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Article

Research on the Thermal Conductivity and Microstructure of Calcium Lignosulfonate-Magnesium Oxide Solidified Loess

by
Yuwen Lu
1,2 and
Wuyu Zhang
1,2,*
1
School of Civil Engineering and Water Resources, Qinghai University, Xining 810016, China
2
Key Laboratory of Energy-Saving Building Materials and Engineering Safety, 251 Ningda Road, Xining 810016, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(8), 4545; https://doi.org/10.3390/app15084545
Submission received: 19 March 2025 / Revised: 11 April 2025 / Accepted: 18 April 2025 / Published: 20 April 2025
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Abstract

:
Loess, characterized by high porosity, a loose structure, and weak cementation, is highly prone to deformation and cracking under thermal stress, which significantly affects the bearing capacity of foundations and the stability of underground engineering structures. This study introduces an innovative approach that utilizes the eco-friendly modifier calcium lignosulfonate (CL) in combination with magnesium oxide (MgO) for the carbonation solidification treatment of loess. The research systematically investigated the thermal conductivity and underlying micro-mechanisms of the treated soil. A series of tests, including analyses of basic physical properties, measurements of thermal conductivity, X-ray diffraction (XRD), and scanning electron microscopy (SEM), were conducted to evaluate the effects of CL dosage, freeze–thaw cycles, moisture content, and dry density on the thermal conductivity of carbonation-solidified loess. The results indicate that carbonated solidified loess absorbed approximately 6% of CO2, while effectively reducing its collapsibility grade to a slightly collapsible classification. Additionally, its thermal conductivity decreased by 16.7%, thereby mitigating the influence of various environmental factors. Based on the experimental results, a microscopic mechanism model was developed. This study presents a sustainable and innovative technical solution for stabilizing loess foundations in cold regions.

1. Introduction

Loess, a predominant soil type in Northwest China, is characterized by large pores, weak cementation, and high collapsibility. These inherent properties lead to rapid reductions in soil strength when exposed to water infiltration or freeze–thaw cycles, often resulting in engineering challenges such as landslides and uneven settlement [1,2]. Additionally, the region’s significant diurnal temperature variations exacerbate issues like frost heave, making the optimization of soil thermal conductivity a crucial research focus [3]. Thermal conductivity is a key parameter influencing heat transfer efficiency and plays a pivotal role in the thermodynamic behavior of underground engineering, ensuring structural stability in cold regions. In such environments, soil thermal conductivity affects heat transfer during freeze–thaw cycles, impacting frost heave deformation and temperature distribution. Consequently, the application of additives to improve soil thermal conductivity has gained increasing attention in recent years. Traditional stabilizers such as cement and lime enhance thermal conductivity by modifying soil particle interactions, altering porosity distribution, and optimizing heat conduction pathways [4,5]. Additionally, they improve soil properties, including strength and stability [6,7,8,9]. However, these conventional methods pose challenges due to environmental pollution and significant CO2 emissions [10,11]. As the demand for sustainable development grows, research has increasingly focused on exploring novel eco-friendly soil stabilization materials to optimize thermal conductivity, while minimizing environmental impact. Wang et al. [12] found that lignin possesses strong chelating properties, which can enhance the efficiency of nutrient utilization in plants, improve soil structure, and increase soil enzyme activity. Additionally, it adsorbs soil pollutants through ion exchange and complexation, promoting the degradation of organic pollutants by microorganisms, thereby facilitating the remediation of contaminated soils. Guo et al. [13] studied the application of lignin-based adsorbents in alleviating soil heavy metal pollution (HMP), focusing primarily on their adsorption performance, mechanisms, kinetics, and thermodynamic models under different environmental conditions. The reusability of lignin-based adsorbents has been emphasized to improve their economic feasibility. Xia et al. [14] demonstrated that reactive oxygen species generated by advanced oxidation technologies can degrade organic pollutants; simultaneously, the use of MgO and its hydrolysis product magnesium hydroxide (Mg(OH)2) can stabilize and immobilize heavy metal ions. Furthermore, this system can also serve as magnesium fertilizer, oxygen fertilizer, and nitrogen fertilizer, thereby increasing crop yields. Zhang et al. [15] concluded that MgO/magnesium hydroxide (Mg(OH)2) nanoparticles successfully combined with soil carbon to form magnesium carbonate (MgCO3), which mitigated CO2 emissions. The high carbon retention of magnesium carbonate (MgCO3) increased cellulase activity, stimulated key carbon-cycling bacteria, and enhanced the expression of genes related to the degradation of starch, sucrose, amino sugars, nucleotide sugars, ascorbic acid, cellulose, and chitin, thus boosting organic carbon mineralization.
Among the various stabilization methods, MgO carbonation technology has emerged as a promising solution due to its dual role as a carbon sequestration technique and soil stabilizer [16,17]. This approach not only enhances soil strength, durability, and thermal conductivity but also reduces carbon emissions, positioning it as an environmentally friendly alternative for stabilizing underground soil environments. Lignin, the second most abundant biopolymer on Earth after cellulose, possesses a phenolic structure that endows it with superior ion-binding capacity and resistance to degradation [18]. Here, the high resistance of lignin to moisture and how this aspect preserves the mechanical properties of this polymer [19] should be mentioned. Lignin has been extensively studied for its potential to enhance the engineering properties of soils [20]. For instance, Liu et al. [21,22,23,24,25] reported that magnesium carbonate crystals formed during MgO carbonation effectively fill soil pores, significantly improving thermal conductivity. Simultaneously, it can enhance soil strength in embankment filling projects, increase the utilization rate of CO2 emissions from construction, and reduce the consumption of natural earth and stone materials. Frazao et al. [26] confirmed a positive correlation between the degree of MgO carbonation and thermal conductivity. Additionally, Jun et al. [27] observed via scanning electron microscopy that when the MgO content reached 8%, the resulting carbonation products fully occupied soil pores, leading to a 31.2% increase in thermal conductivity. In the context of lignin modification, Wu et al. [28] demonstrated that incorporating modified lignin significantly reduces soil porosity, while forming stable interfacial structures that reduce thermal conductivity. Furthermore, Long et al. [29] found that lignin modification decreased soil thermal conductivity by approximately 15–25%. These findings underscore the potential of lignin as an eco-friendly material for optimizing the thermal properties of loess and other soils. Liu et al. [30,31] tested lignin application for improving subgrade silty soil on-site, revealing that it effectively bonds soil particles and fills voids, enhancing the engineering properties of the improved soil. In conclusion, CL and MgO are two environmentally friendly soil-improvement materials, and each offers unique advantages for enhancing the thermal conductivity of loess. Their synergistic application presents a promising approach for precisely regulating soil thermal conductivity, providing an innovative technical solution to the challenges of cold-region engineering construction, while aligning with sustainable development goals.
Building on this contextual analysis, this study investigates the synergistic effects of CL and MgO carbonation in enhancing the thermal conductivity of loess, addressing a critical theoretical gap in soil improvement research. The interaction mechanisms between CL, an environmentally friendly biopolymer, and MgO carbonation technology remain largely unexplored in the existing literature. Elucidating these synergistic mechanisms will significantly advance fundamental thermal conduction theory in modified soils. Employing a comprehensive multi-scale approach—ranging from macroscopic performance to microscopic structure—this study systematically characterized the thermal conductivity properties of CL and MgO carbonated stabilized loess. By revealing the underlying heat transfer mechanisms and their intricate interrelationships, the findings establish a strong theoretical foundation for optimizing the thermal performance of subgrade engineering developments in loess regions. Moreover, this research provides essential scientific evidence supporting the adoption of sustainable and eco-friendly soil stabilization technologies in engineering practice.

2. Materials and Methods

2.1. Test Materials

The loess samples were collected at a 4 m depth below the ground surface at a site in Xining, China. The undisturbed loess, exhibited a brownish-yellow hue, characterized by a loose structure and relatively uniform texture. Table 1 summarizes the laboratory-determined index properties of the loess samples. Notably, the natural water content was lower than its plastic limit, indicating that the in situ loess was in a relatively dry state. The grain size distribution of the loess is illustrated in Figure 1. Based on the index properties presented in Table 1 and the grain size distribution results in Figure 1, the loess was classified as lean clay under the Unified Soil Classification System (USCS). However, the grain size distribution curve reveals that silt-sized particles constituted approximately 90% of the soil, underscoring their dominant role in influencing the material’s thermal conductivity behavior.
The CL (C20H24CaO10S2) used in the experiments was a yellowish-brown, powdery solid with a distinct aromatic odor. Fourier-transform infrared spectroscopy (FT-IR) analysis, as shown in Figure 2a, identified several characteristic absorption peaks corresponding to its functional groups and molecular vibrations. The absorption peak at 3403 cm−1 corresponds to the stretching vibration of hydroxyl (O-H) groups, while the peak at 1634 cm−1 reflects the bending vibration of H-O-H in crystal water molecules. The peak at 1195 cm−1 is associated with the symmetric stretching vibration of the sulfonic group (S=O), and the range of 613–523 cm−1 corresponds to the characteristic vibrations of the Ca-O bond. These spectral peaks confirmed the presence of distinct functional groups and the chemical structure of CL, contributing to its effectiveness as a soil modifier. The MgO used in the experiments was light MgO with a purity of at least 99% and a particle size of 200 mesh (0.075 mm). It appears as a white powder that is highly soluble in water and grease. Figure 2b, derived from FT-IR analysis, shows a prominent absorption peak at 426 cm−1, corresponding to the lattice vibration of the Mg-O bond. The unique absorption peaks identified in both materials provide critical insights into their chemical structures and functional groups. These structural features are essential for effective soil enhancement, particularly for improving the thermal properties of treated soils.

2.2. Experimental Tests

2.2.1. Sample Preparation Plan

To prepare the soil samples, the collected loess was oven-dried at 105 ± 2 °C for over 8 h until weight stabilization. After cooling, the dried soil was sieved through a 2 mm mesh. Based on the experimental design, specified amounts of CL at varying dosages and MgO at a fixed 10% were weighed using a high-precision balance (accuracy: 0.01 g). The soil and modifiers were thoroughly mixed for 5 min to ensure uniformity. Distilled water was sprayed onto the mixtures to achieve the target moisture content, then sealed with plastic wrap and conditioned for 24 h under controlled temperature and humidity to ensure uniform water distribution and minimize evaporation. The mixtures were molded into ring-knife specimens (61.8 mm diameter, 20 mm height) using a two-layer compaction method to achieve 95% relative compaction, ensuring sample consistency.
Table 2 outlines the experimental protocol for investigating the synergistic improvement of thermal conductivity in loess using CL and MgO. The experimental framework involved several geotechnical tests: liquid and plastic limits were determined according to ASTM D4318-17 [32], compaction was assessed using ASTM D698-12e1 [33], collapsibility was evaluated based on ASTM D5333-14 [34], carbonation followed ISO 1920-12:2015 [35], freeze–thaw cycles adhered to ASTM D560-16 [36], and thermal conductivity was measured under controlled conditions following ASTM D5334-14 [37]. CL dosages of 0.5%, 1%, 1.5%, 2%, and 3% were selected to capture the full range of modification effects, while MgO was maintained at 10% based on its optimal carbonation efficiency and soil stabilization performance [38]. According to the preliminary experiments, the strength was highest when the CL content was 1%. Therefore, the thermal conductivity tests were conducted using this dosage as the standard. Moisture content levels of 8%, 12%, 16%, and 20% were chosen to represent conditions ranging from near-dry to near-saturated states, while dry densities of 1.4, 1.5, 1.6, and 1.7 g/cm3 spanned loosely packed to highly compacted conditions, allowing for a comprehensive evaluation of their effects on thermal conductivity. Freeze–thaw cycles of 0, 2, 8, 15, and 20 were applied to simulate seasonal environmental fluctuations in loess regions, providing insights into the durability and long-term stability of the modified material. The experimental design integrated the physicochemical interactions between CL and MgO, while accounting for environmental stressors, generating reliable data to elucidate the mechanisms of CL–MgO synergy in enhancing thermal conductivity in loess. The findings offer a foundation for developing improved soil stabilization techniques under challenging conditions.

2.2.2. Testing Methods

The Freeze–Thaw Cycle experiment utilized a three-dimensional freezing closed-system model in which the specimens were sealed with plastic wrap to simulate a closed environment. This setup ensured stable testing conditions and enhanced the reliability of the experimental results (refer to Figure 3). The experimental parameters were determined based on the average monthly minimum temperature in Xining during the winter season (−14.4 °C). Accordingly, the freezing temperature was set to −15 °C and the thawing temperature to 15 °C, replicating natural environmental conditions, while maintaining the structural integrity of the soil specimens. Preliminary tests indicated that freezing the specimens to −15 °C required 11.36 h, while thawing them to 15 °C took 9.02 h. To meet experimental requirements and maintain consistency, each freeze–thaw cycle was standardized to 12 h of freezing and 12 h of thawing. This alternating temperature regime enabled researchers to monitor changes in the soil’s physical and thermal properties under conditions that closely simulated the natural freeze–thaw environment. Each experiment required the preparation of three test samples with consistent specifications and uniform quality. These samples were tested separately according to the experimental requirements to ensure the reliability and statistical significance of the test data.
The carbonation test was conducted using a YMS9015 carbonation chamber (Figure 4), carbonization chamber purchased from Hangzhou Jiuen Automation Technology Co., Ltd., Hangzhou, China, designed to provide precise control over CO2 concentration, humidity, and temperature. The chamber was connected to a CO2 gas cylinder and a water tank, allowing for adjustments to the desired experimental conditions. Sensors integrated into the system continuously monitored temperature, humidity, and CO2 concentration in real time. Specimens were placed on trays inside the chamber, and the control panel located on the top of the chamber was used to configure the experimental conditions: temperature (20 ± 2 °C), humidity (96%), and CO2 concentration (30%). The equipment was able to reach these set conditions and operate stably within 15 min. Preliminary tests indicated that the specimens achieved near-complete carbonation after 12 h of exposure. Based on these observations, a carbonation cycle of 24 h was selected to ensure full carbonation of the specimens. It was confirmed that no additional curing was necessary after the 24 h carbonation cycle, as the experimental requirements were fully met within this period. Each set of experiments required the preparation of three test specimens with consistent specifications and uniform quality. Carbonation tests were conducted on each specimen in accordance with test requirements to ensure the accuracy of the degree of carbonation and subsequent carbonation test data.
Thermal Conductivity Test Method: The experimental equipment used in this study was a TPS1500 Hot Disk Thermal Constants Analyzer, purchased from Kegonas Instrument Trading Co., Ltd., Shanghai, China (Figure 5), designed for precise measurement of thermal properties. Test specimens were prepared using a ring cutter, producing cylindrical samples with a diameter of 61.2 mm and a height of 20 mm. For each experimental group, two parallel specimens were prepared and used during testing. During the test, a nickel disk probe was securely placed between the two specimens (Figure 5) to measure thermal conductivity. To minimize external interference and ensure accurate results, a windproof cover was placed over the setup. The system was then left undisturbed for 15 min to allow thermal equilibrium to be achieved. Each group of specimens was tested three times, and the average of the three measurements was recorded as the final result, ensuring the consistency and reliability of the experimental data.

3. Results and Discussion

3.1. Basic Physical Properties Test

3.1.1. Atterberg Limits

The moisture content that marks the transition of soil from a semi-solid state to a plastic state is called the plastic limit (WL), while the moisture content that indicates the transition from a plastic state to a liquid state is referred to as the liquid limit (WP). The plasticity index (IP) is defined as the difference between the liquid limit and the plastic limit. They are important indicators for evaluating clayey soil. The liquid limit, plastic limit, and plasticity index of CL-MgO carbonized and solidified loess are presented in Figure 6. The results indicate that the plastic limit of the treated loess samples increased with higher additive content, showing a notable increase of approximately 13%. Conversely, both the liquid limit and the plasticity index exhibited decreasing trends, with the liquid limit decreasing by 11% and the plasticity index decreasing significantly by 33%. These findings align with the results of Cai et al. [39], who reported similar behavior in silt treated with MgO. From a chemical reaction perspective, when reactive MgO undergoes hydration, it releases a significant amount of OH− ions. These ions interact with the charged surfaces of soil particles, triggering ion exchange and modifying the electrochemical properties of the soil. This reaction alters the structure of the double electric layer surrounding the soil particles, thereby affecting the interaction strength between the soil and water molecules [40]. From a microstructural standpoint, the hydration of MgO produces magnesium hydroxide (Mg(OH)2), which acts synergistically with CL [41,42]. This magnesium hydroxide (Mg(OH)2) is distributed in a flocculent form within the soil pores and interacts with CL on the surface of soil particles. This synergy promotes particle rearrangement and aggregation, reducing the soil’s specific surface area and decreasing its water adsorption capacity. The combined physical and chemical processes lead to significant macroscopic changes in soil properties. Specifically, the plasticity range of the solidified soil is reduced, reflecting a decrease in soil plasticity. These changes enhance the engineering performance of carbonized and solidified loess, making it more suitable for construction and environmental applications.

3.1.2. Compaction Curves

The compaction test results (Figure 7) illustrate the compaction behavior of the carbonized and solidified loess under varying additive dosages. The findings indicate that the optimum moisture content of the treated loess increased significantly compared to untreated loess, showing a consistent upward trend as the additive dosage was increased. Conversely, the maximum dry density exhibited a slight decrease with increasing additive dosage; however, the variation was minor and not statistically significant. Based on a comprehensive analysis of the test results, this study established a representative optimum moisture content of 16.5% and a maximum dry density of 1.63 g/cm3 as standard parameters for soil preparation, ensuring experimental reliability and practical applicability.

3.1.3. Collapsibility Test

To evaluate the effectiveness of CL combined with MgO carbonated solidified in improving the collapsibility of loess, indoor water immersion confined compression tests were conducted under various loading pressures (50 kPa, 100 kPa, 150 kPa, and 200 kPa). The results, illustrated in Figure 8, indicate that under all loading conditions, the collapse coefficient of the samples decreased significantly as the stabilizer content was increased. Further analysis revealed that when the stabilizer content reached 1.0%, 1.5%, and 2.0%, the collapse coefficient (δs) of the samples remained below 0.015 across all tested loading pressures. Compared to the moderate collapsibility (Grade II) of the original remolded loess, the improved carbonated solidified loess exhibited slight collapsibility (Grade I), effectively demonstrating the stabilizer’s enhancement effect. The improvement mechanism underlying this phenomenon can be attributed to two primary factors. First, CL enhances soil structure by reinforcing the cementation between soil particles, thereby increasing structural integrity. Second, the synergistic effect of magnesium hydroxide (Mg(OH)2), produced through the hydration of reactive MgO, along with carbonation products, efficiently fills soil pores. This process reduces porosity and simultaneously strengthens the bonding between soil particles, significantly decreasing the soil’s susceptibility to collapse. In summary, the results confirm that carbonated solidified soil technology, combining CL and MgO, offers substantial engineering application potential for the treatment and stabilization of collapsible loess.

3.1.4. Carbonation Test

Figure 9a presents the variations in the degree of carbonation as a function of carbonation duration for loess stabilized with 10% MgO alone. The data reveal that the carbonation process occurred rapidly within the first 3 h, reaching a plateau at approximately 6% after 10 h. This observation suggests that a minimum carbonation duration of 10 h is required to achieve effective stabilization of loess through MgO carbonation. Figure 9b examines the influence of CL on the carbonation efficiency of loess stabilized with 10% MgO. The results indicate that the addition of CL had no measurable effect on the degree of carbonation. This behavior can be attributed to the inherent properties of CL, which has a low calcium content (typically 4% to 6%) and a chemical structure where calcium ions are bound to sulfonic acid groups (–SO3H) through ionic bonds or electrostatic interactions. Consequently, these calcium ions are not available to participate in the carbonation reaction [43]. Additionally, the carbonation reaction of CL itself requires elevated temperatures exceeding 400 °C, well beyond the conditions used in this study. As a result, the carbonation process under these conditions was predominantly driven by MgO [44]. Approximately 6% of the dry soil mass was converted into stable magnesium carbonate during carbonation, effectively stabilizing the loess, while simultaneously reducing carbon emissions. To ensure complete carbonation and optimize both stabilization and environmental benefits, subsequent experiments adopted a carbonation duration of 24 h.

3.2. Thermal Conductivity Test

3.2.1. Admixture Ratios

Figure 10 illustrates the variation in thermal conductivity of carbonized and solidified loess as the CL content increased, showing a notable reduction of 16.7%. The thermal conductivity was tested using one-way analysis of variance (ANOVA) to determine whether there were significant differences among the different CL dosage groups. The F-statistic was 5.37, and the p-value was 0.0101 (<0.05), indicating that there was a statistically significant difference in the average thermal conductivity values among the different CL dosage groups. This reduction can be analyzed from two key perspectives: the constitutive properties of the materials used, and the evolution of the soil’s microstructure. From a thermal conductivity standpoint, the thermal properties of individual components play a crucial role. Under normal temperature conditions (20 °C), the thermal conductivity of soil particles in porous media typically ranges from 1 W/(m·K) to 5 W/(m·K), whereas water and air exhibit much lower thermal conductivities of 0.6 W/(m·K) and 0.026 W/(m·K), respectively [45]. CL, a natural organic polymer, has an even lower thermal conductivity, typically ranging between 0.1 W/(m·K) and 0.5 W/(m·K), making it significantly less thermally conductive than soil particles. Similarly, MgO, with a thermal conductivity of 0.04–0.07 W/(m·K) [46], is classified as an effective insulating material. Consequently, the introduction of these components inherently lowers the thermal conductivity of loess, due to their substantially lower heat transfer capacities. From a microstructural evolution perspective, the addition of CL and MgO induces significant changes in the internal structure of loess. The carbonation of MgO produces carbonate compounds that, together with CL, create a pore-filling effect, modifying the soil’s microstructure. This process results in the formation of an irregular pore network within the soil, which disrupts heat transfer pathways. The combination of this altered microstructure with the inherent insulating properties of MgO and lignosulfonate further reduces the thermal conductivity of carbonized and solidified loess, thereby improving its insulating performance.

3.2.2. Moisture Content

Figure 11 illustrates the impact of water content variation on soil thermal conductivity, showing that as the water content increased, the thermal conductivity of the carbonized solidified loess rose by 16–24%, while that of the untreated loess increased by 31%. This indicates that carbonized solidified loess effectively suppresses the rate of thermal conductivity growth. The thermal conductivity was tested using one-way analysis of variance (ANOVA) to determine whether there were significant differences between the different moisture content levels. Statistical calculations were performed for each dosage, and the average p-value obtained for all dosages was 0.00854, which is less than 0.05. This indicates that there were statistically significant differences in the average thermal conductivity values among the different water content levels. This phenomenon can be attributed to changes in material composition and heat conduction mechanisms at both macroscopic and microscopic scales. From a compositional perspective, the samples consisted of three distinct phases: soil particles, pore water, and gas. Under normal temperature conditions, the thermal conductivity of water (0.6 W/(m·K)) is significantly higher than that of air (0.026 W/(m·K)). As water content increases and dry density rises, the low-thermal-conductivity air in soil pores is gradually replaced by water, leading to an overall increase in soil thermal conductivity. Therefore, the replacement of air by water is a key factor driving this increase. Additionally, higher water content facilitates the formation of “water bridges” [3] between soil particles. These water bridges act as pathways for heat conduction, enhancing heat transfer efficiency at the microscopic scale. This water-mediated heat conduction mechanism significantly improves the soil’s ability to transfer heat, further contributing to the rise in thermal conductivity with increasing dry density. Furthermore, during the carbonation process, magnesium hydroxide (Mg(OH)2) colloids react with carbon dioxide (CO2) to form hydrated carbonates, primarily magnesium carbonate (MgCO3) [47]. This reaction consumes a portion of the initial water content, thereby inhibiting the overall heat transfer capacity of the material. In summary, the interplay between air displacement by water, the formation of water bridges, and the production of hydrated carbonates contributed to the increase in thermal conductivity. However, the carbonized solidified soil effectively suppressed the rate of thermal conductivity growth, demonstrating its potential for thermal performance regulation.

3.2.3. Dry Density

At a specific moisture content, the thermal conductivity of soil is strongly influenced by variations in dry density. Figure 12 illustrates that as dry density increases, thermal conductivity also rises. The thermal conductivity of the uncured loess increased continuously, with a growth rate of 7%, as the dry density was increased. For the carbonated solidified soil, when the dry density exceeded 1.6 g/cm3, the growth rate of the thermal conductivity decreased from 6.5% to 2.1%. Similarly, a one-way ANOVA was conducted to analyze the thermal conductivity and determine whether significant differences existed across the various dry density levels. Statistical calculations were performed for each dosage, and the resulting p-values for all dosages were less than 0.05. This indicates statistically significant differences in the average thermal conductivity values among the different dry density levels. As the dry density of the sample increased, the soil became more compact, reducing the amount of air within the pores. Since air has an extremely low thermal conductivity (0.026 W/(m·K)) compared to soil particles, the reduction in pore air significantly enhances heat transfer within the soil matrix. Additionally, at higher dry densities, particle contact becomes tighter, facilitating more direct heat transfer through the solid phase and further increasing thermal conductivity. In carbonated solidified soil, the pores are filled with carbonation products. At high densities, the limited number of pores results in minimal changes in thermal conductivity, maintaining its stability. Therefore, soils with higher dry density exhibit greater thermal conductivity compared to lightly compacted soils, while carbonated solidified loess demonstrates the ability to maintain stable thermal conductivity.

3.2.4. Freeze–Thaw Cycles

Figure 13 illustrates the relationship between the number of freeze–thaw cycles and soil thermal conductivity, showing a gradual decline in thermal conductivity as the number of freeze–thaw cycles was increased. The thermal conductivity of untreated loess decreased by 9.2%, while that of carbonated solidified loess decreased by 5%. One-way ANOVA was used to test the thermal conductivity, in order to investigate whether there were significant differences between the different numbers of freeze–thaw cycles. Statistical calculations were performed for each dosage, and the p-value obtained for all dosages was less than 0.05, indicating that there were statistically significant differences between the mean thermal conductivity values for the different numbers of freeze–thaw cycles. This reduction was primarily attributed to structural changes in the soil induced by repeated freeze–thaw cycles, which progressively disrupt heat transfer pathways. During the freezing process, free water in the soil undergoes a liquid-to-solid phase transformation, forming ice. This phase change causes an approximately 9% volumetric expansion, exerting stress on the surrounding soil particles. Such stress may lead to structural damage, including loosening of the soil matrix and the enlargement of existing pores. Additionally, the pressure generated by the expanding ice may create new cracks, further compromising soil stability [48]. These structural alterations reduce connectivity between soil particles, thereby diminishing their ability to conduct heat efficiently. During the thawing process, the melting of ice and the redistribution of water exacerbate structural damage. The movement of water within soil pores weakens the bonding between particles, preventing the soil from fully restoring its original structure. Over repeated freeze–thaw cycles, these damages accumulate, leading to a gradual increase in pore size, expansion of pore volume, and an increase in the number of cracks. These changes significantly raise the air content within the soil, and since air has an extremely low thermal conductivity (0.026 W/(m·K)), this adversely affects the soil’s overall thermal conductivity. In carbonated solidified loess, CL plays a cementing role by effectively binding soil particles and carbonation products while the carbonation products fill the pores, collectively reducing the impact of freeze–thaw cycles on soil structure. From a macroscopic perspective, the cumulative effects of freeze–thaw cycles over time lead to significant structural degradation. The progressive loosening of the soil matrix, increased porosity, and reduced integrity of the solid phase collectively weaken the heat transfer efficiency. However, carbonized stabilized loess mitigates structural degradation, reducing the destructive impact of freeze–thaw cycles on soil thermal conductivity.

3.3. Microstructure and Mineralogical Analyses

3.3.1. Scanning Electron Microscope (SEM) with Particle and Crack Analysis System (PCAS)

The microstructural evolution of different freeze–thaw cycle conditions was analyzed using SEM and a PCAS. SEM imaging, at 500× magnification, distinctly revealed point-to-point, point-to-surface, and surface-to-surface contact configurations between soil particles and stabilizing agents. These contact arrangements offered key insights into the microstructural characteristics of the soil matrix. PCAS analysis applied binarization to SEM images, enabling automatic noise removal, segmentation, and identification of particles and pores. This technique facilitated the quantification of various geometric parameters, including particle content, porosity, fractal dimension, and probability distribution index. By quantifying these parameters, the pore structure characteristics of the geotechnical materials were analyzed with high accuracy. This method provides a powerful approach for the quantitative evaluation of pore features, contributing to a deeper understanding of the microstructural properties and deterioration mechanisms of carbonated solidified loess under freeze–thaw cycling conditions.
Figure 14 illustrates the microstructural changes in carbonated solidified loess after 0 and 15 freeze–thaw cycles. This research indicates that under optimal additive conditions, the loess particles and magnesium-based carbonate products formed strong bonds through the connecting action of CL, resulting in relatively fewer pores in the soil. However, after 15 freeze–thaw cycles, the bonding force between soil particles weakened, microcracks easily developed on the surfaces of soil particles and magnesium carbonate products, and the pores enlarged. This result provides a microscopic explanation for the deterioration process of the soil structure. By analyzing the pore characteristics in the SEM images using PCAS V2.3 software, the pattern of structural changes could be visually demonstrated: as the nuber of freeze–thaw cycles increased, the number of pores rose significantly, and the porosity increased accordingly. This trend highlights the destructive effect of repeated freeze–thaw cycles on the soil’s microstructure. Table 3 presents the variations in pore characteristics and related parameters of the carbonated solidified loess under 0 and 15 freeze–thaw cycles. Combined with Figure 14, it is evident that during the freeze–thaw process, the soil pores enlarged, the number of pores increased, cracks developed in soil particles, and structural damage occurred, leading to a significant decline in the soil’s thermal conductivity. Further analysis revealed that the probability entropy values for both sets of images exceed 0.9, indicating minimal changes in the arrangement and orientation characteristics of microstructural units before and after freeze–thaw cycles. This consistency suggests a high degree of randomness and complexity in the soil’s structure. Additionally, probability entropy values close to 1 imply that the direction of pore expansion exhibited strong randomness, with low orderliness in the arrangement of soil particles, resulting in a highly complex structure. The impact of freeze–thaw cycles on the fractal dimension was relatively minor; however, after 15 freeze–thaw cycles, the fractal dimension increased by approximately 0.02. This increase reflects a reduction in the uniformity of the soil particle size distribution and a rise in heterogeneity within the soil. Such microstructural changes contributed to the progressive deterioration of the soil’s macroscopic thermal conductivity.

3.3.2. XRD and FT-IR

Figure 15 illustrates the X-ray diffraction (XRD) patterns of the unsolidified loess, uncarbonated solidified loess, and carbonated solidified loess. The diffraction spectra of the unsolidified loess and uncarbonated solidified loess are nearly identical, except for the emergence of new peaks corresponding to brucite magnesium hydroxide (Mg(OH)2) in the stabilized loess. This observation suggests that the MgO reacted with water in the absence of carbonation, leading to the formation of magnesium hydroxide (Mg(OH)2) [49]. In contrast, the XRD pattern for the carbonated solidified loess reveals prominent peaks corresponding to hydromagnesite (Mg5(CO3)4(OH)2·5H2O) and nesquehonite (MgCO3·3H2O), indicating that carbonation induced new mineral phases. Furthermore, the absence of MgO peaks in the uncarbonated solidified loess implies that MgO hydration was nearly completed through carbonation reactions. This conclusion aligns well with the findings of Cai et al. [50]. To gain a deeper understanding of the chemical reactions, Fourier-transform infrared (FT-IR) spectroscopy was conducted, as this technique applies to both crystalline and amorphous materials. The results are presented in Figure 16. For unsolidified loess (Figure 16a), a peak appeared at 3426.92 cm−1, corresponding to O-H stretching vibrations, indicating the presence of free and structural water in the loess. Peaks at 2519.01 cm−1, 1437.04 cm−1, and 875.52 cm−1 are attributed to C-O and CO3− vibrations in carbonate minerals (e.g., calcite). The peak at 1028.49 cm−1 corresponds to Si-O vibrations, reflecting the abundance of silicate minerals. Figure 16b presents the FT-IR results after stabilization and carbonation. The peak at 3428.94 cm−1 weakened, indicating a reduction in magnesium hydroxide (Mg(OH)2). Conversely, the peaks at 1432.21 cm−1 and 878.66 cm−1 intensified, suggesting the formation of a significant amount of magnesium carbonate (MgCO3), which markedly enhanced the stability of the solidified system. FT-IR analysis further confirmed the process of CO2 mineralization and the formation of magnesium carbonate, consistent with the SEM analysis results.

3.3.3. MIP

A mercury intrusion porosimetry (MIP) test was employed to assess the pore distribution and structure of the remolded loess and carbonated solidified loess subjected to different numbers of freeze–thaw cycles (Figure 17). The cumulative pore volume analysis indicated that the remolded loess had a significantly larger pore volume. In contrast, in carbonated solidified loess, substances such as CL and magnesium carbonate (MgCO3) effectively fill the pore spaces, leading to smaller pore diameters and lower overall pore volume. The pore size distribution analysis reveals that remolded loess, as a low-plasticity clay, typically exhibits a bimodal pore size distribution. This characteristic arises from the presence of fine particles within the loess, which partially fill and block the pores, influencing pore connectivity. Consequently, two primary mercury intrusion pathways are formed: one dominated by pores between soil particles, and another by fewer inter-aggregate pores. As the number of freeze–thaw cycles increases, several key changes occur in the pore structure. Some soil particles and hydration products break into smaller fragments, reducing the number of large pores, while increasing the abundance of small pores. Additionally, structural damage to soil particles and carbonation reaction products leads to particle rearrangement and reorganization, further increasing the proportion of small pores. The experimental results reveal distinct differences in the pore characteristics of remolded and carbonated solidified loess. The remolded loess contained more pores, directly impacting its thermal conductivity. Under freeze–thaw conditions, the denser structure of carbonated solidified loess reduces structural degradation, leading to a smaller decline in thermal conductivity compared to remolded loess. Consequently, carbonated solidified loess exhibits superior thermal performance stability under freeze–thaw cycling conditions.

3.4. Proposed Mechanistic Model

Based on the micro-mechanism analysis in Figure 18, the functional mechanism of CL-MgO carbonated solidified loess can be classified into two processes: the carbonated solidified mechanism, and the thermal conductivity mechanism. The carbonated solidified mechanism progresses through multiple stages. Initially, under uncarbonated solidified conditions, CL forms hydrogen bonds with soil particles via hydroxyl groups, enhancing inter-particle cohesion. Simultaneously, MgO undergoes hydration to form magnesium hydroxide (Mg(OH)2), and these combined processes improve structural integrity by filling voids and increasing compaction. Following the introduction of CO2 and 24 h carbonation, MgO reacts with CO2 to produce magnesium carbonate (MgCO3). These magnesium carbonate crystals effectively fill void spaces and establish a new solid framework. Additionally, their interaction with CL optimizes the microstructure, substantially enhancing soil stability and strength. The thermal conductivity mechanism explains how various factors influence thermal performance. Increased moisture content enhances thermal conductivity as water, with its superior thermal conductivity, replaces air in void spaces. A higher dry density promotes thermal conductivity by expanding particle contact areas and reducing porosity, thereby improving heat transfer pathways dominated by solid particles. The addition of CL modifies the material’s microstructure and pore connectivity characteristics, though the inherently low thermal conductivity of CL and MgO complexes inhibits overall thermal conductivity. Furthermore, successive freeze–thaw cycles progressively increase pore formation and size during freezing, disrupting internal heat conduction pathways. These analyses offer comprehensive insights into the micro-carbonated structure and thermal conductivity performance of CL-MgO carbonated solidified loess, providing a theoretical foundation for material design and engineering applications.

4. Conclusions

This study addressed critical engineering challenges in loess regions by systematically investigating the enhancement effects of CL-MgO carbonation solidification technology on the thermal conductivity and structural performance of loess. Through systematic experiments and microscopic mechanism analysis, the findings provide innovative solutions and new research perspectives for improving engineering stability in loess areas. The key outcomes and significance of the research are summarized as follows:
  • Carbonation Solidification Effect and Environmental Engineering Benefits: The CL-MgO composite material facilitated the solidification of loess, while simultaneously mineralizing and sequestering CO2. After 24 h of carbonation, the degree of carbonation reached 6%, with approximately 0.6 tons of CO2 fixed per ton of MgO, forming stable magnesium carbonate minerals (hydromagnesite and magnesite). The improved soil exhibited a collapsibility coefficient below 0.015, significantly enhancing stability. This dual-effect process effectively mitigated collapsible loess issues, while achieving CO2 emission reduction, demonstrating substantial environmental and engineering value.
  • The Combination of CL and MgO Effectively Reduces Soil Thermal Conductivity Through Microstructural Modification: The combination of CL and MgO effectively reduced the soil thermal conductivity through microstructural modification. During carbonation, MgO’s carbonated products and CL molecules alter the soil’s pore structure, increasing porosity and the tortuosity of heat flow paths. Since both additives possess lower thermal conductivity than soil particles, this carbonation technique not only decreases the thermal conductivity of loess but also limits conductivity increases from environmental factors such as moisture. The improved thermal insulation properties benefit applications ranging from heating pipelines to cold-region infrastructure, while mitigating frost heave in permafrost areas. These findings can contribute to the development of sustainable building materials in loess regions.
  • Microscopic Mechanism Elucidation: The primary products of MgO carbonated solidified loess are hydromagnesite and magnesite. The hydration of MgO, combined with the cementation effect of CL, optimizes the soil structure by enhancing inter-particle bonding and filling pores, resulting in a denser soil matrix. This structural densification suppresses thermal conduction pathways, leading to lower thermal conductivity than the loosely structured remolded loess, thereby exhibiting excellent thermal insulation properties. During carbonation, CO2 sequestration is achieved, contributing to carbon emission reduction. Even after undergoing freeze–thaw cycles, although cracks may develop between particles and carbonation products at the microscopic level, the soil maintains high stability and favorable thermal performance at the macroscopic level. This stability reflects the synergistic optimization of both microscopic and macroscopic properties.
  • Future Outlook: Future research directions include validating the long-term performance of materials under cyclic freeze–thaw and wet–dry conditions to assess durability and CO2 mineralization stability in challenging environments; integrating molecular dynamics simulations with macro-scale performance data through machine learning for precise optimization of MgO–CL interactions; and exploring the co-utilization of industrial by-products to develop hybrid stabilization systems, while conducting lifecycle assessments to ensure scalable net-zero implementations. This would provide theoretical support for embankment engineering.
This study systematically elucidated the microscopic mechanisms and thermal performance regulation characteristics of CL-MgO carbonated solidification technology. It highlights the dual benefits of achieving carbon emission reduction through CO2 mineralization, while significantly enhancing the mechanical properties, thermal performance, and moisture collapse resistance of loess. The findings demonstrate broad application potential in subgrade engineering projects in loess regions. It not only provides theoretical support for improved loess subgrade engineering techniques, but also offers valuable insights for the development and application of low-carbon and environmentally friendly improvement materials.

Author Contributions

Conceptualization, Y.L. and W.Z.; formal analysis, Y.L.; investigation, Y.L.; writing—original draft preparation, Y.L.; writing—review and editing, W.Z.; funding acquisition, W.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Qinghai Provincial Central Guidance Local Science and Technology Development Fund Project, by 2024ZY004, The National Natural Science Foundation of China, by 52168054, 52468054, and The Fund of China Scholarship Council, by 202308630007.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data, models, and code generated or used during the study appear in the published article. Experimental results from other studies can be found in the references cited.

Conflicts of Interest

The authors declare no potential conflicts of interest with respect to the research, authorship, and publication of this article.

Abbreviations

The following abbreviations are used in this manuscript:
CLCalcium Lignosulfonate
MgOMagnesium Oxide

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Figure 1. Particle size distribution curve of loess.
Figure 1. Particle size distribution curve of loess.
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Figure 2. Improved material of FT-IR: (a) CL; (b) MgO.
Figure 2. Improved material of FT-IR: (a) CL; (b) MgO.
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Figure 3. Freeze–thaw chamber.
Figure 3. Freeze–thaw chamber.
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Figure 4. Carbonation box.
Figure 4. Carbonation box.
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Figure 5. Thermal constants analyzer.
Figure 5. Thermal constants analyzer.
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Figure 6. Atterberg limits of carbonated solidified loess.
Figure 6. Atterberg limits of carbonated solidified loess.
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Figure 7. Compaction curves of uncarbonated solidified loess with CL contents.
Figure 7. Compaction curves of uncarbonated solidified loess with CL contents.
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Figure 8. Wetting collapse coefficient of carbonated solidified loess.
Figure 8. Wetting collapse coefficient of carbonated solidified loess.
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Figure 9. Variations in degree of carbonation with (a) carbonation duration for MgO solidified loess and (b) carbonation duration for CL-MgO solidified loess with different CL dosages.
Figure 9. Variations in degree of carbonation with (a) carbonation duration for MgO solidified loess and (b) carbonation duration for CL-MgO solidified loess with different CL dosages.
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Figure 10. Thermal conductivity of different mixing amounts.
Figure 10. Thermal conductivity of different mixing amounts.
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Figure 11. Thermal conductivity of different moisture contents.
Figure 11. Thermal conductivity of different moisture contents.
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Figure 12. Thermal conductivity of different dry densities.
Figure 12. Thermal conductivity of different dry densities.
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Figure 13. Thermal conductivity of different freeze–thaw cycles.
Figure 13. Thermal conductivity of different freeze–thaw cycles.
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Figure 14. Microscopic characteristics of carbonated solidified loess under different freeze–thaw cycle conditions: (a) SEM: CL = 1% + MgO = 10%, FT = 0; PCAS: CL = 1% + MgO = 10%, FT = 0; (b) SEM: CL = 1% + MgO = 10%, FT = 15; PCAS: CL = 1% + MgO = 10%, FT = 15.
Figure 14. Microscopic characteristics of carbonated solidified loess under different freeze–thaw cycle conditions: (a) SEM: CL = 1% + MgO = 10%, FT = 0; PCAS: CL = 1% + MgO = 10%, FT = 0; (b) SEM: CL = 1% + MgO = 10%, FT = 15; PCAS: CL = 1% + MgO = 10%, FT = 15.
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Figure 15. XRD of uncarbonated and carbonated solidified loess.
Figure 15. XRD of uncarbonated and carbonated solidified loess.
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Figure 16. FT-IR test results: (a) unsolidated loess; (b) carbonated solidified loess.
Figure 16. FT-IR test results: (a) unsolidated loess; (b) carbonated solidified loess.
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Figure 17. MIP test results of solidified loess before and after carbonation: (a) cumulative pore volume; (b) differential pore volume.
Figure 17. MIP test results of solidified loess before and after carbonation: (a) cumulative pore volume; (b) differential pore volume.
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Figure 18. Microscopic mechanism.
Figure 18. Microscopic mechanism.
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Table 1. Basic properties of loess samples.
Table 1. Basic properties of loess samples.
Natural Water
Content (%)		Liquid Limit (%)Plastic Limit (%)Plasticity
Index (%)		Dry Density (g/cm)Optimum Water
Content (%)		pH
14.527.715.312.41.7215.18.87
Table 2. The test program.
Table 2. The test program.
TestsBinder Content *Freeze–Thaw
(F-T) Cycles		Carbonation (h)Moisture Content (%)Dry Density
(g·cm−3)
Calcium Lignosulfonate (%)MgO
(%)
Basic Physical
Properties Test		0, 0.5, 1, 1.5, 2, 310--U **16.51.63
Thermal Conductivity Test0, 0.5, 1, 1.5, 2, 3--2416.51.63
1--8, 12, 16, 201.63
--16.51.4, 1.5, 1.6, 1.7
0, 2, 8, 15, 2016.51.63
Microscopic tests10, 1516.51.63
* The percent binder content is compared to the dry mass of loess; ** U = uncarbonated.
Table 3. Pore-related parameters of carbonated solidified soil.
Table 3. Pore-related parameters of carbonated solidified soil.
Parameters1%CL + 10%MgO; FT = 01%CL + 10%MgO; FT = 15
Number of Pores144305
Porosity3.08%6.66%
Probabilistic Entropy0.95590.9689
Fractal Dimension1.18171.2099
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Lu, Y.; Zhang, W. Research on the Thermal Conductivity and Microstructure of Calcium Lignosulfonate-Magnesium Oxide Solidified Loess. Appl. Sci. 2025, 15, 4545. https://doi.org/10.3390/app15084545

AMA Style

Lu Y, Zhang W. Research on the Thermal Conductivity and Microstructure of Calcium Lignosulfonate-Magnesium Oxide Solidified Loess. Applied Sciences. 2025; 15(8):4545. https://doi.org/10.3390/app15084545

Chicago/Turabian Style

Lu, Yuwen, and Wuyu Zhang. 2025. "Research on the Thermal Conductivity and Microstructure of Calcium Lignosulfonate-Magnesium Oxide Solidified Loess" Applied Sciences 15, no. 8: 4545. https://doi.org/10.3390/app15084545

APA Style

Lu, Y., & Zhang, W. (2025). Research on the Thermal Conductivity and Microstructure of Calcium Lignosulfonate-Magnesium Oxide Solidified Loess. Applied Sciences, 15(8), 4545. https://doi.org/10.3390/app15084545

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