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Article

Research on Solidification Methods and Stabilization Mechanisms of Sulfate Saline Soils

by
Sining Li
1,
Yong Huang
1,2,*,
Jian Sun
3,4,*,
Rui Yu
2,
Yubin Liu
2 and
Qiushuang Cui
1
1
State Key Laboratory of Chemistry and Utilization of Carbon Based Energy Resources, College of Chemistry, Xinjiang University, Urumqi 830017, China
2
College of Civil Engineering and Architecture, Xinjiang University, Urumqi 830017, China
3
Department of Automobile Road Construction and Maintenance, Kharkiv National Automobile and Highway University, 61002 Kharkiv, Ukraine
4
Faculty of Architect and Civil Engineer, Pavement Construction and Maintenance, University of Wuppertal, 42119 Wuppertal, Germany
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2024, 14(16), 7246; https://doi.org/10.3390/app14167246 (registering DOI)
Submission received: 1 July 2024 / Revised: 6 August 2024 / Accepted: 14 August 2024 / Published: 17 August 2024
Browse Figures
Versions Notes

Abstract

:
In cold regions, saline soils can cause dissolution, settlement, and salt expansion of the roadbed under the influence of freeze–thaw cycles, so they need to be stabilized during road construction. In this study, lime, fly ash (FA), and polyacrylamide (PAM) were used to stabilize sulfate saline soils, and the stabilized saline soils were subjected to the unconfined compressive strength test (UCS), splitting test, and freeze–thaw cycle tests (FTs). The stabilization mechanism of the three materials on saline soils was also studied via scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), and X-ray photoelectron spectroscopy (XPS). The test results showed that the addition of lime, FA, and PAM to saline soils can improve the mechanical properties and frost resistance of saline soils. After 28 d of curing, the UCS of FA-, PAM-, and lime-stabilized saline soils increased by at least 55%, 23%, and 1068%, respectively, and the splitting strength increased by at least 161%, 75%, and 2720%, respectively. After five freeze–thaw cycles, the residual strength ratios (BDRs) of the UCS of L2 (lime 8%), F2 (FA 11%), and P2 (PAM 1%) stabilized soils and saline soils were 71.78%, 56.42%, 39.05%, and 17.95%, respectively, and the decreasing trend tended to be stable. The saline soils stabilized by lime and FA were chemically stabilized, and their mechanical properties and frost resistance were better than the physical stabilization of PAM.

1. Introduction

Saline soils are soils with a soluble salt content exceeding 0.3%, which can lead to issues such as settlement, subsidence, and salt swelling of roadbeds under the influence of groundwater and freeze–thaw cycles [1,2]. These soils are widely distributed worldwide, covering more than 109 million hectares, and are especially widespread in Northwestern China [3]. As external conditions such as temperature and humidity change, the salt in saline soils repeatedly crystallizes and hydrates, leading to significant changes in their physical and mechanical properties [4]. This can lead to increased pore space in the soil, soil softening, or volume expansion, which can lead to pavement cracking, subsidence, and other disorders [5]. Soluble salts can react chemically at changing ambient temperatures, and phase changes can occur, causing structural damage to saline soils [6]. For example, at low environmental temperatures, the solubility of Na2SO4·10H2O in sulfate saline soils decreases, leading to the absorption of water and the crystallization of mirabilite (Na2SO4·10H2O), causing soil expansion [7]. Rainfall dissolves soluble salts in saline soils, causing soil settlement. This is due to the fact that the salt in saline soils reduces the bonding of soil particles through softening or crystalline expansion, resulting in a loose soil structure [8].
The Xinjiang region of China has a dry climate with low precipitation, high temperature variability, and high evaporation rates [9]. Sulfate saline soils are widely distributed in Xinjiang [10]. If sulfate saline soils are used directly as a roadbed material, salt expansion and collapse may result in damage to the roadbed, affecting the normal service life of the road [11]. In addition, sulfate saline soils are highly sensitive to moisture, and the compressive strength of the soil decreases significantly upon water saturation. Moreover, Xinjiang is located in the middle and high latitudes, and the freeze–thaw cycles is frequent in late winter and early spring. When water freezes, it expands by about 9% in volume, generating high pressure [12] and forming ice crystals that separate and break the bonds of soil particles [13]. Therefore, in order to improve the frost resistance and stability of saline soils as roadbed materials, they need to be stabilized.
In road engineering, inorganic binding materials such as lime, fly ash, and cement are usually used to stabilize the soil [14]. FA is a byproduct of coal’s combustion in power plants [15] and is widely used in the road construction industry [16]. FA contains large amounts of reactive SiO2 and Al2O3, which can be used as binding materials for stabilizing saline soil geopolymers. In the Xinjiang region, there are many thermal power plants that generate large amounts of FA [15,17]. FA-stabilized saline soils can not only solve the problems of environmental pollution and massive waste accumulation occupying land but also reduce the cost of road construction [18,19]. Some researchers conducted mechanical tests on FA-stabilized saline soils and found that the strength of the stabilized soils increased significantly, and the chemical composition of the stabilized soils was evaluated using XRD [20].
Some researchers have also used mixed inorganic binders to improve the properties of saline soils, such as using cement lime to stabilize saline soils [21,22]. Lime undergoes an ion-exchange reaction with the ions in the saline soils, causing the saline soils to solidify [23]. Liu L investigated the strength properties of steel slag, cement, and kaolin composite binder under different clay fractions, moisture contents, and curing times, and they found that the composite binder could effectively enhance the strength of saline soils and provide properties similar to those of hydraulic soil [24]. In cold regions, the combination of freeze–thaw cycles and salt leads to the destruction of cement, soil, interfacial zones, and pore structure, resulting in the deterioration of the durability of stabilized soils [25].
Polyacrylamide (PAM) is a linear, water-soluble organic polymer containing a large number of amide groups [26]. The amide groups form hydrogen bonds that can strengthen the soil by increasing the cohesion between soil particles [27,28]. PAM is widely used in applications such as preventing irrigation-induced erosion and improving the permeability of agricultural soils, as well as for stabilizing subgrade and highway roadbeds in road engineering. However, it is also susceptible to cracking due to the drying and degradation of aggregate soil stability [29,30]. Zhang T [31] investigated the basic physical properties, densities, microstructures, and crack patterns of untreated and PAM-stabilized soil samples and found that PAM improved the physical properties and densities of stabilized soil. Miao et al. [32,33] found that the strength and wind erosion resistance of arid soil stabilized with PAM were significantly improved. However, the “shell” formed by the polymer can break down in harsh environments, reducing the strength of the stabilized soil [34]. PAM is commonly used to improve agricultural soils, but few studies have investigated its feasibility for stabilizing saline soils for use in roadbeds.
This study compared the stabilization mechanisms, mechanical properties, and frost resistance of sulfate saline soils stabilized by lime, FA, and PAM. Sulfate saline soils and three binding-material-stabilized saline soil specimens were prepared separately and tested for unconfined compressive strength (UCS), splitting strength, and freeze–thaw cycles. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetry (TG), and X-ray photoelectron spectroscopy (XPS) were used to investigate the micromorphology and characteristics of the three binder-stabilized saline soils. The results of this study can provide theoretical and technical support for the use of stabilized saline soils as roadbeds in Xinjiang.

2. Materials and Methods

2.1. Saline Soils

The shallow soil showed dispersion in a depth range of 0~100 cm due to salt migration aggregation, and the dispersion decreased with increasing depth of saline soil samples. Therefore, the saline soil samples were taken from a depth of 100–120 cm below the surface at coordinates 43°50′46″ N, 87°44′35″ E, at an elevation of 821 m. The samples were collected from the surface of the soil at a depth of 5.5 cm. The chemical composition of saline soils is shown in Table 1. The basic physical properties of saline soils is shown in Table 2. The XRD analysis of the saline soils, as shown in Figure 1a, indicates the presence of mainly quartz (SiO2), calcite (CaCO3), gypsum (CaSO4), and Margarite (KAl2(AlSi3O10) (OH)2). Figure 1b shows the particle size distribution curve of saline soils. The greatest particle size of saline soil particles is 20.71 μm, while the majority of particles are smaller than 75 μm.

2.2. Fly Ash, Lime, PAM and Sodium Sulfate

The FA was sourced from Class II FA of the 102nd Regiment of ZhunDong in Xinjiang, with relatively low total oxide content of SiO2 and Al2O3. The chemical composition of FA is shown in Table 3. The primary components of lime were Ca(OH)2 and a small amount of CaCO3, as indicated by the XRD analysis of FA and lime in Figure 2. Polyacrylamide was sourced from Shanghai Macklin Biochemical Technology Co., Ltd. in powder form with a purity of 99.0%. The NaSO4 was provided by General-Reagent Company in China in powder form with a purity of 99.0%.

3. Experimental Design and Methods

3.1. Sample Preparation

The saline soil was washed several times with repeated water and dried to obtain the soil, which was crushed and passed through a 2.00 mm sieve. Different proportions of binding materials (lime, FA, PAM) were added to the soil and sprayed with sodium sulfate solution calculated proportionally. After mixing well, the soil was placed in a sealed bag and simmered for 12 h to evenly distribute the moisture and sulfate. Experiments used the molding method with reference to the standard [35], the standard for the inorganic binding material using two-way static compression method, and then stabilized pressure for 2~3 min. Stabilized soil is divided into 2~3 layers and put into the mold for compaction. Then, 100 × 100 mm cylindrical specimens were prepared via the hydrostatic press method, wrapped in cling film, and labeled. The specimen was placed in a closed, constant-temperature environment for curing. The prepared sample had a moisture content of 12.5% and a dry density of 1.894 g.cm−3.
Stabilized soil consists of four components: 1% sodium sulfate, soil, binding material, and water. Stabilized soils with Wt of 6.0%, 8.0%, and 10.0% lime are denoted as L1, L2, and L3, respectively; those with Wt of 9.0%, 11.0%, and 13.0% FA are denoted as F1, F2, and F3, respectively; and those with Wt of 0.5%, 1.0%, and 1.5% PAM are denoted as P1, P2, and P3, respectively.

3.2. Unconfined Compressive Strength and Splitting Strength Test

The stabilized soil specimens, cured for 28 d and subjected to freeze–thaw cycles, were tested for UCS and splitting strength using a cement flexural-compressive testing machine (YAW-SERIES), as shown in Figure 3. The loading rate was 1.0 mm/min during the experiment, and the peak load at the time of specimen destruction was recorded.

3.3. Frost Resistance Test

Referring to the “Test Code for Hydraulic Concrete (SL352-2006)”, the freeze–thaw cycles test without water replenishment was performed with a constant-temperature box. Specimens cured for 28 d were subjected to a freeze–thaw cycles in environments of −20 °C and 20 °C, with a freezing time of 12 h and a thawing time of 12 h in each cycle. Their residual strength ratios (BDR) were calculated after undergoing different numbers of freeze–thaw cycles (FTs = N, N = 1, 3, 5, 10 and 20). The residual strength ratio (BDR) was used to evaluate the frost resistance of stabilized soil, as shown in Equation (1):
DBR = q r q p × 100 %
where q p is the strength before freeze–thaw cycles; q r is the strength after freeze–thaw cycles.

3.4. Material Characterization

In this study, SEM, XRD, FTIR, TG−DSC, and XPS were used to characterize the structural changes in stabilized soil and to infer the stabilization mechanism of different binders. The soil after stabilization for 28 d and the freeze–thaw cycles was soaked in anhydrous ethanol for no less than 24 h. After hydration was terminated, it was put into an oven at 60 °C for drying. Subsequently, the specimens were ground, sieved, and tested by XRD, SEM, FTIR, XPS, and TG−DSC. Functional groups of the materials were evaluated using an FTIR spectrometer (Great 10) over a measurement range of 400 cm−1 to 4000 cm−1 with a 200:1 KBr-to-material ratio and 32 scans. The dried stabilized soil was crushed and scanned with a Cu-Kα (λ = 1.5418 Å) X-ray diffractometer (D8 Advance model) with an X-ray diffractometer 2θ range of 10~80°, a scanning speed of 5°/min, and a scanning step of 0.02°. The microstructure of stabilized soil at different magnifications was characterized using a Hitachi S4800 scanning electron microscope. TG−DSC measurements were performed on a Hitachi STA7300 by heating 10 mg of powdered material and ramping from room temperature to 1000 °C at 10 °C/min. The elemental composition and valence states of the FA materials were analyzed using the ESCALAB250Xi energy spectrometer model from Thermo Fisher Scientific. All analyses for this experiment were adjusted to c1s (284.6 eV), and the data integrals were then fitted with Thermo advantage 5.5.2 analysis software.

4. Results and Analysis

4.1. Mechanical Performance Analysis

The UCS and splitting strength of saline soils stabilized by lime, FA, and PAM after 28 d of curing are shown in Figure 4.
As can be seen in Figure 4, after 28 d of curing, saline soils stabilized with FA, PAM, and lime increased their UCS by 55%, 23%, and 1068%, respectively, and their splitting strengths by 161%, 75%, and 2720%, respectively, compared to saline soils. These three binders improve the bond strength between the soil particles, fill the voids between the soil particles, and increase the mechanical properties of the stabilized soil. The strength of stabilized saline soils reached its maximum value when the concentrations of lime, FA, and PAM were 10%, 13% and 1.0%, respectively. PAM had the lowest stabilizing effect on saline soils, and lime had the best stabilizing effect on saline soils.
The UCS of lime-stabilized saline soils increases gradually with increasing lime admixture within the required range. The main reasons for the series of physicochemical reactions involving lime in the soil were ion exchange, Ca(OH)2 crystallization, Ca(OH)2 carbonation, and reactivity with volcanic ash. Na2SO4 is highly temperature-sensitive, and when it does not react with lime or FA, it crystallizes into Na2SO4·10H2O crystals [7,36]. During the FTs phase, these crystals interact with ice crystals to break up the gel between soil particles [37], resulting in soil failure. Na2SO4·10H2O is almost three-times the volume of Na2SO4, and the dilution of solubility leads to the destruction of soil structure and a decrease in density. It was found that the formation of salt crystals directly disrupts the cementation between soil particles, leading to the swelling of pores, and Na2SO4 greatly reduces soil strength [38].

4.2. Frost Resistance

The residual UCS ratio and splitting strength ratio (BDR) of stabilized soil after different numbers of freeze–thaw cycles are shown in Figure 5.
It can be seen from Figure 5 that the UCS and splitting strength residual strength value BDR of saline, lime, FA, and PAM-stabilized saline soils decreased and then stabilized as the number of freeze–thaw cycles increased. Among them, the decreasing trend of the residual value of the UCS is the most obvious, indicating that the freeze–thaw cycles have the greatest influence on the UCS. A comparison of the effects of three binders on the frost resistance of saline soils showed that the BDR of UCS decreased more rapidly than the BDR of splitting strength. Among them, lime-stabilized soil showed the smallest decrease in residual strength ratio (BDR); PAM-stabilized soil showed the largest decrease, which was close to that of saline soils; and FA-stabilized soil showed a decrease in residual strength ratio (BDR) between that of lime and PAM.
When the number of freeze–thaw cycles is low, the water film bonding between soil particles is transformed into ice bonding, which enlarges the volume of the ice crystals and reduces the contact area between soil particles around the ice crystals. This leads to an increase in porosity and the first disintegration of the bonded material [39]. After more than three freeze–thaw cycles, the stabilizing material between the soil particles is continuously stretched during the ice crystal expansion and contraction cycles, resulting in continuous damage to the gel film and the soil particle material, a decrease in the bond strength, an increase in the number of voids and cracks, and a macroscopic manifestation of a rapid decrease in strength. After five freeze–thaw cycles, most of the PAM- and FA-stabilizing fractions between soil particles were destroyed, and the rate of decrease in the strength of stabilized soil slowed down. After 10 cycles of freeze–thaw cycles, the gel at the edges of soil particles decreased, while the pores increased and penetrated each other, destroying the integrity of the soil [40]. The gel material between the soil particles was repeatedly torn and destroyed by the contraction and expansion of the ice crystals, leaving the soil particles completely exposed and leading to a decreasing trend in the strength of stabilized soils. In addition, the solubility of Na2SO4 is temperature-dependent. The solubility decreases with decreasing temperature. Na2SO4 crystallizes into Na2SO4·10H2O crystals when it does not react with lime and FA. These crystals along with ice crystals break the gel between the soil particles, leading to a strength failure of the soil.

4.3. Microstructure Analysis of Stabilized Soils

4.3.1. XRD Analysis

Figure 6 shows the XRD test results of saline soils and stabilized saline soils cured for 7 d and 28 d using three binders.
Figure 6a shows the XRD patterns of specimens L1, L2, and L3 after 7 days of curing. Diffraction peaks of quartz (SiO2), calcium carbonate (CaCO3), muscovite (KAl2(AlSi3O10)(OH)2), and lime (Ca(OH)2) may be seen in the lime-stabilized soil, but no saltpeter peaks can be found. The composition and proportion of components differ from the XRD of saline soils. Weak diffraction peaks of CaSO4, Ca(OH)2, and CaCO3 can be seen in L1, L2, and L3, showing the early reaction of sulfate with lime to generate CaSO4 and lime with CO2 to form CaCO3. The higher the lime content, the more CaSO4, Ca(OH)2, and CaCO3 are present, demonstrating a link between the strength of the stabilized soil and these three substances.
Figure 6b shows the XRD patterns of specimens F1, F2, and F3 after 7 d of curing. FA-stabilized soil had micro-peaks of hematite (Fe2O3). The content and percentage of each component in the stabilized soils changed, although it was not possible to determine whether FA chemically reacted with sodium sulfate. Figure 6c shows the XRD patterns of the P3 specimens after 7 d and 28 d of curing. No new minerals were produced compared to the saline soils, which indicates that the PAM did not interact with the saline soils.
Figure 6d shows the XRD pattern of specimens L3, F3, and P3 stabilized after 28 d of curing. PAM did not change the composition or proportions of the stabilized soil. FA and lime converted the sulfates in the saline soils into other minerals, reducing the salt content in the stabilized soil and increasing its strength.
The reaction products generated during the stabilization processes of lime, FA, and PAM may not be detectable by XRD. The main reason for this is that these compounds are either in a low crystalline state or they are too small to be distinguished by XRD.

4.3.2. SEM Analysis

Figure 7a,b show SEM images of sulfate saline soils. The microstructure of the saline soils shows the following characteristics: the skeletal particles are dominated by single particles, and the interstices of the particles are dominated by pores, which are basically unfilled; the soil particles are in point contact before solidification, with little bonding material, large voids, and small contact surfaces.
Figure 8a shows an SEM image of L3 cured for 28 d. Trace amounts of bound material produced by the reaction of soil particles with Ca(OH)2 can be seen in the area indicated by an arrow. Fibrous-like AFt can be seen in the area indicated by white circles, indicating interaction between lime and saline soils. The pores of L3 are filled with a large amount of filler material, which increases the contact area between particles from point to face contact. The structural and overall integrity of saline soils is improved when soil particles are almost completely bound together.
Figure 8b,c show SEM images of F3 cured for 28 d. The saline soil surface is covered with FA-hydrated binding material, and hydrated gel material is visible in the area shown in the white circle. In the binder-stabilized saline soils, some granular crystals were observed on the surface of the samples, which were salted to the generated C−A−H and C−S−H [41]. The resulting C−A−H and C−S−H gels [42] fill the pores between the soil particles and connect the soil particles together to form a cohesive whole. In saline soils, the bonding mechanism between soil particles shifts to face contact, where the bonding area is larger than point contact. After 28 d of hydration, the relatively low content of amorphous hydrated calcium (aluminum) silicate (C−(A)−S−H) gel [43] in FA reacted with sulfate to produce a small amount of Aft [24], which was not observed in the results of XRD experiments. Various gels encapsulate the saline soil particles and fill the pores, thereby increasing the strength of the specimen.
Figure 8d shows an SEM image of P3 cured for 28 d. The PAM gel fills the pores [31] between the saline soils particles, but the particles continue to make point-to-point contact, resulting in an increase in the strength of the saline soils [44]. These compounds fill some of the pores on the surface and above the sample, reducing the porosity and increasing the density.
Figure 9 shows SEM images of the stabilized soil after 3 freeze–thaw cycles. Li Jinze carried out a characterization of stabilized soil specimens with a lime content of 3% and 9% after 5 freeze–thaw cycles. It was found that with 9% lime content, not only the surface material was damaged but also some of the particles were directly crushed [41]. In Figure 9a, it can be seen that there is significant cracks in saline soils [40]. No small cracks were observed in L3 (Figure 9b), and the bonding mechanism between the soil particles was not altered, suggesting that the early strength loss of the lime stabilized soil after freeze–thaw cycles was negligible. Figure 9c shows that there are no visible cracks between soil grains in F3 stabilized saline soils. This indicates that the FA-stabilized soil has less strength loss after three freeze–thaw cycles. The strength of the PAM-stabilized soil with low salinity is remarkable, as shown in Figure 9d, where the film on the soil particles disappears and the connection of the soil particles in point-to-point contact is changed.
Figure 10 shows SEM images of stabilized soil subjected to 20 freeze–thaw cycles. When subjected to 20 freeze–thaw cycles, the L3-stabilized soil showed microcracks at the locations designated by arrows in Figure 10a,b, but the bonding pattern remained unchanged. The area shown by the white circle in Figure 10c shows damage to the gel material on the surface with the formation of large cracks, suggesting a large change in particle morphology after 20 freeze–thaw cycles. As a result, after 20 freeze–thaw cycles, FA-stabilized soil significantly loses strength. In Figure 10d, after 20 freeze–thaw cycles, the film on the surface of soil particles almost disappeared, leading to the dispersion of soil particles and loss of strength in PAM-stabilized soil.
The lime stabilization of saline soils, after 28 d of curing, resulted in the production of CaSO4 and CaCO3 binding substances, which made the saline soils denser, with enhanced mechanical properties and frost resistance. FA hydration generates C−(A)−S−H gel material, which partially reacts with SO42− to generate calcite, which covers the saline soil particles surface and enhances the saline soils’ properties. The PAM film wraps around the soil particles and fills the pores, which enhances the mechanical properties of the saline soils [45]. The three materials provide different kinds of cementing substances so that the soil particles are connected in a way that changes them into face–face, side–face, and side–side, and become a whole with definite structural characteristics. This improves the mechanical properties and frost resistance of stabilized saline soils.

4.3.3. FTIR Analysis

The FTIR of stabilized soil L3, F3, and P3 after curing for 1, 3, 7, and 28 d is shown in Figure 11.
Figure 11a shows the main reactive functional groups in L3-stabilized saline soils. The unique absorption peak near 3490 cm−1 relates to the stretching vibration of hydroxyl (−OH) groups [46], attributed to Ca(OH)2−OH, which weakens as it increases in time, suggesting the transformation and reduction of Ca(OH)2 into other compounds. The vibration at 1603 cm−1 is caused by the bending vibration of adsorbed water (−OH). The absorption peaks near 463–473 cm−1 and 779–797 cm−1 correspond to the asymmetric bending vibrations of Si−O−Si and Al−O, respectively [47]. In addition, a band at 1360 cm−1 indicates O−C−O production in carbonate compounds [47]. The deformation of C−O causes the creation of CO32− when lime reacts with CO2 in the air. XRD and SEM showed that lime combined with Na2SO4 to form a binding compound, which is chemically solidified.
Figure 11b shows the FTIR spectrum of the F3-stabilized soil. All specimens exhibit steadily strengthening vibration peaks between 3490 cm−1 and 1601 cm−1, which belong mainly to the −OH stretching vibrations of adsorbed water and C−S−H, showing an increase in C−S−H production, agreeing with the SEM results. The peak at 1360 cm−1 was attributable to the stretching vibration of C−O, showing that CO32− originates via the reaction of alkaline chemicals in FA with CO2 in the air. The absorption bands at 970 cm−1 and 450 cm−1 were characteristic of C−S−H gels [46]. The asymmetric bending vibration range of Si−O−Si is 463 cm−1 to 473 cm−1 originates mainly from C−S−H and unhydrated SiO2 [48]. The absorption peaks near 779–797 cm−1 were attributed to the asymmetric bending vibrations of Al−O [17], which may have formed as a consequence of the substitution of Si4+ by Al3+ in C−S−H gels. The combination of XRD and SEM results revealed that FA stabilized to form C−S−H and (hydrated calcium aluminate) C−A−H, which are classified as being chemically solidified.
Figure 11c shows the FTIR spectrum of the P3-stabilized soil. The absorption peaks at 3490 cm−1 and 1603 cm−1 are the characteristic −OH stretching vibrations of adsorbed water. The peak at 1360 cm−1 was attributable to the stretching vibration of C−O, showing CO32− absorption in the carbonate of saline soils. The absorption peaks near 463 cm−1 to 473 cm−1 and 779 cm−1 to 797 cm−1 correspond to the asymmetric bending vibrations of Si−O−Si and Al−O in soil, respectively. During the stabilization of saline soils by PAM, no new functional groups were formed, indicating that PAM did not react with the soil components, resulting in physical solidification.

4.3.4. TG Analysis

The TG−DSC of L3-stabilized soil cured for 28 d, FA, FAS (FA hydrated in deionized water for 28 d), and FAS1 (FA hydrated in a solution of Na2SO4:H2O at a ratio of 1:12.5 for 28 d) is shown in Figure 12.
Figure 12a shows the TG−DSC curve of L3-stabilized soil after 28 d of curing. The dehydration of Ca(OH)2, gypsum (CaSO4), CaCO3, and adsorbed and crystalline water in cementitious materials was responsible for the approximately 1.3% mass loss in the 0–380 °C range [49]. XRD and FTIR data confirm the existence of Ca(OH)2 and explain the 1.28% mass loss in the 380–420 °C range [50]. The breakdown of CaCO3 [51] causes a mass loss of about 2.33% in the 550–700 °C range, which was consistent with the XRD data.
Figure 12b shows the TG−DSC curve of FA-stabilized saline soils. There was no mass loss between 0 and 120 °C, meaning no adsorbed water in FA. The oxidation of carbon and organic materials in FA accounts for the 5.3% mass loss in the 400–500 °C range, as well as the exothermic peak observed. Carbonate breakdown results in a 0.51% mass loss between 650 and 750 °C.
Figure 12c,d show the TG−DSC curves of FAS and FAS1. The 1.07% mass loss in the 0–120 °C range can be attributed to the dehydration of the hydration products C−S−H, C−(A)−S−H, and AFt [50,52]. The mass loss in the range of 120–200 °C was attributable to the dehydration of the bound water in the hydration product C−S−H gels and AFt, indicating the generation of hydration products from FA [52], agreeing with the SEM and FTIR results. The mass loss in the 400–500 °C range was due to carbon oxidation in FA, which contradicts the endothermic dehydroxylation reaction of Ca(OH)2 [53,54]. Carbonate breakdown, mostly produced by the interaction of Ca2+ and Na+, accounts for the mass loss between 650 and 750 °C [50]. The hydration products created in the FA-stabilized soil, such as C−A−H, C−S−H and C−(A)−S−H, were the primary components that contributed to the soil’s strength. The strength increases with increasing C−A−H, C−S−H and AFt content.

4.3.5. XPS Analysis

Figure 13a shows the XPS complete spectra of FA, FAS, and FAS1. Peaks for Ca 2p, Si 2p, Al 2p, Fe 2p, Na 1s, Mg 1s, and O 1s can be seen, suggesting the presence of elements, like Ca, Si, O, Al, Na, and Mg, in the samples. Each sample spectrum has peaks for the same elements, with noteworthy variations in width and height, meaning that the elements in the samples are the same, but different types of compounds are present.
Figure 13b shows that there are variances in the binding energies of oxygen in the samples, indicating differences in the chemicals bound to oxygen in the three samples [55].
Figure 13c shows that the Ca 2p binding energies in FA and FAS1 are 346.61 eV and 350.27 eV, respectively [56,57]. The consistent Ca binding energies show that both have similar chemical stability, implying the presence of molecules such as CaCO3, CaSO4•2H2O, and C−S−H. The decreased binding energy of Ca in FAS indicates that Ca remains more active, making it more susceptible to substance binding, perhaps involving CaCO3 and C−S−H.
Figure 13d shows that the binding energies in the S 2p fitting spectra of FA, FAS, and FAS1 were 167.76 eV, 167.84 eV, and 168.10 eV, respectively [46,58]. The chemicals for FAS and FAS1 are gypsum and AFt. The greater binding energy of S molecules in FAS1 as compared to FA and FAS suggests that FA and FAS can continue to react with Na2SO4. The Si 2p fitting spectra of FA, FAS, and FAS1 show binding energies of 100.83 eV, 101.06 eV, and 101.26 eV, respectively [46,58]. The comparable chemicals include SiO2, C−S−H, and AFt. FA contains active SiO2, which hydrates to provide increased binding energy C−S−H, resulting in more stable AFt in the presence of sulfate.
Furthermore, the Al 2p fitting spectra of FA, FAS, and FAS1 show binding energies of 72.78 eV, 73.20 eV, and 73.30 eV, respectively [58]. The analogous chemicals may be Al2O3 and C−A−H. Al molecules in FA have a lower binding energy than FAS and FAS1, meaning that they remain more reactive.
Compared to FA, the binding energies of O 1s, Ca 2p, Si 2p, and Al 2p in FAS and FAS1 are lower. From the results of XRD, SEM, FTIR and TG−DSC, it is evident that the high reactivity of FA leads to the formation of C−S−H and C−A−H gel materials through hydration reactions and the solidification of sulfate compounds.

5. Discussions and Conclusions

5.1. Discussions

Figure 14 shows an SEM image of L3 cured for 14 d. Lime particles adhere uniformly to the surface of the saline soil particles. Lime can be turned into CaSO4 and CaCO3 over time via ion exchange and carbonation reactions, resulting in a stronger connection between saline soil particles. The adhesion of lime to the surface of the saline soil particles makes it easier to fill the gaps between them, resulting in a cohesive solid with higher UCS and splitting strength. Li Jinze’s research showed that, when the quicklime content was 3% and the FTs was 0, the surface pores of the sample were reduced, and most of the pores were filled with gels, making it difficult to observe the exposed soil particles [41]. This procedure eventually improves the strength of the lime-stabilized soil.
Upon dissolution in water, FA raises the pH of the solution to an alkaline level, resulting in the production of flocculated hydration products such as C−S−H and C−A−H. As shown in Figure 15, after 7 days of FA hydration, a flocculating gel is formed, in which some of the material solidifies by interacting with sulfate to form needle-like AFt. Some of these materials react with sulfates to solidify and produce needle-like AFt. The surface of saline soil particles is wrapped by these flocculated gel materials, which fill the voids and make the microstructure of the stabilized soil denser, thus increasing the strength of FA-stabilized soil. XRD analysis showed that a large amount of unhydrated SiO2 was still present, resulting in a relatively poor stabilization effect of FA on saline soils.
PAM is a water-based gel material that does not react with sulfates during the curing process of saline soils, and it wraps around the surface of saline soil particles. Under the action of freezing and thawing cycles, salt expansion and freezing expansion work together to destroy the PAM gel, leading to the overall destruction of the soil particles and a reduction in strength. Therefore, PAM-stabilized soil has the worst mechanical properties and frost resistance.

5.2. Conclusions

In this study, saline soils were stabilized by incorporating different proportions of lime, FA, and PAM binders and were subjected to UCS, splitting strength, freeze–thaw cycle tests, XRD, SEM, and TG tests. The major findings of this study are as follows:
(1)
The addition of lime, FA, and PAM significantly increased the strength of saline soils. The UCS and splitting strength of stabilized saline soils gradually increased with an increase in binding material content. The best performance was obtained for lime, followed by FA, and the worst performance was obtained for PAM. The results of XRD, FTIR, SEM, and TG−DSC analyses showed that lime chemically solidified the soil by generating CaSO4 and CaCO3 as binding substances through ion exchange and carbonation.
(2)
The number of freeze–thaw cycles has a greater effect on the strength of stabilized soil; with an increase in the number of freeze–thaw cycles, the UCS and splitting residual value of stabilized soil decrease. SEM test results show that the internal structure of stabilized soil is more compact before freeze–thaw cycles. With an increase in the number of freeze–thaw cycles, the porosity of the stabilized soil increased, the contact between soil particles began to change, and the structure became loose; after 20 freeze–thaw cycles, a small amount of voids appeared in the lime-stabilized soil, a large crack appeared in the FA-stabilized soil, and the structure of the PAM-stabilized soil was destroyed.
(3)
Saline soils have a granular structure, and the contact mode is point to point. Lime-stabilized soil changes the cementing substances from Ca(OH)2 and Na2SO4 to CaSO4 and CaCO3 through ion exchange and carbonation, which fill the pores between soil particles and change the contact between particles to face-to-face contact, thus enhancing the mechanical properties of saline soils.
(4)
XRD, SEM, FTIR, and TG analyses of FA-stabilized soil were consistent with the observed mechanical properties. The hydration of FA to form a C−(A)−S−H gel altered the microstructure of saline soils.
This study mainly investigated the mechanical properties and frost resistance of stabilized soil with 1% sulfate content, and we did not investigate the shrinkage characteristics of stabilized soil, which can be further explored in future studies.

Author Contributions

Conceptualization, S.L., R.Y. and Y.L.; methodology, S.L.; validation, S.L., Y.L. and Q.C.; formal analysis, Y.L.; investigation, R.Y.; data curation, Q.C.; writing—original draft preparation, S.L.; writing—review and editing, S.L.; visualization, J.S.; funding acquisition, Y.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Scientific and Technological Research Programs in key Areas of Xinjiang Production and Construction Corps Science and technology Bureau, grant number [2023AB013-01], Major Science and Technology Special Projects in Xinjiang Uygur Autonomous Region, grant number [2023A03004-04], and Xinjiang Uygur Autonomous Region Science and Technology Department, grant number [2023B03011-3].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Applsci 14 07246 g001
Figure 1. (a) XRD of saline soils and plain soil; (b) grain size distribution of saline soils.
Figure 1. (a) XRD of saline soils and plain soil; (b) grain size distribution of saline soils.
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Figure 2. XRD pattern of FA and lime.
Figure 2. XRD pattern of FA and lime.
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Figure 3. (a) Before UCS test; (b) after UCS test; (c) before splitting strength test; (d) after splitting strength test.
Figure 3. (a) Before UCS test; (b) after UCS test; (c) before splitting strength test; (d) after splitting strength test.
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Figure 4. Mechanical properties of stabilized soil at 28 d: (a) UCS; (b) splitting strength.
Figure 4. Mechanical properties of stabilized soil at 28 d: (a) UCS; (b) splitting strength.
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Figure 5. Strength residual ratio after freeze–thaw cycles: (a) UCS DBR of stabilized soils; (b) splitting strength DBR of stabilized soils.
Figure 5. Strength residual ratio after freeze–thaw cycles: (a) UCS DBR of stabilized soils; (b) splitting strength DBR of stabilized soils.
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Figure 6. XRD results for saline soil solidification: (a) L1, L2, L3 stabilized for 7 d; (b) F1, F2, F3 stabilized for 7 d; (c) P3 stabilized for 7 d and 28 d; (d) L3, F3, P3 stabilized for 28 d.
Figure 6. XRD results for saline soil solidification: (a) L1, L2, L3 stabilized for 7 d; (b) F1, F2, F3 stabilized for 7 d; (c) P3 stabilized for 7 d and 28 d; (d) L3, F3, P3 stabilized for 28 d.
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Figure 7. (a,b) SEM of saline soils.
Figure 7. (a,b) SEM of saline soils.
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Figure 8. SEM images of the three binders stabilized for 28 d: (a,b) L3; (c) F3; (d) P3.
Figure 8. SEM images of the three binders stabilized for 28 d: (a,b) L3; (c) F3; (d) P3.
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Figure 9. SEM of after 3 freeze–thaw cycles: (a) saline soils; (b) L3; (c,d) F3.
Figure 9. SEM of after 3 freeze–thaw cycles: (a) saline soils; (b) L3; (c,d) F3.
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Figure 10. SEM after 20 freeze–thaw cycles: (a,b) L3 FTs = 20; (c) F3 FTs = 20; (d) P3 FTs = 20.
Figure 10. SEM after 20 freeze–thaw cycles: (a,b) L3 FTs = 20; (c) F3 FTs = 20; (d) P3 FTs = 20.
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Figure 11. FTIR spectra of stabilized saline soils for 1, 2, 7, and 28 d: (a) L3; (b) F3; (c) P3.
Figure 11. FTIR spectra of stabilized saline soils for 1, 2, 7, and 28 d: (a) L3; (b) F3; (c) P3.
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Figure 12. TG−DSC curves of stabilized saline soils: (a) L3 stabilized for 28 d; (b) FA (c) FAS (d) FAS1.
Figure 12. TG−DSC curves of stabilized saline soils: (a) L3 stabilized for 28 d; (b) FA (c) FAS (d) FAS1.
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Figure 13. XPS spectra of O, Ca, S, Si, and Al in FAS, FAS1 hydration 28 d, and FA: (a) wide range; (b) O 1s; (c) Ca 2p; (d) S 2p; (e) Si 2p; (f) Al 2p.
Figure 13. XPS spectra of O, Ca, S, Si, and Al in FAS, FAS1 hydration 28 d, and FA: (a) wide range; (b) O 1s; (c) Ca 2p; (d) S 2p; (e) Si 2p; (f) Al 2p.
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Figure 14. SEM of lime solidification at 14 d: (a) enlarged 2000-times; (b) enlarged 10,000-times.
Figure 14. SEM of lime solidification at 14 d: (a) enlarged 2000-times; (b) enlarged 10,000-times.
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Figure 15. SEM of FAS hydration at 7 d: (a) needle-like AFt and gel material; (b) short rod AFt and net gel material.
Figure 15. SEM of FAS hydration at 7 d: (a) needle-like AFt and gel material; (b) short rod AFt and net gel material.
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Table 1. Chemical compositions of saline soils (wt%).
Table 1. Chemical compositions of saline soils (wt%).
Saline SoilsSiO2Al2O3CaOFe2O3MgOK2ONa2OClSO3
Content (%)61.7815.638.574.973.392.791.660.170.09
Table 2. Basic physical properties of saline soils.
Table 2. Basic physical properties of saline soils.
Grain Size Distribution/%Maximum Dry Density (g.cm−3)Optimum Water Content (%)Salt Content (wt%)
>50 μm20~50 μm10~20 μm<10 μmNa2SO4NaClNa2CO3
saline soils2.4126.6433.3631.591.89412.50.860.110
Table 3. Chemical compositions of FA (wt%).
Table 3. Chemical compositions of FA (wt%).
FASiO2Al2O3CaOFe2O3MgOK2ONa2OClSO3
Content (%)37.3117.3813.1415.075.951.282.600.275.23
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Li, S.; Huang, Y.; Sun, J.; Yu, R.; Liu, Y.; Cui, Q. Research on Solidification Methods and Stabilization Mechanisms of Sulfate Saline Soils. Appl. Sci. 2024, 14, 7246. https://doi.org/10.3390/app14167246

AMA Style

Li S, Huang Y, Sun J, Yu R, Liu Y, Cui Q. Research on Solidification Methods and Stabilization Mechanisms of Sulfate Saline Soils. Applied Sciences. 2024; 14(16):7246. https://doi.org/10.3390/app14167246

Chicago/Turabian Style

Li, Sining, Yong Huang, Jian Sun, Rui Yu, Yubin Liu, and Qiushuang Cui. 2024. "Research on Solidification Methods and Stabilization Mechanisms of Sulfate Saline Soils" Applied Sciences 14, no. 16: 7246. https://doi.org/10.3390/app14167246

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