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Article

Unlocking the Potential of RFA and Stabilizers in High Moisture Geotechnical Applications

by
Kaiqing Zhou
1,
Xuliang Wang
1,
Jun Hu
1,2,
Qinxi Dong
1 and
Hui Zeng
3,*
1
School of Civil Engineering and Architecture, Hainan University, Haikou 570228, China
2
Collaborative Innovation Center of Marine Science and Technology, Hainan University, Haikou 570228, China
3
School of Civil Engineering and Architecture, Wuyi University, Jiangmen 529020, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(3), 1270; https://doi.org/10.3390/app15031270
Submission received: 3 January 2025 / Revised: 23 January 2025 / Accepted: 24 January 2025 / Published: 26 January 2025
Browse Figures
Versions Notes

Abstract

:
In recent decades, rapid urbanization has generated a large amount of waste soft soil and construction debris, resulting in severe environmental pollution and posing significant challenges to engineering construction. To address this issue, this study explores an innovative approach that synergistically applies recycled fine aggregate (RFA) and soil stabilizers to improve the mechanical properties of soft soil. Through laboratory experiments, the study systematically examines the effects of different mixing ratios of RFA (20%, 40%, 60%) and soil stabilizers (10%, 15%, 20%) with red clay. After standard curing, the samples underwent water immersion maintenance for varying durations (1, 5, 20, and 40 days). Unconfined compressive strength (UCS) tests were conducted to evaluate the mechanical performance of the samples, and the mechanisms were further analyzed using scanning electron microscopy (SEM) and particle size distribution (PSD) analysis. The results indicate that the optimal performance is achieved with 20% RFA and 20% stabilizer, reaching the highest UCS value after 40 days of water immersion. This improvement is primarily attributed to the formation of a dense reticulated structure, where RFA particles are effectively encapsulated by clay particles and stabilized by hydration products from the stabilizer, forming a robust structural system. Unconsolidated undrained (UU) tests reveal that peak deviatoric stress increases with confining pressure and stabilizer content but decreases when excessive RFA is added. Shear strength parameter analysis demonstrates that both the internal friction angle (φ) and cohesion (c) are closely related to the content ratios, with the best performance observed at 20% stabilizer and 20% RFA. PSD analysis further confirms that increasing stabilizer content enhances particle aggregation, while SEM observations visually illustrate a denser microstructure. These findings provide a feasible solution for waste soft soil treatment and resource utilization of construction debris, as well as critical technical support and theoretical guidance for geotechnical engineering practices in high-moisture environments.

1. Introduction

During rapid urbanization over the past few decades, extensive foundation excavations and construction activities have generated a large volume of discarded soft soil [1]. Due to its high compressibility, high water content, poor permeability, and low bearing capacity, soft soil is challenging to use directly in engineering projects, often causing settlement, deformation, and stability issues [2,3,4,5]. Traditional disposal methods, such as landfilling or open-air stacking, not only occupy vast land resources but also lead to environmental pollution, including dust emissions, soil erosion, and damage to soil structures, sometimes even causing land subsidence [6,7,8]. Thus, improving the mechanical properties of soft soil and reducing its reuse difficulty have become critical problems in construction engineering.
At the same time, urbanization has also generated a significant amount of construction waste [9]. To enhance the utilization efficiency of construction waste, various management strategies and policies have been implemented worldwide [10,11,12]. Recycled fine aggregate (RFA) is one of the primary reuse forms of construction waste, typically produced by crushing and grading discarded concrete, stones, and bricks in specific proportions. This sustainable material offers notable environmental and economic benefits [13]. However, compared to natural aggregates, RFA generally has a higher water absorption rate, lower density, and certain limitations in mechanical performance and stability [14,15,16]. Enhancing the application effectiveness of RFA has become a research focus. Currently, RFA is widely used in fields such as recycled aggregate concrete (RAC), road pavements, and foundation reinforcement [17,18,19,20,21].
With the increasing accumulation of construction waste and discarded soft soil, there is an urgent need for an environmentally friendly, low-carbon, and cost-effective treatment method. Some studies have shown that RFA can be used not only as conventional building material but also for soft soil stabilization to achieve the required mechanical properties for engineering. For example, research on RFA-based granular piles for soft soil treatment demonstrated that RFA could effectively replace natural aggregates, enhancing the bearing capacity and settlement resistance of soft soil. The addition of binders such as cement and fly ash further improved the performance [22]. Furthermore, studies have indicated that combining soil stabilizers with RFA achieves peak strength when the RFA content reaches 30%. Microstructural analysis revealed that RFA promotes the formation of hydration products and provides effective mechanical support, but excessive content may lead to large pores in the matrix [23]. These findings suggest that RFA holds significant potential for soft soil reinforcement. However, most current methods for soft soil stabilization rely mainly on stabilizers such as silicates, lime, and cement [24,25,26,27] which are limited in practical applications due to high energy consumption, pollution, and cost [28,29,30]. Therefore, the synergistic use of RFA and soil stabilizers for treatment is a promising direction worth exploring.
This study introduces an innovative approach that combines the use of recycled fine aggregates (RFA) and soil stabilizers to improve the mechanical properties of soft soils, particularly red clay, which is characterized by high plasticity, low strength, and high moisture content. While previous studies have primarily focused on either recycled aggregates or soil stabilizers individually, the synergistic effects of combining these two materials have not been extensively explored. This research aims to address this gap by investigating the combined influence of RFA and stabilizers on soil strength and structural stability, providing a sustainable solution for soft soil stabilization. This study aims to address this gap by investigating the combined effects of RFA and stabilizers on soil strength and structural stability to provide a sustainable solution for stabilizing red clay soils with high plasticity, low strength, and high water content. The effects of recycled aggregate content (R), soil stabilizer content (C), and water curing age (M) on the mechanical properties of stabilized soft soil were analyzed through unconfined compression tests and triaxial tests. Additionally, microscopic techniques such as scanning electron microscopy (SEM) and laser diffraction particle size distribution (PSD) analysis were employed to investigate the microstructure, pore characteristics, and morphological changes in stabilized soft soil. The findings reveal the reinforcement mechanisms by which recycled aggregates and soil stabilizers synergistically enhance the performance of stabilized soft soil.

2. Materials and Methods

2.1. Materials

Red clay: Red clay samples were collected from a specific region in Hainan Province. The soil was processed by drying, crushing, and sieving through a 2 mm mesh. Particles smaller than 2 mm were selected for the experiments. Following the Standard for Soil Test Methods (GB/T 50123–2019) [31], the particle size distribution and basic physical properties of the red clay were measured, as shown in Figure 1 and Table 1.
Recycled aggregate: The recycled aggregate used in the experiment was sourced from construction waste generated at a building site in Hainan Province. The material was processed by manually removing debris, crushing it with a crusher, and sieving it through a 1–2 mm mesh, as illustrated in Figure 2.
Soft soil stabilizer: The stabilizer used in this study was provided by a specific company. The stabilizer is a milky white powder. Its chemical composition was determined through XRD analysis with a measurement range of 10–80°, and the results are presented in Figure 3.

2.2. Sample Preparation

The specimen preparation process followed the Standard for Soil Test Methods (GB/T 50123–2019) [31]. Specimens for unconfined compressive strength (UCS) and triaxial tests were cylindrical, with a diameter of 39.1 mm and a height of 80 mm. Specimens that deviated by ±5 g in weight or ±1 mm in height were deemed invalid and required re-preparation, curing, and inspection.
Preliminary Preparation: Red clay and recycled aggregate were manually treated to remove debris, dried, and crushed using a crusher. The materials were sieved through a 2 mm mesh. Particles between 1 mm and 2 mm of recycled aggregate and particles below 2 mm of red clay were used as experimental materials.
Material Mixing: The proportions of the stabilizer and recycled aggregate were expressed as the mass ratio to red clay. The required amounts of stabilizer, red clay, recycled aggregate, and water were weighed according to the experimental design and mixed thoroughly.
Specimen Molding and Curing: The mixed materials were compacted in a lubricated mold in three layers using a standard compaction method. The compaction energy and layer count (three layers) were controlled, and the compaction strokes were back-calculated to 23 per layer. The specimens were demolded and cured in a standard curing room for 28 days. Subsequently, water immersion curing was conducted for 1, 5, 20, and 40 days, as per the experimental plan. During immersion curing, the specimens were fully submerged in water, and those unsuitable for UCS testing after curing were excluded. The final specimens were cylindrical with a diameter of 39.1 mm and a height of 80 mm.

2.3. Test Equipment

Unconfined Compressive Strength (UCS) Test Equipment: The UCS tests were conducted using a fully automatic UCS testing machine produced by Zhejiang Geo-Tech Co., Ltd. (Ningbo, China). The machine has a maximum load capacity of 10 kN, a range of 50 mm, an accuracy of 0.01 mm, and a compression rate of 1 mm/min. Following the Standard for Soil Test Methods (GB/T 50123–2019) [31], the test was terminated either at peak axial force or when axial strain increased by 3–5% beyond the strain at failure. For specimens with a peak value, the peak strength was recorded as the UCS. For specimens without a peak, the strength corresponding to 20% axial strain was taken as the UCS.
The triaxial tests were conducted using a fully automated triaxial apparatus (model TKA-TTS-1) manufactured by Nanjing TKAO Instruments (Nanjing, China). The equipment can apply a maximum confining pressure of 2 MPa and withstand an axial load of up to 10 kN. The adjustable shear rate ranges from 0.0001 mm/min to 2.4 mm/min. During the tests, the failure axial strain was set at 15%, and the tests were terminated once this strain level was reached.
Scanning Electron Microscope (SEM): A Hitachi SU8010 SEM model was used (Hitachi, Tokyo, Japan). After specimen preparation, gold sputtering was performed to ensure sufficient conductivity. Samples were then placed on the test platform, and the height was adjusted for observation under different magnifications.
Laser Diffraction Particle Size Analyzer: The Malvern MS3000 (Malvern, UK) was used to analyze the particle size distribution (PSD) of soil samples. The device has a working range of 0.01 μm to 3500 μm and classifies particles into clay (<1.5 μm), silt (1.7–75 μm), and sand (>75 μm) according to the USCS particle size standard [32].

2.4. Experimental Programs

Unconfined compressive strength tests and triaxial tests are methods used to determine the mechanical properties of materials such as soil and rock, primarily for evaluating the natural strength of soil and its sensitivity index. This study aimed to investigate the effects of stabilizers and recycled aggregate on the mechanical properties, strength characteristics, and water stability of soft soil under standard curing and immersion curing conditions. The moisture content was set at the optimal value of 20% for red clay.
Mixing Ratios: When the recycled aggregate content was 20%, stabilizer contents were set at 10%, 15%, and 20%. When the stabilizer content was 20%, recycled aggregate contents were set at 40% and 60%. Standard curing was conducted for 28 days, with water immersion curing times of 1, 5, 20, and 40 days. Parameter settings were adjusted based on related research and preliminary results. The selected mixing ratios of RFA (20%, 40%, 60%) and stabilizer (10%, 15%, 20%) were based on optimal ranges reported in previous studies. Stabilizer content typically does not exceed 20% to avoid diminishing strength improvement, while RFA content is limited to 60% to prevent excessive voids. Preliminary experiments validated that these ratios achieve a balance between strength enhancement and material stability, confirming their suitability for this study [33,34].
Scheme Description: The contents of the stabilizer, recycled aggregate, and water were expressed as mass ratios to red clay. The experimental scheme notation used M (immersion curing age), C (stabilizer content), and R (recycled aggregate content). For example, “R20C20-M40” denotes a recycled aggregate content of 20%, a stabilizer content of 20%, and an immersion curing age of 40 days. Specific details are shown in Table 2.

3. Results

This study conducts unconfined compressive strength (UCS) tests on specimens made by mixing recycled aggregate, soft soil stabilizer, and red clay after standard curing and water immersion curing. The results of the triaxial test from various experimental groups are analyzed for the impact of different experimental factors.

3.1. UCS Stress–Strain Curve

The test results show a correlation between UCS results and water immersion curing age, recycled aggregate content, and stabilizer content. Similar studies have indicated that these factors influence the peak strength qu [35,36,37,38]. Figure 4 and Figure 5 illustrate the effects of water immersion curing age on UCS.
The stress–strain curves for all the specimens exhibited strain-softening behavior. At water immersion curing times of 1 day, 5 days, 20 days, and 40 days, the R20C20 specimen consistently achieved the highest peak stress, while the R20C10 specimen consistently achieved the lowest.
The peak UCS of the specimens increased with the extension of curing age (1 day, 5 days, 20 days, 40 days). Furthermore, when the recycled aggregate content was 20%, increasing the stabilizer content (R20C10, R20C15, R20C20) raised the peak UCS. Conversely, when the stabilizer content was 20%, increasing the recycled aggregate content (R20C20, R40C20, R60C20) reduced the peak UCS. This phenomenon was attributed to the hydration reaction producing cementitious materials randomly distributed within the specimen, forming distinct structural features with red clay. These random distributions influenced both peak stress and strain characteristics, leading to irregular strain behaviors.

3.2. Influence of Various Factors on Mechanical Properties

3.2.1. Effect of Water Immersion Age on Mechanical Properties

As shown in Figure 6, the peak strength of the specimens with the same mix ratio increased with the extension of curing age, reaching a maximum at 40 days. With extended curing age, the peak strength qu improved correspondingly. For the R20C10, R20C15, R20C20, R40C20, and R60C20 specimens, qu values were 811.8 kPa, 1181 kPa, 1407 kPa, 1149.8 kPa, and 844.2 kPa at 1 day, respectively. These values increased after 5 days and 20 days of curing and peak at 40 days, showing 1.45-, 1.05-, 2.2-, 2.16-, and 3.25-fold increases compared to the 1-day values.
Discussion reveals that the addition of a stabilizer consumes internal moisture through hydration reactions. Water immersion curing effectively replenished this moisture, further facilitating hydration reactions to produce cementitious materials that agglomerate and bind red clay particles with recycled aggregate, forming stable clusters and increasing qu. During shorter curing periods (1 and 5 days), reactions remained incomplete. After 20 days, hydration reactions proceeded gradually, producing a denser internal structure. Consequently, UCS peaked at 40 days, indicating that the synergistic effect of recycled aggregate and soft soil stabilizer under full immersion conditions significantly improved the stability of high-water-content soft soil foundations, providing valuable references for engineering practices.

3.2.2. Effect of Recycled Aggregate on Mechanical Properties

As shown in Figure 6, under the same curing age and stabilizer content (20%), qu decreased with increasing recycled aggregate content, peaking at 20%. For instance, at 40 days of curing, qu values of R20C20-40d, R40C20-40d, and R60C20-40d were 4505 kPa, 3642.4 kPa, and 3589 kPa, respectively. The R20C20-40d specimen showed a 0.24- and 0.26-fold increase in peak strength compared to the latter two.
The analysis reveals that increased stabilizer content enhanced hydration reactions, producing cementitious materials that synergistically interacted with red clay, forming a denser internal structure and filling voids. At 20% stabilizer content, hydration reactions were most complete, yielding the most stable skeletal structure and maximum qu.

3.3. Analysis of Triaxial Test Results

The triaxial test is a widely used experimental method in geotechnical engineering, primarily employed to study the mechanical properties of soils, rocks, and other granular or powdered materials. Research indicates that the shear strength parameters and peak stress of treated soils vary with the type and content of additives [39,40,41]. Therefore, based on the preceding experiments, this study conducted unconsolidated undrained (UU) triaxial tests on specimens with different mix ratios and confining pressures, which were subjected to water curing for 40 days. The test results on the shear performance of the specimens were analyzed and discussed in detail.

3.3.1. Analysis of Factors Affecting Peak Deviatoric Stress

Unconsolidated undrained (UU) triaxial tests were conducted under confining pressures of 200–400 kPa on specimens with various mix ratios cured in water for 40 days. The peak deviatoric stress results are presented in Figure 7. As shown in the figure, the peak deviatoric stress of the specimens increased with higher confining pressure. Additionally, under the same confining pressure and recycled aggregate content, the peak deviatoric stress rose with an increase in stabilizer content, reaching its maximum when the stabilizer content was 20%. For confining pressures of 200–400 kPa, the peak deviatoric stress of R20C20 was 72.74%, 77.38%, and 97.57% higher than that of R20C10, respectively. Conversely, under the same confining pressure and stabilizer content, the peak deviatoric stress decreased as the recycled aggregate content increased. For confining pressures of 200–400 kPa, the peak deviatoric stress of R20C20 was 68.68%, 61.59%, and 59.45% higher than that of R60C20, respectively.
The peak deviatoric stress results of different specimens indicate variability in the combined effects of stabilizer and recycled aggregate at varying dosages. Within acceptable limits, increasing the stabilizer content positively influenced shear strength. This effect was attributed to the hydration reaction of the stabilizer, which generated a significant amount of cementitious material, effectively filling the inter-particle voids and enhancing inter-particle cohesion. However, increasing the recycled aggregate content negatively impacted shear strength. This was likely due to slippage and displacement between recycled aggregate particles, which reduced the specimen’s overall shear strength.

3.3.2. Analysis of Shear Strength Parameters

The shear strength parameters (cohesion c and internal friction angle ϕ) of the mixed soil containing recycled aggregate and stabilizer vary with changes in their respective dosages. Figure 8 illustrate the comparison of shear strength parameters under different dosages of recycled aggregate and stabilizer.
In Figure 8a, it can be observed that ϕ increased overall with the increase in stabilizer content. This was likely due to the hydration reaction of the stabilizer, which generated cementitious material that formed stronger inter-particle contacts, thereby enhancing the specimen’s shear resistance. Conversely, as the recycled aggregate content increased, ϕ slightly decreased at lower dosages and declined further at higher dosages. This was likely because excessive recycled aggregate increased internal voids within the specimen, leading to insufficient particle contact and reduced mechanical performance.
Figure 8b shows that c significantly increased with higher stabilizer content, reaching its maximum when the stabilizer content was 20%. This was likely due to the large amount of cementitious material produced by the stabilizer’s hydration reaction, which effectively fills inter-particle voids and strengthens particle bonding. The effect of recycled aggregate content on ccc indicates that it was maximized at 20% dosage. However, as the dosage increased to 40% and 60%, c gradually decreased. This decline was likely caused by the inability of cementitious material to fully fill the voids between recycled aggregate particles at high dosages, resulting in a loose overall structure.

3.4. Analysis of Mechanical Performance Mechanism

This section analyzes and discusses the mechanical performance mechanism of specimens made from different proportions of recycled aggregates and stabilizers mixed with red clay, after 28 days of standard curing followed by 40 days of water curing. Figure 9 shows a schematic diagram of the variation in the particle skeleton. When the stabilizer content was 10–15%, as the recycled aggregate content increased (e.g., R20-C10, R40-C10, R60-C10), the red clay could not completely encapsulate and fill the voids between recycled aggregate particles. Meanwhile, insufficient stabilizer content led to inadequate flocculation, resulting in insufficient cementitious materials to aggregate more red clay particles, consequently causing unconfined compressive strength to decrease with increasing recycled aggregate content. When the recycled aggregate content was 40–60%, as stabilizer content increased (e.g., R20-C10, R20-C15, R20-C20), the cementitious materials produced by hydration could not completely fill the voids between the recycled aggregates and between the red clay particles. When stabilizer content reached 20%, hydration products reached maximum levels, resulting in peak unconfined compressive strength. When both recycled aggregate and stabilizer contents were 20%, red clay could fully encapsulate the limited recycled aggregates while forming a dense skeletal structure with hydration products, allowing the specimens to withstand higher pressure and achieve maximum peak strength.

3.5. Particle Size Distribution

Particle size distribution (PSD) is a crucial parameter describing particle size distribution patterns, significantly affecting soil’s physical and mechanical behavior. Based on previous unconfined compressive strength test results, PSD tests were conducted on the specimens (R20-C10, R20-C15, R20-C20, R40-C20, R60-C20), and the results were analyzed and discussed. Figure 10 shows the PSD curves and changes in silt–clay content ratios for different proportions of stabilizer and recycled aggregate contents after 40 days of water curing.
PSD test curves for the R20-C10, R20-C15, and R20-C20 specimens show that with fixed recycled aggregate content, PSD curves shift rightward as stabilizer content increases. This indicates strengthening aggregation effects due to hydration reactions, leading to increased coarse particle content. Specifically, the R20-C10 specimens contained 24.08% clay, 19.33% silt, and 56.59% sand particles; while the R20-C15 and R20-C20 specimens showed decreased or stable clay and silt contents, with sand content increasing by 16.94% and 27.58%, respectively. This leftward to rightward curve shift indicates the formation of large particle aggregates after stabilizer mixing, enhancing specimen density.
For the R20-C20, R40-C20, and R60-C20 specimens’ PSD test curves, with fixed stabilizer content, PSD curves showed different trends as recycled aggregate content increased. R20-C20 and R60-C20 curves appeared more rightward compared to R40-C20. Specifically, the R40-C20 specimens contained 63.20% sand particles, while R20-C20 and R60-C20 contained 84.43% and 87.48%, respectively. R40-C20’s lower sand content was due to stabilizer hydration binding more silt–clay particles, reducing silt content and relative sand content. Additionally, increased recycled aggregate content in the R60-C20 specimens led to higher coarse particle content, affecting mechanical properties.

3.6. Field Emission Scanning Electron Microscopy

Field Emission Scanning Electron Microscopy (SEM) clearly observes material microstructure and organization, crucial for revealing mechanisms of soil improvement through a stabilizer and recycled aggregate combination. This section discusses and analyzes SEM microstructure images of the specimens (R20-C10, R20-C15, R20-C20, R40-C20, R60-C20).
Figure 11a,b show the R20-C10 specimen microstructure at 500× and 2000× magnification. Figure 11a shows recycled aggregates fully encapsulated by red clay, but with notable void structures and cracks; Figure 11b shows limited cementitious materials from soil stabilizer hydration between red clay and recycled aggregates, partially filling inter-particle voids. However, large cracks and wide voids remained, forming aggregates that constitute the main skeletal structure supporting the specimens and bearing unconfined compressive strength.
Figure 12a,b show the R20-C20 specimen microstructure at 500× and 2000× magnification, revealing recycled aggregates fully encapsulated by red clay and voids between soil particles and recycled aggregates filled with hydration products, forming denser aggregates. This explains the R20-C20 specimens’ highest unconfined compressive strength.
Figure 13a,b show the R60-C20 specimen microstructure at 500× and 2000× magnification, where increased recycled aggregate particles created large voids that red clay particles could not completely fill and encapsulate, leading to unstable skeletal structure due to movement and sliding between recycled aggregate particles, resulting in the R60-C20 specimens’ lowest unconfined compressive strength.

4. Discussions

This study demonstrates the synergistic effects of recycled fine aggregate (RFA) and stabilizers in improving the mechanical properties of soft soils, particularly red clay. The results indicate that both the stabilizer content and recycled aggregate content play critical roles in enhancing strength. Specifically, higher stabilizer content contributes to the formation of cementitious products like calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H), which improve bonding between particles and densify the soil structure. Conversely, excessive recycled aggregate content may introduce voids, leading to reduced strength. Curing conditions further impact the stabilization process. Extended curing periods allow for more complete hydration reactions, resulting in improved compressive and shear strength. This study also highlights the significance of particle size distribution in enhancing particle interlocking and minimizing void spaces, contributing to overall stability.
The primary practical application of this research lies in the stabilization of soft soils, particularly in urban areas where construction waste, such as recycled fine aggregate (RFA), is abundant. The use of RFA, in combination with soil stabilizers, presents a sustainable approach for improving the mechanical properties of soft soils, making them more suitable for construction purposes. This method offers an environmentally friendly solution by recycling construction waste and reducing the need for natural aggregates, thereby conserving natural resources. Additionally, this research contributes to the field of sustainable construction and waste management by providing a feasible option for recycling construction waste, such as RFA, in soil stabilization. It also opens new avenues for future research to explore the feasibility and economic viability of large-scale applications.
While the results are promising, limitations exist. Variability in recycled aggregate properties and site-specific soil conditions may affect the generalizability of these findings. Future studies should explore long-term performance under diverse environmental conditions and assess the economic feasibility of large-scale applications.

5. Conclusions

This study discusses and analyzes the mechanical performance of recycled aggregate in conjunction with soft soil stabilizers after curing under specified water-immersion periods. By mixing varying proportions of recycled aggregate (20%, 40%, 60%) and soft soil stabilizers (10%, 15%, 20%) with red clay at optimal moisture content, and conducting standard curing and water-immersion curing, the following conclusions were drawn from unconfined compressive strength tests (UCS), Field Emission Scanning Electron Microscopy (SEM), and particle size distribution tests (PSD):
(a) The UCS test results show that when the recycled aggregate content is fixed, the unconfined compressive strength (qu) increases with higher stabilizer content. Conversely, when the stabilizer content is fixed, qu decreases as the recycled aggregate content increases. The highest qu is achieved when both stabilizer and recycled aggregate contents are at 20%. Additionally, qu increases with curing time under different water-immersion periods (1 day, 5 days, 20 days, 40 days) and peaks at 40 days. These results indicate that combining recycled aggregates with stabilizers can maintain stable strength under water-immersion conditions, providing useful insights for engineering applications.
(b) The triaxial test results reveal that the peak deviatoric stress increases with confining pressure. For the same confining pressure and recycled aggregate content, both peak deviatoric stress and shear strength parameters increase with higher stabilizer content. However, under constant confining pressure and stabilizer content, these values decrease as recycled aggregate content increases.
(c) The mechanism for improving soft soil through the combination of recycled aggregates and soft soil stabilizers is primarily attributed to the interaction between the recycled aggregates and the stabilizer, which leads to the encapsulation and agglomeration of the aggregates by red clay. During the hydration reaction of the stabilizer, cementitious substances such as calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H) are formed. These products act as binding agents, filling the voids between the soil particles and recycled aggregates and significantly enhancing the internal cohesion of the mixture. The formation of these hydration products not only strengthens the bond between the particles but also improves the overall compactness of the soil structure. As a result, the sample exhibits an enhanced internal structure that contributes to the increase in unconfined compressive strength and overall mechanical stability. This process results in a more robust, dense, and stable mixture, offering an effective method for stabilizing soft soils.
(d) PSD test results show that when recycled aggregate content is fixed, PSD curves shift to the right as stabilizer content increases, reaching a maximum at 20% stabilizer content. This indicates that the agglomeration effect caused by hydration reactions is strengthened, increasing the proportion of coarse particles and making the sample more compact internally.
(e) The SEM analysis reveals significant changes in the microstructure of the stabilized soil, with detailed measurements and comparisons providing deeper insights into the structural improvements. The random distribution of recycled aggregates is encapsulated and agglomerated by red clay, and cementitious substances such as calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H) form during the stabilizer’s hydration reaction. These products fill the voids between the soil particles, leading to a denser and more cohesive structure. Quantitative measurements of the particle size distribution, pore volume, and the density of cementitious products further emphasize the structural enhancement. Compared to untreated soil, the stabilized samples exhibit significantly reduced pore volume and improved particle bonding, contributing to the increased unconfined compressive strength.

Author Contributions

Conceptualization, K.Z. and X.W.; methodology, K.Z. and X.W.; validation, K.Z. and X.W.; data curation, K.Z.; writing—original draft preparation, K.Z.; writing—review and editing, Q.D., J.H. and X.W.; project administration, Q.D., J.H., H.Z. and X.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Key Projects for Basic and Applied Basic Research of Jiangmen City in 2022 (Project Number: 2220002000176); the High Technology Direction Project of the Key Research and Development Science and Technology of Hainan Province, China (Grant No. ZDYF2024GXJS001); the Hainan University Collaborative Innovation Center Project (Grant No. XTCX2022STB09); the Key Research and Development Projects of the Haikou Science and Technology Plan for the Year 2023 (2023-012); and the Enterprise Entrusted Project of Hainan University (HD-KYH-2024340, HD-KYH-2024153).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Gradation of red clay particles and the red clay used.
Figure 1. Gradation of red clay particles and the red clay used.
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Figure 2. Red clay, soft soil stabilizers, and recycled aggregate.
Figure 2. Red clay, soft soil stabilizers, and recycled aggregate.
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Figure 3. XRD plots of soft soil stabilizers.
Figure 3. XRD plots of soft soil stabilizers.
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Figure 4. (a) The stress–strain curve at an age of 1 d of submerged conservation; (b) the stress–strain curve at an age of 5 d of submerged conservation.
Figure 4. (a) The stress–strain curve at an age of 1 d of submerged conservation; (b) the stress–strain curve at an age of 5 d of submerged conservation.
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Figure 5. (a) The stress–strain curve at an age of 20 d of submerged conservation; (b) the stress–strain curve at an age of 40 d of submerged conservation.
Figure 5. (a) The stress–strain curve at an age of 20 d of submerged conservation; (b) the stress–strain curve at an age of 40 d of submerged conservation.
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Figure 6. Peak stresses at different ages of inundation conservation.
Figure 6. Peak stresses at different ages of inundation conservation.
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Figure 7. Peak deviatoric stress of specimens with different mix ratios under confining pressures of 200–400 kPa.
Figure 7. Peak deviatoric stress of specimens with different mix ratios under confining pressures of 200–400 kPa.
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Figure 8. (a) Angle of internal friction of specimens with different ratios; (b) adhesion and cohesion of specimens with different ratios.
Figure 8. (a) Angle of internal friction of specimens with different ratios; (b) adhesion and cohesion of specimens with different ratios.
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Figure 9. Schematic diagram of particle skeleton changes.
Figure 9. Schematic diagram of particle skeleton changes.
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Figure 10. Distribution curves of grain sizes of specimens with different ratios and the proportion of powdery and viscous sand at 40 d of immersion conditioning.
Figure 10. Distribution curves of grain sizes of specimens with different ratios and the proportion of powdery and viscous sand at 40 d of immersion conditioning.
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Figure 11. R20C10-SEM micrographs (a,b).
Figure 11. R20C10-SEM micrographs (a,b).
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Figure 12. R20C20-SEM micrographs (a,b).
Figure 12. R20C20-SEM micrographs (a,b).
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Figure 13. R60C20-SEM micrographs (a,b).
Figure 13. R60C20-SEM micrographs (a,b).
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Table 1. Basic physical properties of red clay.
Table 1. Basic physical properties of red clay.
Relative Density of Soil/(g·cm−3)Liquid Limit/%Plastic Limit/%Plasticity IndexOptimum Moisture Content/%
2.7150.122.727.420.1
Table 2. Design solutions for unconfined compressive tests and triaxial tests.
Table 2. Design solutions for unconfined compressive tests and triaxial tests.
Moisture ContentRecycled Aggregate Admixture (R)Dosage of Soft Soil Curing Agent (C)Age of Conservation (M)
20%20%10%Standard curing 28 d + immersion curing 1 d, 5 d, 20 d, 40 d
15%
20%
40%20%
60%20%
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Zhou, K.; Wang, X.; Hu, J.; Dong, Q.; Zeng, H. Unlocking the Potential of RFA and Stabilizers in High Moisture Geotechnical Applications. Appl. Sci. 2025, 15, 1270. https://doi.org/10.3390/app15031270

AMA Style

Zhou K, Wang X, Hu J, Dong Q, Zeng H. Unlocking the Potential of RFA and Stabilizers in High Moisture Geotechnical Applications. Applied Sciences. 2025; 15(3):1270. https://doi.org/10.3390/app15031270

Chicago/Turabian Style

Zhou, Kaiqing, Xuliang Wang, Jun Hu, Qinxi Dong, and Hui Zeng. 2025. "Unlocking the Potential of RFA and Stabilizers in High Moisture Geotechnical Applications" Applied Sciences 15, no. 3: 1270. https://doi.org/10.3390/app15031270

APA Style

Zhou, K., Wang, X., Hu, J., Dong, Q., & Zeng, H. (2025). Unlocking the Potential of RFA and Stabilizers in High Moisture Geotechnical Applications. Applied Sciences, 15(3), 1270. https://doi.org/10.3390/app15031270

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