chemical composition of lime is presented in Table 1. Also, the physical properties of nano-Al₂O₃ used in this research are presented in Table 2.

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Grain size distribution curve of the CH clayey soil.

Table 1 Chemical composition of lime.
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Table 2 Characteristics of nano-Al2O3.
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Sample preparation

In this study, lime was used at 0%, 3%, 6%, 9%, and 12% by dry weight of soil, and nano-Al₂O₃ was added at 0%, 0.6%, 1%, 1.2%, and 1.4% by dry weight of lime. The soil was first oven-dried at 105 °C for 24 h. Then, the dried soil and additives were mixed in a dry state for approximately 10 min using a laboratory mixer to achieve an initial uniform distribution. After that, water equivalent to the optimum moisture content obtained from the standard compaction test was gradually added to the mixture in the form of a spray. At the same time, hand-mixing was performed simultaneously to ensure uniform moisture distribution and prevent the formation of clay aggregates. Thereafter, the materials were combined with a mechanical mixer for an additional 5 min to achieve final homogeneity18,19. The specimens were compacted at 95% of the MDD and at the OMC to represent practical field compaction conditions. This level ensures efficient utilization of compaction energy and realistic strength behavior of stabilized soils, as commonly recommended in previous studies18,19,20,21,22,23,24,25,26,27,28. The UCS test samples were prepared in molds with a diameter of 50 mm and a height of 100 mm, while the ITS test samples were prepared in molds with a diameter of 50 mm and a height of 75 mm. All specimens were prepared in five layers using a static compaction method. After compaction, the samples were sealed in two plastic bags at a temperature of 22 ± 2 °C for 7, 28, and 90 days. This procedure is consistent with common practices in lime stabilization studies, as in field conditions, the surfaces of stabilized soils are also typically maintained under moisture-retaining covers18,19,20,29,30. The curing times of 7, 28, and 90 days were selected to investigate the time-dependent evolution of soil stabilized with lime and nano-Al₂O₃. The formation and growth of hydration and pozzolanic products associated with lime-based reactions progress gradually over time, particularly at longer curing ages. The continuation of these reactions improves the structural stability and integrity of the cementitious matrix. For this reason, a 90-day curing period was selected to examine the long-term behavior of the stabilized matrix. This curing duration was not intended for classical durability evaluation based on wet–dry cycles, freeze–thaw cycles, or cyclic loading. The focus was on the microstructural and chemical evolution of the cementitious matrix over curing time.

For each test condition, three samples were prepared and tested, and the average results were reported. The standard deviation of the measurements ranged from 3 and 5%, indicating acceptable accuracy and repeatability of the results.

Testing program

This research performed the standard Proctor compression test, unconfined compressive strength test, and indirect tensile strength test according to ASTM D69831, ASTM D216632 and ASTM D396733 based on strain control. After curing the sample and before the UCS and ITS tests, a non-destructive ultrasonic pulse velocity test was performed according to ASTM D284534. In this test, piezoelectric transducers operating at a frequency of 54 kHz were used in through-transmission mode. To ensure proper contact between the specimen surfaces and the transducers, silicone gel was applied as a coupling agent to minimize energy loss during wave transmission. The device was calibrated before each testing using an acrylic reference bar with a known pulse travel time of 26 μs to ensure the accuracy and consistency of the measurements30,35,36,37. In this study, the UPV test was used as a non-destructive method to assess the mechanical and microstructural behavior of stabilized soils. The results were compared with those from destructive tests to serve as reference data.

In addition, scanning electron microscopy (SEM) analysis was performed to investigate the microstructural characteristics of soils stabilized with lime and nano-Al₂O₃. Furthermore, X-ray diffraction (XRD) analysis was conducted on selected specimens cured for 90 days to identify the mineral phases and reaction products at a mature curing stage.

Results and discussion

Standard proctor compaction test

The results of standard Proctor compaction tests on CH clay containing different contents of lime (0, 3, 6, 9, and 12% by weight of dry soil) and nano-Al₂O₃ (0, 0.6, 1, 1.2 and 1.4% weight of lime) is presented in this section. At first, standard proctor compaction and UCS tests were performed to select the optimum content of lime. In the following, based on the presented results, different contents of lime were selected for the experiments. The results of standard proctor compaction tests include density change curves for different moistures, optimum moisture content (OMC), and maximum dry density (MDD) based on different lime and nano-Al₂O₃ contents.

Figure 2 shows the effect of moisture content on the graphs of changes in dry density according to different moisture contents. Figure 2 shows that adding lime to CH clay soil reduces the MDD and increases the OMC.

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Effect of lime content on soil density curve.

The reduction of the MDD of the soil due to stabilization with lime is caused by the reduction of the diffuse-double water layer thickness. Adding 12% of lime reduces the MDD of CH soil by 8%. The creation of a flocculated structure due to the addition of lime causes a part of the compaction energy to be used to break this structure, and the MDD decreases. The lower specific gravity (Gs) of lime grains compared to soil is another reason for the reduction of MDD due to the addition of lime. This research’s Gs of soil and lime are 2.72 and 2.3, respectively. The standard proctor compaction test results show that due to the addition of 12% lime, the OMC increases by 20%. The immediate reactions that occur due to the addition of lime to the soil cause additional water, and as a result, the OMC increases. Various researchers reported similar effects of adding CaO-based stabilizers on soil compaction characteristics. Shirvani and Noorzad, R.38 evaluated the effect of adding sludge ash of wood with high CaO properties on the compaction properties of clay soil. This research showed that adding 7% of SAW decreases the MDD by 7% and increases the OMC by 23%. Noorzad, R., & Motevalian, S.39,40 also reported a decrease in MDD and an increase in OMC of clay soil due to adding lime.

The effect of nano-Al₂O₃ addition on MDD and OMC of lime-stabilized soil is investigated. Figures 3 and 4 illustrate the influence of nano-Al₂O₃ addition on the maximum dry density (MDD) and optimum moisture content (OMC) of soils stabilized with 3, 6, and 9% lime, respectively.

The evaluation of the results of Figs. 3 and 4 shows that adding nano-Al₂O₃ reduces the MDD and increases the OMC of clay stabilized with lime. The immediate reactions between lime and nano-Al₂O₃ will strengthen the bonds created in the soil, and as a result, more energy will be used to break these bonds instead of moving the particles, and MDD will decrease. On the other hand, due to the very high specific surface of nano-Al₂O₃, its density is low. From this point of view, adding nano-Al₂O₃ also reduces the MDD of the soil. For example, adding 1.4% nano-Al₂O₃ to soil stabilized with 9% lime reduces MDD by 5%. On the other hand, nanoparticles have a considerable specific surface. This issue causes more water to be needed to lubricate the particles, and as a result, the OMC of lime-stabilized soil increases. Also, creating the flocculated structure increases the water retention potential due to adding aluminum nanoparticles, which also increases the OMC. Adding 1.4% nano-Al₂O₃ to soil stabilized with 9% lime increased OMC by 24%.

Fig. 3
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Effect of nano-Al₂O₃ addition on MDD for samples containing 3, 6 and 9% lime.

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OMC changes in terms of nano-Al₂O₃ content different for soil stabilized with 3, 6, and 9% lime.

Several studies have reported similar compaction trends for soils stabilized with nano-scale additives in CaO-based systems. Choobbasti et al.21,22 showed that stabilizing CL clay with nano-calcium carbonate increased the optimum moisture content (OMC) and reduced the maximum dry density (MDD). This behavior was attributed to enhanced flocculation and increased water demand. Kutanaei and Choobbasti41, based on standard Proctor compaction tests on cement-stabilized sand, reported that adding nano-silica increased OMC by about 21% while decreasing MDD by approximately 4%. Similarly, Karimiazar et al.42 found that incorporating 1.5% nano-Al₂O₃ into cement-stabilized CL clay increased OMC by 26% and reduced MDD by 6%. These findings indicate that nano-scale additives generally increase water demand and limit dense packing due to their high specific surface area and strong interparticle interactions.

In contrast, studies using silica-rich natural pozzolana as a source of reactive SiO₂ report a different compaction response. Previous research has shown that natural pozzolana typically requires much higher replacement ratios, commonly in the range of 15–20%, to noticeably affect MDD and OMC. Moreover, the resulting changes are usually gradual and strongly dependent on curing conditions. Harichane et al.43 reported that adding up to 20% natural pozzolana to lime-stabilized clay led to progressive changes in compaction characteristics due to slow pozzolanic reactions. Likewise, Gadouri et al.44,45 showed that similar replacement levels were necessary to modify the compaction behavior of lime-treated clayey soils, especially under sulfate-bearing conditions. Compared with these systems, the present results demonstrate that nano-Al₂O₃ alters compaction behavior at very low dosages. This effect is mainly related to its extremely high specific surface area and its strong interaction with lime hydration products, which produce noticeable changes in MDD and OMC even at contents close to 1%.

Unconfined compressive strength and indirect tensile strength tests

In this section, the UCS test first calculates the optimum content of lime. Then, according to the obtained optimum amount, the combined effect of the content of lime and nano-Al₂O₃ on the UCS of clay soil is investigated. The parameters investigated in this section include lime content, nano-Al₂O₃ content, and curing time.

Figures 5 and 6 show the stress–strain curves and UCS for the samples stabilized with different contents of lime in the curing time of 28 days.

Fig. 5
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Stress–strain curves for samples stabilized with different contents of lime in the curing time of 28 days.

Fig. 6
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Changes of UCS in terms of contents of lime in the curing time of 28 days.

As shown in Fig. 5, the addition of lime significantly alters the stress–strain behavior of the clay soil. The untreated soil (LC = 0%) exhibits a more ductile response, characterized by a gradual increase in stress followed by a smooth post-peak reduction at relatively high strain levels. In contrast, lime-treated samples (LC = 3–15%) exhibit a steeper initial slope and higher peak stress but fail at noticeably lower strains, indicating a more brittle response. The sharper peak observed in lime-stabilized specimens indicates an increase in material stiffness and strength. The subsequent rapid post-peak stress reduction reflects a limited plastic deformation capacity and the predominance of brittle failure at relatively low axial strains.

This brittle behavior can be attributed to the pozzolanic reactions between lime and the silica and alumina present in the clay, leading to the formation of cementitious compounds such as calcium silicate hydrate (C–S–H) and calcium aluminate hydrate (C–A–H). These reaction products create rigid interparticle bonds that significantly enhance compressive strength but simultaneously restrict particle rearrangement and plastic deformation. As a result, stress concentration increases within the cemented structure, and failure occurs abruptly once the bonding strength is exceeded, resulting in low failure strain in lime-treated soil. Similar observations regarding reduced failure strain and a more brittle mechanical response due to cementation effects have been reported for highly reactive lime-stabilized clays14,46,47.

Figure 6 illustrates the UCS behavior of the CH clay before and after lime stabilization. The untreated soil (LC = 0%) exhibits a relatively low UCS of about 250 kPa, which is typical of remolded high-plasticity clays and consistent with values reported in the literature14,39,40,46,47. As lime is added, the UCS increases markedly due to the formation of cementitious bonds, reaching a maximum at a lime content of 9%. Beyond this optimum content, a reduction in UCS is observed, which can be attributed to the development of a more flocculated structure and the associated decrease in dry density.

In this study, lime contents of 3%, 6%, and 9% were used to investigate the effect of nano-Al₂O₃ on the behavior of clay soil. Furthermore, the combined effects of lime and nano-Al₂O₃ contents, along with curing time, on the UCS and ITS of the clay were evaluated. Figures 7 and 8 illustrate the variations in UCS and ITS of samples stabilized with 3%, 6%, and 9% lime and 0%, 0.6%, 1%, 1.2%, and 1.4% nano-Al₂O₃ at curing periods of 7, 28, and 90 days.

Fig. 7
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UCS values of lime–nano-Al₂O₃ stabilized soil at different nano contents and curing times (7, 28, and 90 days).

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ITS values of lime–nano-Al₂O₃ stabilized soil at different nano contents and curing times (7, 28, and 90 days).

Figures 7 and 8 show that adding nano-Al₂O₃ to lime-stabilized soil causes UCS and ITS. The pozzolanic and hydration reactions between calcium hydroxide released due to the contact of water and lime with silica and alumina particles in clay are as follows:

$$Ca^{ + + } + 2(OH)^{ - } + SiO_{2} \to CSH$$
(1)
$$Ca^{ + + } + 2(OH)^{ - } + Al_{2} O_{3} \to CAH$$
(2)

The conceptual model of the stabilization mechanism of lime and nano-Al₂O₃ in high-plasticity clay is presented in Fig. 9. This figure schematically summarizes the main stages of the stabilization process, including cation exchange, flocculation and agglomeration of clay particles, and the formation of calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH) gels, which lead to a denser structure and enhanced strength of the treated soil.

Fig. 9
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Conceptual model illustrating the stabilization mechanism of lime and nano-Al₂O₃ in high-plasticity clay (CH).

The creation of high-strength CSH and CAH gels causes the addition of lime to the soil to increase strength. On the other hand, the significant amount of Al2O3 in nano-Al₂O₃ and the reactions with CH will cause the formation of CSH high-strength gel. As a result, the UCS will increase. CH crystals are less strength than CAH gel and are primarily placed in the contact area between cement paste and grains. The presence of these products with low strength in this area causes the bond between the grains to be destroyed due to the application of load, and eventually, the stabilized soil will fail. Adding 1.2% nano-Al₂O₃ to CH soil stabilized with 9% lime in the curing time of 7 days increases UCS and ITS by 42% and 26%, respectively. As can be seen, the effect of nano-Al₂O₃ on increasing the UCS of CH soil stabilized with lime is greater than that of ITS.

Considering both performance improvement and cost efficiency, adding 1.2% nano-Al₂O₃ to CH clay stabilized with 9% lime appears to be a practical and economical choice for field applications. Although the unit cost of nano-Al₂O₃ is relatively higher than that of conventional additives, its small dosage requirement (about 1–1.2%) keeps the total treatment cost around 15–16 USD per ton of soil. This value is comparable to lime–fly ash mixtures (10–14 USD/ton) and significantly lower than nano-silica (approximately 49 USD/ton) or nano-MgO (approximately 366 USD/ton). Hence, the lime–nano-Al₂O₃ combination achieves a reasonable balance between strength improvement and economic feasibility for real-world soil stabilization.

Beyond economic considerations, the lime–nano-Al₂O₃ system offers clear eco-sustainability advantages. The strength gains achieved at low nano-Al₂O₃ contents allow partial reduction of lime dosage while maintaining or even improving mechanical performance. Since lime production is energy-intensive and associated with significant CO₂ emissions, reducing lime consumption directly contributes to lowering the carbon footprint of soil stabilization works. In addition, the rapid strength development observed in nano-Al₂O₃-modified mixtures may shorten curing periods and reduce construction time, further decreasing energy demand and environmental impact. Therefore, the combined use of lime and nano-Al₂O₃ represents a more sustainable alternative to traditional lime- or cement-based stabilization methods.

Other researchers also reported similar effects due to the addition of active particle-type nanomaterials to soils stabilized with CaO-containing materials. Studies on nano-silica-modified systems generally report noticeable strength gains at relatively low dosages. For example, Choobbasti et al.48 showed that adding 5% nano-silica to sand stabilized with 5% cement increased UCS by about 17%, while Bahmani et al.49 reported substantial UCS enhancement at nano-silica contents below 1% for cement-stabilized clay soils. Similar improvements have been reported for nano-ZnO additives in lime-treated soils. Khodaparast et al.15 showed that adding 1.5% nano-ZnO to lime-stabilized silty clay increased UCS by approximately 17%, with no further benefit observed at higher contents. In addition, Ghorbani et al.13 reported that combining 6% lime with 2% nano-silica increased the UCS of high-plasticity clay by more than five times after 28 days of curing, highlighting the high reactivity of silica-based nanoparticles in lime-treated systems.

In contrast to nanomaterials, studies using silica-rich natural pozzolana as a source of reactive SiO₂ indicate that much higher replacement ratios are required to achieve comparable strength improvements. Harichane et al.43 and Gadouri et al.44,45 reported that lime–natural pozzolana systems typically require 15–20% pozzolana to produce significant strength gains, and the improvement is generally gradual and strongly dependent on curing duration. Compared with these approaches, the present study demonstrates that a relatively small dosage of nano-Al₂O₃ (1.2%) can produce a rapid and substantial increase in UCS and ITS, particularly at early curing times, emphasizing the efficiency and novelty of the proposed lime–nano-Al₂O₃ stabilization system.

Examining the UCS and ITS test results shows that increasing the nano-Al₂O₃ content beyond a certain level does not continuously improve the mechanical performance of CH clay. An excessive increase of nanoparticles limits their proper distribution in the soil matrix and leads to the formation of unstable agglomerates, resulting in a reduction in UCS and ITS. For example, increasing the nano-Al₂O₃ content from 1.2% to 1.4% at a curing time of 7 days reduces the UCS and ITS of the soil stabilized with 9% lime by approximately 10% and 8%, respectively.

To further evaluate the effect of nano-Al₂O₃ content, statistical analysis was performed on the UCS and ITS results. One-way ANOVA indicated that nano-Al₂O₃ content has a statistically significant effect on both UCS and ITS under all curing times and lime contents (p < 0.01). However, post-hoc analysis showed that the strength increase becomes statistically non-distinguishable between adjacent nano-Al₂O₃ contents in the range of approximately 1.0–1.4%, indicating the formation of a plateau region near the peak response. Quadratic trend analysis further supports this interpretation, showing that the peak UCS response occurs within a narrow range around 1.1–1.2% nano-Al₂O₃, and the corresponding 95% confidence intervals consistently include the 1.2% dosage (for example, x* = 1.09 with a confidence interval of 0.99–1.25 and x* = 1.22 with a confidence interval of 1.03–1.68). Based on this statistical behavior and the observed mechanical trends, the nano-Al₂O₃ content of 1.2% can be defined as a practical optimum for achieving the maximum or near-maximum UCS and ITS in lime-stabilized CH clay.

A review of previous studies clearly indicates that the existence of an optimum nanoparticle content is a common feature in soils stabilized with CaO-containing binders. This optimum value is primarily governed by soil type, nanoparticle mineralogy, particle size, and the nature of the stabilizing agent. For nano-silica-modified systems, Bahmani et al.49 reported an optimum content of about 0.8% for cement-stabilized CL soil, while Ghasabkolaei et al.50 identified an optimum nano-silica content of approximately 1.5%, beyond which particle agglomeration limited further UCS improvement.

Similar optimum ranges have been reported for other nanomaterials. Karimiazar et al.42 found that the optimum content of both nano-Al₂O₃ and nano-silica for CL clay stabilized with cement was close to 1%, whereas Khodaparast et al.15 reported an optimum nano-ZnO content of 1.5% for lime-stabilized silty clay, corresponding to a UCS increase of about 17%. These studies consistently show that exceeding the optimum nanoparticle dosage leads to reduced dispersion efficiency and diminished mechanical performance due to agglomeration effects.

In contrast, silica-rich natural pozzolans exhibit a fundamentally different optimum behavior, as their lower specific surface area and reactivity require much higher contents to achieve effective stabilization. Harichane et al.43 and Gadouri et al.44,45 demonstrated that optimum pozzolana contents typically fall in the range of 15–20%, with strength development occurring progressively over long curing periods. Within this context, the optimum nano-Al₂O₃ content of 1.2% identified in the present study is fully consistent with trends reported for other nanomaterials, while offering the advantage of lower dosage and faster strength development in lime-stabilized CH clay.

Furthermore, Figs. 7 and 8 display that the increase in curing time due to the development of hydration and pozzolanic processes increases the UCS and ITS of the soil stabilized with lime and nano-Al₂O₃. The increase in curing time from 7 to 90 days for the sample stabilized with 9% lime and 1.2% nano-Al₂O₃ results in a 97% and 117% increase in UCS and ITS, respectively. On the other hand, adding nano-Al₂O₃ increases the UCS in the low curing times. For example, adding 1.2% nano-Al₂O₃ to soil stabilized with 9% lime, in curing times of 7 and 90 days, increases the UCS by 42% and 17%, respectively. Due to its specific surface and high reactivity, nano-Al₂O₃ reacts quickly with the hydration products of cement and causes a significant increase in strength in low curing times. Kutanaei and Choobbasti41 reported similar results in adding nano-silica to cement-stabilized sand.

Ultrasonic pulse velocity (UPV)

Figure 10 illustrates the three-dimensional representation of UPV values for samples stabilized with 3%, 6%, and 9% lime and varying nano-Al₂O₃ contents at curing times of 7, 28, and 90 days.

Fig. 10
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UPV values of lime–nano-Al₂O₃ stabilized soil at different nano contents and curing times (7, 28, and 90 days).

Figure 10 exhibits that soil stabilization with lime increases UPV. For example, increasing the lime content from 3 to 9% during a 7-day curing period increases the UPV by 24%. The addition of lime provides a dense path for the waves to pass due to the bonding between the soil grains, and as a result, the UPV will increase. On the other hand, as the lime content increases to its optimum level (9%), UPV will increase due to the increase in CaO hydration products. Other researchers also achieved similar results in the field of other CaO-based stabilizers on UPV. Jalali et al. (2021) investigated the effect of stabilization with industrial sewage sludge ash on CH clay soil’s mechanical properties and durability by conducting UCS and UPV tests. This research demonstrated that during a 28-day curing period, increasing the ISSA content from 5 to 10% resulted in a 13% increase in UPV. It should be noted that increasing the optimum stabilizer content, by reducing MDD and creating a distance between soil grains, can reduce UPV. Figure 10 shows that increasing the curing time increases the UPV. For example, for the sample stabilized with 9% lime, increasing the curing time from 7 to 90 days has increased the UPV by 113%. Increasing the curing time due to the hydration reactions increases the hydration products and the density of the bonds created.

On the other hand, with the development of the hydration process, inappropriate calcium hydroxide crystals are transformed into highly-strength CSH and CAH gels. This issue provides a more suitable connection between the grains, and on the other hand, it will cause the cement paste to become denser; as a result, the UPV will increase. Kutanaei and Choobbasti41 reported that increasing the curing time for sand stabilized with cement increased the UPV by 35%. It should be noted that, unlike cement, lime does not contain active materials, so the passage of time and the reactions that occur with SiO2 and Al2O3 in the soil cause a more significant increase in UPV than soils stabilized with cement.

Increasing the content of nano-Al₂O₃ up to the optimum value of 1.2% increases the UPV for all contents of lime and at all curing times. For example, for the soil stabilized with 9% lime, adding 1.2% nano-Al₂O₃ increases the UPV by 72%. The main reason for this issue is the role of nanomaterials in improving the microstructural characteristics, such as increasing the amount of CAH gel formation and compacting the cement paste. Due to their micro-filling property, nanomaterials can fill the voids in the cement paste with nano dimensions. On the other hand, nanomaterials provide a suitable place for the growth of hydration products and act like the nucleus of an atom. Improving the structure of the transition zone between cement paste and soil grains, which is very weak due to the high ratio of water to cement, is another reason for the increase in UPV caused by the addition of nano-Al₂O₃. The presence of a large amount of CH crystals makes the addition of nanomaterials contribute significantly to changing this inappropriate structure. It ultimately leads to an increase in mechanical properties and UPV. Kutanaei and Choobbasti41 showed that adding 8% nanomaterials to sand stabilized with 4% cement increased the UPV by 16%. Choobbasti et al.21,22, by performing tests of UCS, unconsolidated-undrained triaxial, and UPV on CL clay stabilized with nano calcium carbonate, reported that the addition of 1.2% of nano calcium carbonate increased the UPV by 30%.

Increasing the content of nano-Al₂O₃ more than the optimum value reduces UPV. For example, for the sample stabilized with 3% lime in the curing time of 7 days, the increase of nano-Al₂O₃ from 1.2% to 1.4% decreased the UPV by 9%. Increasing more than the optimum amount of nano-Al₂O₃ causes the hydration reactions to be prevented. On the other hand, due to the high reactivity of nanomaterials, adding it in high contents causes these particles to bond with each other and creates a weak and heterogeneous structure. Therefore, the UPV will be deviation and broken due to collision with this heterogeneous structure, and finally, the UPV will decrease. Kutanaei and Choobbasti41 showed by SEM images that by increasing the content of nano-silica from 8 to 12% in cement-stabilized soil due to the formation of unstable mass caused by the bonding of nano-silica particles, UPV decreased to 9%.

Since soil stabilization in the field occurs in various environmental conditions and to increase the trend of mechanical parameters, a large number of stabilized soils must be sampled and tested. Using the results of non-destructive tests is an efficient way to control the gaining strength due to stabilization. Since UPV depends on soil density, grain bond strength, and stabilized soil microstructure, it seems logical to establish a relationship between UPV and mechanical and shear parameters. This section discusses relationships between UCS, ITS, effective cohesion, and effective internal friction angle with UPV. Figure 11 shows the changes of UCS and ITS for stabilized samples with different contents of lime at different curing times in terms of UPV. The research investigation shows that the relationship between mechanical properties and UPV is exponential. The relationships between UPV and ITS with UPV are as follows:

$$UCS \, = \, 423.03e^{0.0025UPV}$$
(3)
$$ITS \, = \, 77.597e^{0.0016UPV}$$
(4)
Fig. 11
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UCS and ITS changes for samples stabilized with different contents of lime at different curing times.

It should be noted that in Eqs. 3 and 4, UCS and ITS are in terms of kPa, and UPV is in terms of m/s. The results in Fig. 11 show that the Regression Eqs. 3 and 4 equal 0.84 and 0.79, respectively, indicating acceptable accuracy for estimating UCS and ITS by UPV.

To further evaluate the reliability of the proposed UPV-based prediction models, the obtained correlations between UPV and both UCS and ITS were compared with those reported in the literature (Table 3). As shown, most studies have reported exponential or power-type relationships between UPV and mechanical strength parameters, with determination coefficients (R2) generally ranging from 0.73 to 0.96.

Table 3 Comparison between UPV–UCS and UPV–ITS correlations from the present study and those reported in the literature.
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The exponential equations proposed in the present study (Eqs. 3 and 4), with R2 values of 0.84 for UCS and 0.799 for ITS, fall well within this range. The results are comparable to those obtained for cement–nanosilica systems (R2 = 0.88–0.9854), lime–nano-Al₂O₃ stabilized soils (R2 = 0.73–0.9351), and other nanomaterial-based stabilizers. This agreement indicates that the proposed models can reliably estimate both compressive and tensile strengths of lime–nano-Al₂O₃ stabilized CH clay using UPV measurements.

Overall, the consistency of the proposed correlations with those reported in previous studies confirms the reliability of the developed UPV-based models for predicting the mechanical behavior of lime–nano-Al₂O₃ stabilized CH clay. Given the demonstrated reliability of these correlations, their application at the field scale is discussed in the following section.

Field-scale implications of UPV monitoring

At the field scale, UPV monitoring can be used as a non-destructive approach to assess the uniformity of soil stabilization and to track strength development in treated layers. UPV measurements are performed after field mixing and compaction. These operations should be carried out under moisture and density conditions consistent with laboratory design parameters, particularly the target maximum dry density (MDD) and optimum moisture content (OMC). Measurements can then be taken at several locations and at different curing ages, such as 7, 28, and 90 days.

This study establishes clear relationships between UPV and the mechanical properties UCS and ITS. These relationships allow rapid estimation of in-situ strength from UPV measurements and facilitate the identification of weak or non-uniform zones within stabilized soil layers.

For reliable field application, an initial site-specific calibration step is recommended before full-scale implementation. This step may include simultaneous UPV measurements and limited destructive testing, such as core sampling for UCS and ITS, to adjust the laboratory-based correlations to field conditions. In addition, field variables including moisture content, temperature, transducer coupling quality, and wave travel path should be carefully controlled. The use of coupling gel, through-transmission measurement techniques, and repeatability checks in accordance with device guidelines can improve measurement reliability.

In practice, UPV monitoring can serve as an effective quality control and quality assurance (QC/QA) tool. It enables continuous assessment of strength development and structural continuity in stabilized soils. As a result, the need for extensive destructive sampling can be substantially reduced. Similarly, in deep stabilization projects such as deep soil mixing (DSM) columns, UPV monitoring can be applied along the column length using embedded or surface-mounted transducers to evaluate uniformity and strength development at depth.

Microstructural and mineralogical analyses

SEM analysis

Figures 12 and 13 present SEM images of CH clay stabilized with 6% lime after 28 days of curing, containing 0% and 1.2% nano-Al₂O₃, respectively. As shown in Fig. 12, the sample without nano-Al₂O₃ exhibits a relatively porous microstructure with visible voids, and its surface is partially covered by plate-like Ca(OH)₂ crystals. The presence of voids and the dominance of CH crystals, rather than well-developed cementitious gels, indicate a limited degree of pozzolanic reaction, which is consistent with the comparatively lower mechanical strength observed for this specimen.

Fig. 12
Fig. 12The alternative text for this image may have been generated using AI.
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SEM image of CH clay stabilized with 6% lime (0% nano-Al₂O₃) after 28 days of curing.

Fig. 13
Fig. 13The alternative text for this image may have been generated using AI.
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SEM image of CH clay stabilized with 6% lime and 1.2% nano-Al₂O₃ after 28 days of curing.

In contrast, the SEM image of the sample containing 1.2% nano-Al₂O₃ (Fig. 13) reveals a much denser and more compact microstructure with significantly fewer voids. The soil particles are embedded in abundant gel-like hydration products, forming a continuous bonding network. This microstructural refinement can be attributed to the micro-filling effect of nano-Al₂O₃ and its role in promoting pozzolanic reactions with Ca(OH)₂. The resulting dense cementitious matrix provides an effective explanation for the observed increases in UCS, ITS, and UPV of the lime–nano-Al₂O₃ stabilized soil.

XRD analysis

Figure 14 presents the XRD patterns of the untreated soil and the soils stabilized with lime and nano-Al₂O₃ after 90 days of curing.

Fig. 14
Fig. 14The alternative text for this image may have been generated using AI.Fig. 14The alternative text for this image may have been generated using AI.
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XRD patterns of (a) untreated CH soil, (b) CH soil stabilized with 9% lime, (c) CH soil stabilized with 9% lime and 1.2% nano-Al₂O₃, and (d) CH soil stabilized with 9% lime and 1.4% nano-Al₂O₃ after 90 days of curing.

The XRD results for the untreated CH soil indicate that the dominant crystalline phases consist of quartz and clay minerals, primarily montmorillonite and kaolinite. In this sample, the high intensity of the clay peaks at 2θ ranges of 6–12° and 20–25° reflects the significant presence of active layered minerals. In contrast, no amorphous regions or hydration-product peaks are observed. This pattern indicates an absence of effective cementitious bonding. This diffraction pattern is consistent with the low compressive and tensile strengths of the untreated soil.

After stabilization with 9% lime and 90 days of curing, noticeable changes in both phase composition and peak intensities are observed. The intensity of the clay mineral peaks decreases significantly, indicating the degradation of their layered structure through cation exchange and flocculation. At the same time, distinct peaks corresponding to Ca(OH)₂ appear at around 2θ = 18°, 34°, and 47°, which account for a significant portion of the total diffraction intensity. In addition, the formation of CaCO₃ at around 2θ = 29–30° confirms the secondary carbonation reactions. In this sample, the amorphous region associated with C–S–H and C–A–H gels, although visible, is limited in extent, indicating that only a part of the Ca(OH)₂ participates in pozzolanic reactions.

As shown, in the sample containing 9% lime and 1.2% nano-Al₂O₃, the semi-quantitative analysis reveals that the relative intensity of the Ca(OH)₂ peaks decreases significantly compared to the lime-only sample. At the same time, the amplitude of the amorphous region in the 2θ range of 25–35° increases significantly. This increase in the amorphous halo indicates enhanced formation of C–S–H and C–A–H gels. Also, the clay-related peaks reach their lowest intensity among all specimens. From a relative quantitative perspective, this sample exhibits the highest proportion of hydrated cementitious products and the lowest amount of unreacted phases, reflecting the greatest advancement of pozzolanic reactions after 90 days of curing.

In contrast, increasing the nano-Al₂O₃ content to 1.4% results in a reduction of the amorphous region relative to the 1.2% sample, despite the continued presence of hydration products. Moreover, a partial increase in the intensity of Ca(OH)₂ peaks is observed. These relative changes indicate a decline in pozzolanic reaction efficiency, attributed to nanoparticle agglomeration and the consequent reduction in effective reactive surface area. The improvement in the mechanical properties of the specimen containing 1.2% nano-Al₂O₃ shows a direct correlation with the semi-quantitative XRD results. The reduction in Ca(OH)₂ intensity and the increase in the proportion of amorphous phase (C–S–H and C–A–H) promote the formation of a more continuous cementitious network between soil particles. This enhanced formation of hydrated products is the primary factor responsible for the significant increases in UCS and ITS, as well as the higher UPV values. Additionally, the reduction in active clay phases reduces volumetric instability and stress concentration, thereby increasing ITS.

Furthermore, the expansion of the amorphous region and the densification of the microstructure provide a more uniform path for ultrasound wave propagation, leading to a slight increase in UPV. In the sample with 1.4% nano-Al₂O₃, the relative decrease in amorphous products and the increase in unreacted phases correspond directly to decreases in UCS, ITS, and UPV. Therefore, the XRD results demonstrate that the combination of 9% lime and 1.2% nano-Al₂O₃ at 90 days of curing produces the highest degree of pozzolanic reaction and the most favorable microstructure. This microstructural development provides a clear explanation for the observed improvement in mechanical properties.

Limitations and future work

In this study, the experiments were conducted on remolded laboratory samples; therefore, the results may not fully reflect in situ field conditions. Moreover, aspects such as wet–dry, freeze–thaw, and leaching resistance were not performed, which are essential for evaluating the long-term behavior of stabilized soil. Additionally, the findings of this study are limited to the UCS, ITS, and UPV tests. Moreover, the UPV–UCS and UPV–ITS correlations proposed in this study are empirical and limited to the tested materials and conditions; further experimental data are required to validate their applicability under broader conditions. For a more comprehensive understanding of the mechanical behavior, future research should include triaxial tests and field investigations.

To this end, future research will focus on evaluating the long-term durability and performance of soils stabilized with lime and nano-Al₂O₃ under wet-dry and freeze–thaw cycles to understand their resistance to environmental conditions better. Furthermore, since the use of nano-Al₂O₃ can reduce the required lime content, investigating the sustainability benefits and carbon footprint reduction of this stabilization approach compared with conventional cement-based methods would represent a valuable direction for future research.

In addition, the microstructural investigation in this study was limited to a selected number of SEM images. The SEM observations were primarily intended to provide qualitative evidence of representative microstructural features. Future studies are therefore recommended to include a larger number of SEM images and more advanced microstructural analyses to achieve a more comprehensive understanding of the microstructural evolution of stabilized soils.

Conclusions

This research investigated the effect of adding lime and nano-Al₂O₃ on CH clay soil’s mechanical and shear properties. For this purpose, standard Proctor compaction, UCS, ITS, UPV, and SEM tests were performed. The following results were obtained:

  • Adding lime to CH clay soil decreases MDD and increases OMC. Adding 12% lime reduces the MDD of CH clayey soil by 8%, while the OMC increases by 20%. Similarly, adding 1.4% nano-Al₂O₃ to soil stabilized with 9% lime reduced MDD by 5% and increased OMC by 24%.

  • The UCS and ITS increased with lime content up to 9%, after which both parameters decreased.

  • Adding nano-Al₂O₃ significantly enhanced strength. The optimum nano-Al₂O₃ content was determined to be 1.2% (by weight of lime), which increased UCS and ITS by 42% and 26%, respectively, at 7 days of curing. Increasing nano-Al₂O₃ beyond this value (to 1.4%) caused a reduction of 10% (UCS) and 8% (ITS) due to nanoparticle agglomeration.

  • The UPV increased with both lime content and curing time. For example, increasing lime from 3 to 9% raised UPV by 24% after 7 days, while increasing curing from 7 to 90 days improved UPV by 113%.

  • The addition of nano-Al₂O₃ up to 1.2% increased UPV by 72% (at 90 days and 9% lime), confirming microstructural densification and higher CAH gel formation. Excessive nano-Al₂O₃ (1.4%) caused a 9% reduction in UPV due to particle clustering.

  • The SEM images of the sample containing 1.2% nano-Al₂O₃ show a dense microstructure without voids.

  • XRD analysis showed that 9% lime combined with 1.2% nano-Al₂O₃ produced the most advanced pozzolanic reaction after 90 days. The associated increase in amorphous hydration products explains the observed improvements in strength and UPV.

  • Optimum design recommendation: For field applications, 9% lime + 1.2% nano-Al₂O₃ and a curing period of at least 28 days are recommended to achieve a balance between strength, density, and durability.

  • In addition to mechanical benefits, the partial replacement of lime with nano-Al₂O₃ can reduce total lime consumption and associated CO₂ emissions, providing a more sustainable and environmentally friendly soil stabilization technique.