Research on the mechanical properties and microstructure of saline soil subgrade improved by steel slag powder and lime

Saline soils typically exhibit poor engineering properties, including low strength and instability, which severely compromising the structural stability of subgrade in arid and semi-arid regions. Based on this, a method for improving subgrade in saline soil areas using a composite of steel slag powder and lime was proposed in this study. Through orthogonal experimental design, the effects of total content, steel slag proportion, moisture content, and curing age on the unconfined compressive strength of saline soil were systematically analysed. Shear performance parameters of the modified soil were obtained in conjunction with direct shear tests. The mechanism of modification was elucidated using Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM-EDS) and X-ray Diffraction (XRD). The results indicate that the optimal improvement scheme is a total admixture content of 15% (comprising 75% steel slag and 25% lime), the moisture content is 4.48%, and the curing age is 28 days. At this point, the unconfined compressive strength reached 403.50 kPa, approximately 3.8 times that of the unmodified soil; the cohesion increased significantly from 11.8 kPa to 54.2 kPa, while the angle of internal friction showed no significant change. Microstructural analysis indicates that the composite modification of steel slag powder and lime generates calcium silicate hydrate gel and calcium carbonate, significantly reducing the porosity of saline soil. This enhances intergranular bonding, resulting in a more compact and stable structure. The study has validated the high efficiency and sustainability of this method, providing reliable technical support for the remediation of subgrade defects in road engineering projects within saline soil regions.

Highlights

• Use steel slag powder and lime to improve the saline soil;

• Determine the optimal scheme of steel slag powder-lime to improve saline soil by orthogonal test;

• Explain the strength improvement mechanism of saline soil from the microstructure level.

1 Introduction

Soil salinization is a global threat, affecting more than 1 billion hectares of land in over 100 countries worldwide, and these figures are still rising (Haj-Amor et al., 2022; Ivushkin et al., 2019; Ma and Tashpolat, 2023; Basak et al., 2022). According to estimates, salinization affects around 25% of all land and 33% of irrigated land globally (Mohanavelu et al., 2021). Saline soil is widely distributed in northwestern China (Liu Y. et al., 2024; Yang et al., 2025). Due to the presence of soluble salts, saline soils often have fragile structures and low bearing capacity, which have always been one of the key factors limiting transportation construction in central and western China. Salinity is not limited to arid areas. Saline soil also occurs in humid areas due to high groundwater levels and natural salt deposits (Nirubana et al., 2021; Thorslund et al., 2021). Soil is prone to degradation due to the effects of salinity, and climate change impacts such as increased evaporation and reduced rainfall exacerbate soil salinization (Corwin, 2020). The problem of saline soil has become a serious challenge faced by many regions around the world (Hassani et al., 2020; Hassani et al., 2021). How to effectively control saline soil subgrade hazards, improve the quality of engineering construction, and explore scientific and reasonable measures to prevent soil salinization are urgent issues to be solved.

At present, research on the improvement of saline soil has made some progress, and many scholars have explored various improvement materials and methods. Wang and Ni (2011) utilised industrial production waste materials such as slag, fly ash and desulphurisation gypsum to reinforce saline soil, demonstrating its favourable mechanical properties. Yang et al. (2010) studied the solidification of saline soil with grass roots, wheat straw, and lime, and proposed the optimal dosage of the modifier by testing the strength and deformation of the modified saline soil. Szostek et al. (2024) used ash from biomass combustion as a soil amendment material, and founded that ash can alter the acid-base balance and salinity of soil, and has a certain effect on improving saline soil. Sahin et al. (2008) added sludge and fly ash to solidify saline soil, thereby reducing the porosity and enhancing soil stability. Li et al. (2024) used a mixture of fly ash, silica fume, and ordinary Portland cement to consolidate saline soil, showing that the mechanical properties, frost resistance, and salt erosion resistance of the saline soil were markedly enhanced after treatment. Han et al. (2025) conducted solidification experiments on saline soil using industrial solid waste such as blast furnace slag, fly ash, coal gangue, kaolin, and founded that it could significantly enhance the salt frost expansion resistance of saline soil in an alkaline environment.

In recent years, significant results have also been achieved in the field of saline soil-steel slag powder research. As the main waste product of steel production, steel slag has been extensively studied in the treatment of saline soil hazards. Ying et al. (2021) conducted field experiments in saline-alkali soil areas of the Yellow River in Inner Mongolia, China, and concluded that flue gas desulfurization steel slag powder can reduce soil alkalinity and improve poor soil physical properties, effectively and rapidly restoring saline soil. Wang (2022) found that the unconfined compressive strength of saline soil increased significantly after adding alkali-activated steel slag, and its microstructure became denser, with soil particles bound together by gel products. Liu J. et al. (2024) researched and discovered that under the coupling effect of salt and alkali, steel slag hydration generates a large number of cementing compounds that help to enhance the stability of soil, thereby improving the mechanical properties of saline soil. The experiment of Zhuo et al. (2025) proved that steel slag powder can significantly improve the mechanical properties of saline soil, thus effectively improving its engineering properties.

Lime as a traditional improvement material, plays an important role in improving saline soil. Liu et al. (2011) used lime to solidify saline soil and found that adding lime could effectively improve the particle size composition of saline soil. Liu et al. (2019) added different contents of lime to saline soil and proved that lime can alter the particle size distribution of soil and enhance the load-bearing capacity of saline soil. Kamon and Gu (2001) found that when a solidifying agent made from a mixture of industrial waste residue and lime was mixed into the soil, the early strength of the soil could be significantly improved, thus making up for the shortcoming of excessively low early strength. Moayed et al. (2012) found that adding 2% lime and 3% microsilica to saline soil can enhance the mechanical properties and water-stability capacity of the soil. Nan et al. (2022) used quicklime to improve saline soil, researched its effects on the mechanical properties and microstructure of the modified saline soil, and drew the conclusion that quicklime can improve the saline soil.

Although some achievements have been made in the study of saline soil-steel slag powder and saline soil-lime, relatively few studies have focused on using steel slag powder and lime jointly to improve saline soil. Given the serious impact of saline soil hazards on engineering construction and the limitations of traditional treatment methods, this study proposes a composite improvement method based on steel slag powder and lime, aiming to improve the stability of the subgrade of saline soil. The research content mainly includes: through mechanical property tests, the influence of factors such as the total steel slag powder-lime content, the proportion of steel slag powder, moisture content, and curing age on the mechanical properties of saline soil will be studied, and the optimal improvement scheme will be determined. Additionally, microstructural analysis methods will be used to investigate the mechanism of steel slag powder and lime in saline soil and reveal their impact on the microstructure and mechanical properties of saline soil subgrade.

2 Materials and methods

2.1 Materials

2.1.1 Steel slag powder

The steel slag powder employed in this study has a fineness of 100–200 mesh and a density of 3.18 g/cm³. The silicate minerals in steel slag powder confer a degree of hydraulic properties upon it. The chemical composition of steel slag powder is shown in Table 1.

Table 1

Table 1. The main chemical composition of steel slag powder.

2.1.2 Quicklime

Table 2 displays the chemical composition of the quicklime employed in this study.

Table 2

Table 2. The main chemical composition of quicklime.

2.1.3 Saline soil

Figure 1 shows the saline soil sample collected from Shanshan County, Xinjiang, China in this study.

Figure 1

Figure 1. Saline soil of shanshan county.

Figure 2 shows the particle size distribution curve of saline soil. The non-uniformity coefficient (C_u) and curvature coefficient (C_c) are calculated according to Equations 1 and 2, respectively:

$C_u=\frac{d_{60}}{d_{10}}(1)$

$C_C=\frac{{\left(d_{30}\right)}^2}{d_{10}\times d_{60}}(2)$

Figure 2

Figure 2. Particle size distribution curve of saline soil.

In the formula, d_i (i = 10, 30, 60) is the characteristic particle size of the soil (mm), indicating that the mass of soil particles smaller than this particle size is 10%, 30%, and 60% of the total soil mass, respectively.

The calculation shows that the saline soil sample has C_u = 4.18 and C_c = 0.93, which belongs to poorly graded sandy soil according to Chinese standard Test Methods of Soils for Highway Engineering (JTG 3430-2020) (JTG 3430-2020, 2007).

This study determined the soluble salt content in accordance with Chinese standard Standard for geotechnical testing method (GB/T 50123-2019) (GB/T 50123-2019, 2019). The test results are presented in Table 3.

Table 3

Table 3. Soluble salt content of saline soil.

According to JTG 3430-2020, saline soils are classified based on the molar ratio of ions in soil, measured in mmol/kg. The Cl⁻/SO₄²⁻ ratio is 1.57, with an average salt content (by mass percentage) of 1.56%. This saline soil is classified as subchloric saline soil based on its properties, and as moderately saline soil according to its degree of salinisation.

Figure 3 shows the compaction curve of saline soil. It can be concluded from Figure 3 that the optimal moisture content of the saline soil sample is 6.48%, and the maximum dry density ρ_d is 2.14 g/cm³.

Figure 3

Figure 3. Compaction curve of saline soil.

2.2 Test methods

2.2.1 Unconfined compressive strength test

2.2.1.1 Specimen production

The saline soil sample was sieved and dried for later use. The saline soil was thoroughly mixed with lime and steel slag powder, then deionised water was added according to the required moisture content, achieving a compaction rate of 95%. Unconfined compressive strength specimens are cylindrical with a base diameter of 50 mm and a height of 90 mm, and at least three parallel specimens are prepared for each group. The specimen size for the direct shear test is the ring cutter size: inner diameter of 61.8 mm, height of 20 mm, and at least 4 parallel specimens are prepared for each group. The formed specimens are shown in Figure 4.

Figure 4

Figure 4. Formed specimens: (a) unconfined compressive strength specimen; (b) direct shear specimen.

2.2.1.2 Test method

The ultimate strength value of a specimen resisting axial pressure without lateral restraint constitutes its unconfined compressive strength. This serves as a technical indicator for determining soil compressive strength and is frequently employed as a key parameter in the acceptance assessment of road subgrade. This study used a strain controlled unconfined compressive strength tester. Conduct the test procedure in accordance with the unconfined compressive strength test specified in T0148-1993, as referenced in JTG 3430-2020. Equation 3 is used to calculate the specimen’s axial stress.

$\sigma =\frac{10CR}{A_{\alpha }}(3)$

In the formula: σ is axial stress (kPa); C is the force gauge calibration coefficient (N/0.01 mm); R is the dial gauge reading (0.01 mm); A_α is the calibrated cross-sectional area of the specimen (cm²).

The unconfined compressive strength is generally taken as the maximum axial stress in the test. Where the maximum axial stress is not clearly discernible, the stress corresponding to 15% axial strain is adopted as the specimen’s unconfined compressive strength.

2.2.1.3 Orthogonal experimental design

Orthogonal experiments constitute a design methodology for investigating multi-factor, multi-level scenarios. Characterised by dispersion and uniform comparability, they enable the scientific and effective representation of all experimental combinations.

The dosage and proportion of steel slag powder and lime directly affect the strength and stability of modified saline soil. A reasonable admixture dosage can both meet the objectives of strength stability and fulfill economic and environmental requirements. Moisture content significantly affects soil stability and compaction effectiveness, and the hydration reactions of inorganic binders are all related to moisture content. Therefore, this study conducted experiments at five moisture content levels around the optimum moisture content of saline soil. For the aforementioned reasons, this study investigates the effects of four factors—total steel slag powder-lime content (A), proportion of steel slag powder (P), moisture content (W), and curing age (D)—on the unconfined compressive strength of saline soil. Each factor is set at five levels. The factors and their levels are listed in Table 4, while the orthogonal experimental design table is shown in Table 5.

Table 4

Table 4. Orthogonal test factor level table.

Table 5

Table 5. Orthogonal array of mix proportions.

2.2.2 Direct shear test

This study employed the rapid shear test within the direct shear test to determine the shear strength parameters (internal friction angle and cohesion) of modified soil sample. The test employed a strain-controlled direct shear apparatus, conducted in accordance with the T0142-2019 rapid shear test specified in JTG 3430-2020. Shear stress was calculated using Equation 4.

$\tau =\frac{10CR}{A_0}(4)$

In the formula: τ is the shear stress, calculated to 0.1 kPa; C is the force gauge calibration coefficient (N/0.01 mm); R is the force gauge reading (0.01 mm); A₀ is the initial area of the specimen (cm²); 10 is the unit conversion factor.

After completing the test, plot the shear stress-shear displacement relationship curve. Use vertical pressure p as the horizontal axis and shear strength S as the vertical axis. Plot the shear strength point for each specimen on graph paper and connect them to form a straight line. The slope of this line represents the angle of internal friction, while the intercept on the vertical axis represents the cohesion.

2.2.3 SEM-EDS test

Scanning Electron Microscope (SEM) is used to characterize the microstructure of saline soil samples. Energy Dispersive X-ray Spectroscopy (EDS) is used for elemental analysis of saline soil samples to identify element types and concentrations. The SEM model used in this study is the Hitachi S-3400N, and the EDS model is IXRF Systems.

The specimen preparation method is as follows: After curing, the specimen is placed in an oven for drying. The central portion of the soil sample is then extracted, cut, and polished into a cube with edges less than 5 mm in length. The cut surfaces of the cubic specimen are treated with gold spraying before being placed on the sample stage for scanning. To obtain more representative images for observing both the overall surface and local morphology of the specimens, this study used two magnifications of 500 and 2,000 for testing.

2.2.4 XRD test

X-ray Diffraction (XRD) test can analyze the mineral composition and relative abundance within saline soil samples, determining mineral content through analysis of diffraction results. By comparing the changes in mineral composition before and after modification, analyze the chemical reactions occurring between the modification materials and the saline soil samples. The X-ray diffractometer used in this study is a Rigaku SmartLab SE model from Japan, with a copper target. Both the unmodified soil sample and the modified soil sample were dried, ground into powder, and set aside for later use.

3 Results and analysis

3.1 Analysis of unconfined compressive strength test

Unconfined compressive strength specimens were prepared according to the orthogonally designed tables for admixture dosage and moisture content, then cured. After curing, tests were conducted as specified. Test results are presented in Table A1 of Appendix A.

This study employed two common methods, range analysis and analysis of variance, in orthogonal experiments to analyze the four influencing factors. By integrating the analytical results to characterise each factor’s impact on unconfined compressive strength, the optimal improvement scheme for saline soil sample under the experimental conditions was determined.

3.1.1 Range analysis

Figure 5 presents the mean values for each factor level, while Table 6 displays the range analysis. Within the table, K is the sum of experimental data for a given factor level, K_avg is the corresponding mean value, and the optimal level indicates the level number associated with the best K_avg value for that factor. R is the range value for the factor, calculated as the maximum K_avg value minus the minimum K_avg value for that factor. This range value facilitates comparative assessment of factor performance.

Figure 5

Figure 5. Mean values for each factor level.

Table 6

Table 6. Range analysis table.

A range analysis was conducted on the orthogonal test results, with the findings as follows: The range values of various factors for the unconfined compressive strength of modified saline soil: R_A = 298.09, R_P = 86.63, R_W = 203.70, R_D = 126.39. R_A is the highest, followed by R_W, R_D, and R_P. The influence of various factors on unconfined compressive strength, from most to least influential, is: total steel slag powder-lime content, followed by moisture content, curing age, and finally the proportion of steel slag powder. The analysis of range table indicates the optimal scheme under the study conditions for improving the unconfined compressive strength of saline soil: total steel slag powder-lime content is 15%, proportion of steel slag powder is 75%, moisture content is 4.48%, and curing age is 28 days.

Figure 6 shows the effect of total dosage on unconfined compressive strength. As shown in Figure 6: the unconfined compressive strength of saline soil exhibited a lower mean value of 105.42 kPa without the addition of lime and steel slag powder. With the addition of lime and steel slag powder, the unconfined compressive strength values demonstrated an upward trend. The mean unconfined compressive strengths at 5% and 10% total admixture ratios were 1.7 and 1.9 times that of the unadulterated group, respectively. When the total admixture content reached 15%, the average unconfined compressive strength attained the maximum value of 403.50 kPa under the test conditions, approximately 3.8 times that of the unadulterated group. As the admixture content continues to increase, the unconfined compressive strength exhibits a decreasing trend. The underlying reason lies in the fact that as the total steel slag powder-lime content increases, the particle grading of the soil improves. This results in a more compact and robust surface on the amended saline soil. Furthermore, the chemical reaction between steel slag powder and lime generates cementitious substances that reinforce the soil structure. The flocculent connections between particle structures enhance the overall integrity of the soil structure. Consequently, the unconfined compressive strength exhibits a synchronous increase trend with rising total dosage. When the total dosage reaches a certain threshold, the steel slag powder-lime mixture begins to exert an adverse effect on soil gradation improvement. Moreover, compared to cement, the hydration reaction of steel slag powder-lime is slower and weaker. Excessive lime and steel slag powder fail to react in time and become entrapped within soil particles. The lower specific gravity of free lime readily forms unstable agglomerates with moisture, thereby diminishing the stability of the soil structure.

Figure 6

Figure 6. Effect of total dosage.

Figure 7 shows the effect of proportion of steel slag powder on unconfined compressive strength. As shown in Figure 7: the group with 0% steel slag powder content (only lime) demonstrated superior improvement in the unconfined compressive strength of saline soil compared to the group with 100% steel slag powder content (only steel slag powder). As steel slag powder replaces lime, the average unconfined compressive strength exhibits a trend of initially decreasing before subsequently increasing. When the proportion of steel slag powder reached 75% (with lime at 25%), the mean unconfined compressive strength was slightly higher than that of the lime-only group. The effect of using steel slag powder only is inferior to that of using lime only, possibly because lime exhibits stronger hydration and cementation properties than steel slag powder. The better mixing effect is due to the high alkalinity environment rich in Ca(OH)₂ provided by the hydration of lime, which is the key to stimulating the activity of steel slag powder. This alkaline environment can destroy the glass silicon aluminum phase structure on the surface of steel slag particles, causing them to dissolve and release active SiO₂ and Al₂O₃. These active components then undergo a pozzolanic reaction with the enriched Ca²⁺ in the system to generate a large number of calcium silicate hydrate and calcium aluminate hydrate gel with cementation ability. In contrast, when steel slag powder is added alone, the alkalinity of the system is insufficient, making it difficult to effectively stimulate the above reactions, resulting in weaker strength development. Therefore, the effect of steel slag powder combined with lime is better than that of steel slag powder added alone. Furthermore, the slight volume expansion of steel slag powder in later stages counteracts the volume shrinkage of lime in later stages. The slight expansion of steel slag powder effectively counteracts internal stresses generated by lime shrinkage, thereby inhibiting the development of micro-cracks caused by contraction. The complementary relationship between the two materials in terms of volume change reduces internal defects within the material, resulting in a more compact soil structure with enhanced integrity.

Figure 7

Figure 7. Effect of proportion of steel slag powder.

Figure 8 shows the effect of moisture content on unconfined compressive strength. As shown in Figure 8: at a moisture content of 2.48%, the average unconfined compressive strength of the saline soil was relatively low, reaching only 103.75 kPa. As the moisture content increases, the mean unconfined compressive strength exhibits an upward trend, reaching its maximum value within this study group at a moisture content of 4.48%. As the moisture content continues to increase, the mean unconfined compressive strength exhibits a decreasing trend. The reason lies in the fact that at extremely low moisture content, the soil exists in a dry state where its strength is primarily maintained by inter-particle friction. This renders it highly susceptible to relative slippage and failure. In addition, based on the material reaction mechanism of steel slag powder and lime, it is inferred that in low moisture environments, lime and steel slag powder do not have enough water to produce chemical reactions, and the chemical reaction process is slow or even stagnant, making it difficult to improve soil strength through bonding. Therefore, the unconfined compressive strength is significantly lower when the moisture content is extremely low. As moisture content increases, the lubricating effect of water facilitates the rearrangement of soil particles, thereby improving soil compaction. Furthermore, capillary action of water generates cohesive forces between particles. Concurrently, an appropriate amount of water undergoes chemical reactions with lime and steel slag powder to produce hydrated cementitious materials, which consolidate soil particles and promote their aggregation. Consequently, the unconfined compressive strength is enhanced. When the moisture content reaches a certain threshold, a liquid film forms on the particle surfaces, excessively lubricating the particles. This weakens both the friction and cohesion between particles, causing the soil structure to become loose. Consequently, the unconfined compressive strength actually decreases at higher moisture contents.

Figure 8

Figure 8. Effect of moisture content.

Figure 9 shows the effect of curing age on unconfined compressive strength. As shown in Figure 9: the unconfined compressive strength of saline soil shows an increasing trend with increasing curing age. The unconfined compressive strength after 28 days of curing was 1.7 times that of the uncured group. This is primarily due to ongoing chemical reactions within the soil matrix—such as hydration reactions—which generate additional cementitious materials. These fill pores, enhancing the compactness and cohesion between particles. Concurrently, environmental factors also contribute to strength development. This process gradually stabilises over time as hydration reactions slow and cementitious material production reaches saturation. Therefore, the strength exhibits an increasing trend with the extension of the curing age.

Figure 9

Figure 9. Effect of curing age.

3.1.2 Analysis of variance

This study includes four factors, each with 5 levels, and each level requires 5 trials. Therefore, 25 trials need to be conducted.

Total degree of freedom: df = 25-1 = 24

Degree of freedom of each factor: df_A = df_P = df_W = df_D = 5-1 = 4

Degree of freedom between groups: df₁ = 5-1 = 4

Degree of freedom within groups: df₂ = 25-5 = 20

Calculate the sum of squares between groups (SSbetween) and within groups (SSwithin) using Equations 5 and 6 respectively:

$SSbetween=\sum n_i{\left(u_i-u\right)}^2(5)$

$SSwithin=\sum \sum {\left(y_{ij}-u_i\right)}^2(6)$

In the formula: n_i is the sample size of the i-th group, u_i is the mean of the i-th group, u is the population mean, and y_ij is the j-th observation value in the i-th group.

Table 7 presents the analysis of variance for the results. At the given significance levels of α = 0.05 and α = 0.01, the critical value table shows that F0.05 (4,20) = 2.87 and F0.01 (4,20) = 4.43. From 4.43 > FA = 4.229 > 2.87 > FW = 1.577 > FD = 0.536 > FP = 0.247, it can be seen that total steel slag powder-lime content has a significant impact on the unconfined compressive strength, followed by the proportion of steel slag powder, moisture content, and curing age. This is consistent with the results of the range analysis.

Table 7

Table 7. Analysis of variance table.

3.2 Analysis of direct shear test

Table 8 presents the results of the direct shear test. It can be observed that, under identical vertical pressure conditions, the shear strength of the steel slag powder-lime modified soil sample consistently exceeds that of the unmodified soil sample. Figure 10 illustrates the relationship between shear strength and vertical pressure for soil samples before and after modification. As shown in Figure 10, the cohesion of the unmodified soil sample was 11.8 kPa, with an internal friction angle of 33.5°; the cohesion of the steel slag powder-lime modified soil sample was 54.2 kPa, with an internal friction angle of 34.6°.

Table 8

Table 8. Direct shear test results.

Figure 10

Figure 10. Relationship diagram between shear strength and vertical pressure: (a) unmodified soil sample; (b) modified soil sample.

Test results for shear strength indicate that the unmodified soil sample exhibits low cohesion, with a cohesion value of merely 11.8 kPa. The modified soil sample demonstrates significantly enhanced cohesion compared to the unmodified soil sample, with a cohesion value of 54.2 kPa, which is 4.6 times that of the unmodified soil sample. The change in the internal friction angle of the soil samples before and after modification was not significant, differing by only 1.1°.

The enhanced cohesion of the modified soil sample is attributable to the incorporation of steel slag powder and lime, which significantly improved the microstructure of the saline soil. The modified saline soil generated new granular material that filled the pores between soil particles, thereby reducing porosity and optimising particle grading. This resulted in a more compact and stable overall soil structure, substantially increasing the sample’s cohesion.

The internal friction angle is primarily determined by the mineral composition, shape, and roughness of soil particles (Stark et al., 2014; Zegzulka et al., 2022). The incorporation of steel slag powder and lime improved the soil’s microstructure, yet did not significantly alter the angularity or hardness of the native soil particles. Consequently, the friction characteristics of the soil particles remained largely unchanged, and the interlocking action between particles showed no significant modification. Therefore, the increase in the internal friction angle was not pronounced.

3.3 SEM-EDS test analysis

In this experiment, the soil samples with a total steel slag powder-lime content of 10% (including 50% steel slag powder and 50% lime), a moisture content of 6.48%, and a curing age of 28 days were compared with the unmodified soil samples. And two magnification levels (500-fold and 2000-fold) were selected for the SEM images analysis of the samples. The 500-fold images characterize the overall microstructure of the soil sample, including its compaction degree and pore distribution, while the 2000-fold images are used to observe the interparticle connections and material morphology at the local level.

Figure 11 shows a comparative SEM images of the saline soil sample before and after modification. Figures 11a,b present SEM images of the saline soil samples magnified 500-fold before and after modification. The unmodified soil sample exhibits numerous surface pores distributed widely and of varying sizes. These pores readily develop into interconnected cracks, resulting in an overall coarse, fragmented, and loose structure. In contrast, the surface of the steel slag powder-lime modified soil sample appears relatively smooth with fewer pores, exhibiting pores only in localized areas. The particles are tightly interconnected, forming a cohesive system, and the overall structure presents a dense and stable state. Figures 11c,d present SEM images of the saline soil samples magnified 2000-fold before and after modification. The unmodified soil sample primarily exhibited interlocking connections between particles, making them highly susceptible to separation due to external factors. In contrast, the modified soil sample exhibits significantly increased flocculent connections between particles. These particles are now interconnected through a flocculent network, forming a cohesive whole. Coarse particles are filled with fine steel slag particles, making the soil less susceptible to dispersion under external influences.

Figure 11

Figure 11. SEM images of saline soil samples before and after modification: (a) unmodified soil sample (500-fold); (b) modified soil sample (500-fold); (c) unmodified soil sample (2000-fold); (d) modified soil sample (2000-fold).

In the 2000-fold SEM image shown in Figure 11d, EDS testing was performed on different regional points. The EDS test results are presented in Figure 12. The EDS spectrum for spot 1 reveals that the primary constituents of the material at this location are Si and O, indicating that the substance at this point consists of soil particles, primarily composed of SiO₂. The EDS spectrum for spot 2 shows that the main constituents of the material at this location are Si, O, Al, Ca, Mg, Fe, K. It can be judged that the substance at this point is steel slag powder particles. The EDS spectrum for spot 3 shows a significant change in the Ca and Si content among the main components, with an increase in Ca and a decrease in Si. This indicates that this spot is most likely calcium silicate hydrate gel formed after a hydration reaction.

Figure 12

Figure 12. EDS test results of different regional points: (a) spot 1; (b) spot 2; (c) spot 3.

3.4 XRD test analysis

Figure 13 shows the XRD results of the soil samples before and after modification. Comparing Figures 13a,b, it can be observed that the modified soil sample exhibits new peaks for calcium carbonate and hydrated calcium silicate compared to the unmodified sample. This indicates that the addition of steel slag powder and lime promotes the formation of calcium carbonate and cementitious materials.

Figure 13

Figure 13. XRD pattern of saline soil samples before and after modification: (a) unmodified soil sample; (b) modified soil sample.

Calcium carbonate fills the pores between soil particles, increasing soil density and reducing the pore ratio. This physical filling action also binds soil particles more tightly together, thereby enhancing the soil’s bearing capacity and shear strength to some extent. This is why the SEM images of the modified soil sample appear more compact.

The calcium silicate hydrate gel formed during the hydration reaction binds soil particles together, creating small soil aggregates with tighter interparticle connections. The modified soil sample exhibits flocculent connections, with a more compact surface structure. The cementitious material simultaneously fills inter-soil pores, tightly bonding the soil into a unified whole and forming a stable, compact soil structure. This robust soil structure and interlocking mechanism effectively suppress the development of saline soil subgrade hazards, providing an excellent support system for road surfaces.

4 Conclusion

This study investigated the mechanical enhancement effects and microcosmic action mechanism of steel slag powder–lime composite conditioners on saline soil subgrade through mechanical property testing and microscopic analysis. The main conclusions are as follows:

1. The optimal improvement scheme is a total admixture content of 15% (comprising 75% steel slag and 25% lime), the moisture content is 4.48%, and the curing age is 28 days. At this point, the unconfined compressive strength reached 403.50 kPa, approximately 3.8 times that of the unmodified soil.

2. The increase in shear strength is primarily reflected in cohesion, which rose from 11.8 kPa to 54.2 kPa; whereas the internal friction angle showed no significant change. This indicates that the strength improvement stems mainly from enhanced interparticle bonding rather than alterations in particle friction characteristics.

3. Microstructural analysis indicates that the incorporation of steel slag powder and lime reduces the porosity of saline soil, improves particle packing conditions, and promotes the formation of hydration products such as alcium silicate hydrate cementitious material and calcium carbonate. These products act as fillers and binders, rendering the soil structure more compact and stable.

4. The steel slag powder–lime composite stabilisation method not only significantly enhances the mechanical stability of saline soil but also offers advantages in resource recycling and environmental sustainability. It holds considerable potential for application in the construction of subgrade within arid and semi-arid saline soil regions.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

PL: Methodology, Conceptualization, Funding acquisition, Writing – review and editing, Supervision. SL: Writing – review and editing. ZH: Writing – review and editing. XS: Writing – review and editing. GL: Writing – original draft, Data curation, Methodology. JS: Writing – review and editing. JZ: Writing – review and editing.

Funding

The author(s) declared that financial support was received for this work and/or its publication. The research was supported by the Nanning South Transit Line (from Liujing to Datang Section and from Wuxu Airport to Long’an Extension Section) Highway Scientific Research Topic (Project·No. EHGS-GC-2023-025).

Conflict of interest

Authors PL, SL, and ZH were employed by Guangxi Nanning Second Ring Expressway Co., Ltd.

Author XS was employed by Nanning Expressway Construction and Development Co., Ltd.

The remaining author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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