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Article

Assessing the Impact of Rice Husk Ash on Soil Strength in Subgrade Layers: A Novel Approach to Sustainable Ground Engineering

by
Abdelmageed Atef
* and
Zakaria Hossain
Graduate School of Bioresources, Mie University, Tsu 514-0102, Japan
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(12), 5457; https://doi.org/10.3390/su17125457
Submission received: 9 March 2025 / Revised: 27 March 2025 / Accepted: 2 April 2025 / Published: 13 June 2025
(This article belongs to the Special Issue Innovative Building Solutions for Decarbonized and Sustainable Construction)
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Abstract

:
The disposal of rice husk ash (RHA) in rice-producing regions poses critical environmental and public health challenges. However, RHA’s high amorphous silica content offers significant potential for soil stabilization, particularly in improving the mechanical properties of weak soils. This study investigates the shear strength of clay soil stabilized with rice husk ash (2%, 4%, 6%) and low cement dosages (2%, 4%, 6%) that incorporate layered subgrade systems (top, bottom, and dual-layer configurations). By optimizing rice husk ash incorporation with reduced cement content, this approach challenges conventional stabilization methods that rely heavily on cement. Sixteen soil-cement-RHA mixtures were evaluated through mechanical testing, supplemented by microstructural and elemental analyses using scanning electron microscopy and energy-dispersive X-ray spectroscopy. Results demonstrated substantial improvements in shear strength across all subgrade layers. The dual-layer system with 2% RHA 6% cement (2%RHA6%C) achieved the highest cohesive strength (115 kN/m2) and maximum deviatoric stress (446 kN/m2). These findings highlight the viability of RHA as a sustainable, low-cement soil stabilizer, offering dual benefits: effective waste valorization and enhanced geotechnical performance. This study advances sustainable ground engineering practices by introducing a resource-efficient novel building material and provides a framework for layered stabilization systems in clay soils. Future investigations will focus on a broader range of soil types and extend the application of this approach to other sustainable ground engineering practices.

1. Introduction

The quest for sustainable solutions in ground engineering is paramount in geotechnical engineering [1,2]. As environmental issues become more widely known and the need for resilient infrastructure increases [3], researchers and practitioners are continuously seeking innovative approaches to enhance soil engineering properties while minimizing adverse environmental impacts. This pursuit has led to exploring alternative materials and techniques [4] to traditional ground improvement methods. Historically, geotechnical engineers have relied on mechanical reinforcement and chemical stabilization [5] with cement and lime to bolster soil strength and stability. However, the environmental implications of cement production, including high energy consumption and CO2 emissions [6,7], have spurred interest in greener alternatives. As such, researchers are searching for supplementary materials [8] that can partially or fully replace cement while maintaining or enhancing soil performance, and this has become a focal point of research.
One such material that has garnered attention in recent years is rice husk ash (RHA). As a by-product of rice milling, RHA is abundant in rice-producing regions and offers a renewable and cost-effective solution [9] for soil enhancement. What sets RHA apart is its high content of amorphous silica [10], which lends itself to pozzolanic reactions when combined with calcium hydroxide in cementitious systems. This reaction forms calcium silicate hydrate (C-S-H) gel [11,12,13,14], improving soil strength and durability. Moreover, RHA presents an opportunity to simultaneously handle waste management and soil enhancement [15,16]. With proper treatment and processing, RHA can be transformed from a waste material into a valuable resource for infrastructure development [17]. By incorporating RHA into subgrade layers, engineers can enhance soil properties and mitigate environmental pollution associated with RHA disposal [18,19]. Despite the potential benefits of RHA, challenges remain in optimizing its utilization for ground improvement [20,21]. The effectiveness of RHA as a soil reinforcement depends on various factors [22], including the optimal mixture proportions and curing conditions. To produce the appropriate strength and durability properties, this must be ascertained in soil–RHA–cement mixtures.
Unlike conventional soil stabilization approaches, the layering method proposed in this study provides unique benefits, such as controlled distribution of stabilizing agents and enhanced geotechnical efficiency. Subgrade layers serve as the primary support for pavement systems, and their reinforcement is essential to withstand long-term mechanical stresses and environmental challenges. By strategically localizing rice husk ash within specific subgrade layers (top, bottom, or dual zones), this research focuses on optimizing key properties like shear strength, compressibility, and load-bearing capacity, thereby improving the overall stability of pavement foundations. This novel methodology addresses a notable gap in existing research, where layered reinforcement techniques for subgrade systems remain understudied. The findings could transform sustainable construction by reducing resource consumption, lowering ecological footprints, and offering adaptable solutions for durable infrastructure. This study aims to determine suitable applications for soil–RHA–cement mixtures in light-load construction scenarios. Specifically, the mixture is recommended for settings with low-stress requirements, such as low-traffic roads, modest foundations, pedestrian walkways, minor access routes, and parking areas with limited vehicular load. In these contexts, using this mixture can offer cost savings and promote sustainability.
Sixteen clay soil mixtures were prepared with varying proportions of RHA (2%, 4%, 6%) and cement (2%, 4%, 6%) in top, bottom, and dual-layer configurations. Consolidated drained (CD) triaxial tests and SEM and EDS analyses were conducted to evaluate how these mixtures influence shear strength parameters, such as cohesion and internal friction angle, and to reveal the microstructural and chemical reinforcement mechanisms.
Aligned with United Nations Sustainable Development Goal 9 (SDG 9) [3], which promotes resilient infrastructure, sustainable industrialization, and innovation, this study contributes to the development of eco-friendly construction materials. By incorporating recycled agricultural waste like rice husk ash into soil stabilization, our research supports sustainable practices that not only enhance infrastructure resilience but also complement SDG 7 on affordable and clean energy and SDG 11 on sustainable cities and communities, thereby advancing broader sustainable development objectives.

2. Research Significance

The global shift toward sustainable infrastructure has intensified the need for innovative, environmentally conscious solutions in geotechnical applications. Although the ability of rice husk ash to stabilize soil is widely recognized, prior research has primarily emphasized homogeneous blending methods, overlooking the strategic benefits of layering. This study pioneers a stratified application of RHA within subgrade systems, a methodology scarcely investigated in contemporary literature. Our paper fills a significant gap in the literature by addressing the paucity of research on layering techniques for soil fortification. Concentrating RHA within the designated top, bottom, or dual subgrade zone layers optimizes material efficiency while enhancing critical engineering properties such as shear strength and deformation resistance. Such targeted reinforcement ensures superior structural integrity in pavement foundations, directly addressing vulnerabilities imposed by dynamic traffic loads and climatic fluctuations. Beyond advancing geotechnical performance, this layered approach reduces resource consumption, offering cost-effective and eco-friendly alternatives to conventional stabilization practices. This research further distinguishes itself by systematically evaluating how varying layer configurations (single or dual placement) influence mechanical behavior, mainly through shear strength analysis of RHA–cement composites relative to untreated soil. These insights bridge a vital knowledge gap in layered soil enhancement strategies, providing actionable guidelines for designing resilient, low-carbon infrastructure. By harmonizing technical efficacy with ecological responsibility, the study contributes a scalable model for sustainable pavement construction, aligning with global imperatives to decarbonize civil engineering practices while bolstering infrastructure longevity.

3. Materials and Methods

3.1. Materials

The Handa region of Tsu City, Mie Prefecture, Japan, provided the soil for this investigation. We selected this region to address geotechnical difficulties arising from its inherently weak, expansive, and poorly consolidated soils. Adhering to typical geotechnical protocols [23], a methodical soil sampling technique was used to obtain representative samples. The collected soil underwent air-drying for three weeks [24] before being sieved through a 2 mm sieve [25] in preparation for the triaxial test. During the three-week air-drying period, uniform moisture content was ensured across all samples, a critical factor for achieving precise, repeatable [26] and similar test outcomes. Sieving through the 2 mm sieve effectively eliminated coarse aggregates and particles, promoting homogeneity in the soil utilized [27] for the triaxial test. This step improved the accuracy of the test results. It guaranteed adherence to specified particle size requirements, facilitating ease of weighing, mixing, and creating the appropriate specimen shapes for the triaxial test by compacting the soil. Additional studies were conducted using sieve, hydrometer, liquid, and plastic limit examinations to classify the soil. The soil’s measured plastic and liquid limits were 28.5% and 48.5%, respectively. The soil comprised 42.7% clay, 57.28% silt, and 0.02% sand. The American Association of State Highway and Transportation Officials (AASHTO) classified the soil as clay soil as A-7–6 (5) clayey soil. Figure 1 shows a thorough grain size examination of the cement, rice husk ash (RHA), and soil. Japan Industrial Standards (JIS) A 1210 were followed in all testing procedures [28]. Further, Table 1 lists the geotechnical characteristics of the soil. The RHA from Make Integrated Technology Co., Ltd. in Osaka, Japan, had a high silica level of 91.1%. Produced through a controlled burning process, the RH was combusted for 25 to 28 h using computer-controlled ash production equipment, maintaining temperatures between 650 and 700 °C. The resulting RHA had particle sizes ranging between 0.07 and 0.3 mm. Figure 1 shows the soil and RHA’s precise particle size distribution. In contrast, Table 2 thoroughly describes rice husk ash’s physical and chemical characteristics. The binding agent utilized in this investigation was cement, which is widely accessible in the market. Other studies describe cement’s complex physical and chemical characteristics [16,29].

3.2. Material Preparation

Specimen preparation, as shown in Figure 2, involved creating a series of specimens to examine the impact of shear strength of soil–RHA–cement combinations. Each specimen was around 12.3 to 12.6 cm tall, weighing between 475 and 485 g, and had a weight-to-height ratio of 38 to 41 g per 1 cm (Figure 3). Each layer had roughly 157–162 g of soil, and the experimental layer was made up of 37–42 g of soil, RHA, and cement mixed with 117–122 g of soil alone to create a 1 cm layer of the soil–RHA–cement blend, as seen in Figure 4. A 4.9 cm diameter hand rammer weighing 1 kg was used to condense the three layers manually inside a mold that measured 5.0 cm in diameter and 12.5 cm in height. Each layer received 20 blows to guarantee even compaction, with a falling height of approximately 28 to 32 cm. Water was added gradually for adequate hydration without saturation, based on the ideal amount of moisture (OMC) determined from compaction tests in accordance with Japan Industrial Standards (JIS) A 1210 [28]. The components were thoroughly mixed in a big mixing bowl to create a uniform soil–RHA–cement blending. A meticulous preparation and compaction process was employed to ensure the integrity and reliability of the subsequent shear strength tests, adhering to standardized methods and precise weight ratios. This approach enhances the validity of the results, allowing researchers to confidently replicate the experiment, verify the findings, or explore additional aspects of the material’s behavior under similar conditions. For a detailed illustration of the process, refer to Figure 2.
In this study, the dosages of RHA were selected as 2%, 4%, and 6% based on the maximum utilization of amorphous silica at low percentages, as indicated by previous research [16,30]. The maximum utilization of amorphous silica based on previous research was 5%; therefore, we selected 4% and 6% to examine the effects of slight variations around this peak value. To ensure a broader evaluation, 2% RHA was chosen as a lower bound to analyze the initial impacts of RHA at a minimal yet effective dosage, which helped determine whether even a tiny amount of RHA contributes positively to soil improvement, allowing for a more comprehensive assessment of the material’s impact across various dosages. This range offers an optimal balance between enhancing soil strength and maintaining cost efficiency, considering the reduction in cement usage, soil stabilization expenses, and the effective utilization of RHA as a waste material.
On the other hand, the selected cement dosages (2%, 4%, and 6%) allow for a systematic evaluation of how increasing cement content improves soil strength and stability. Many studies on soil stabilization have used similar dosage ranges [16,19,30], making it easier to compare our results with existing research. A minimal cement dosage is used as a pozzolanic reaction activator to assess the synergistic effects of cement and RHA on sustainable soil enhancement. Since RHA is a partial replacement for cement, selecting these specific percentages enables a practical analysis of how different cement levels interact with RHA. Additionally, higher cement dosages can have negative environmental impacts, such as increased carbon emissions and the depletion of natural resources during production. This study explores a cost-effective approach to achieving strength and stability by incorporating alternative binding materials with cementitious properties, such as RHA.
Furthermore, using minimal cement dosages helps reduce overdependence on cement and promotes the adoption of sustainable cement-reducing alternatives. Incorporating moderate cement into soil reinforcement enhances the process by ensuring optimal calcium ion (Ca2+) levels necessary for pozzolanic reactions. This strategy is consistent with the broader objectives outlined in the introduction, such as minimizing carbon emissions and promoting environmental sustainability. It also highlights the shift in focus from traditional concrete-based methods to more soil-centered solutions, emphasizing eco-friendly practices.

3.3. Testing Methods

3.3.1. Compaction Tests

The compaction characteristics of each material mixture listed in Table 3 were evaluated using the standard Proctor test to determine the maximum dry density and the corresponding optimum moisture content. For this purpose, the mixtures were compacted in a cylindrical mold with a diameter of 10 cm and a height of 12.7 cm. A 2.5 kg rammer, released from approximately 28 to 32 cm height, was used to compact the material in three layers, ensuring uniform compaction effort. Tests were conducted at varying moisture levels, and each case’s dry densities were calculated. The resulting compaction curves plotted dry density against moisture content and were analyzed to identify the peak values representing the maximum dry density and optimum moisture content. All procedures strictly followed the Japan Industrial Standards (JIS) A 1210 [28]. The outcomes, including the optimum moisture content and dry density for all combinations, are illustrated in Figure 5.

3.3.2. Triaxial Compression Test

The testing methodology was conducted per the Japan Geotechnical Society standards 0520~0524 [31]. Consolidated drained (CD) triaxial tests (Marui Co., Ltd., Osaka, Japan) were performed on cylindrical samples, each measuring 5 cm in diameter and 12.5 cm in height. The samples were prepared by compacting the material blends in three layers using a 1 kg hammer dropped from a 30 cm height into appropriately sized molds. Once prepared, the specimens were enclosed in rubber sleeves and placed in the triaxial chamber, as shown in Figure 6. The setup for the CD triaxial tests consisted of several critical elements: a load cell to measure axial force, a piston to apply the axial load, a dial gauge to track axial displacement, a pressure cylinder containing chamber fluid to apply hydrostatic pressure (σ3), and pathways for drainage/pore water pressure monitoring, confining fluid supply, and air pressure input. A detailed schematic of the setup, including labeled components, is provided in Figure 6. Tests were conducted under three confining pressures: 50 kPa, 100 kPa, and 150 kPa. The drainage route was left open during testing to ensure drained conditions and prevent pore water pressure buildup. The specimens were sheared at a controlled rate of 0.5 mm/min, with deviatoric stress and axial strain continuously recorded via a data logger. Testing was halted when the axial strain reached two-thirds or 10% of its starting value, whichever came first, or when the peak deviatoric stress was reached.

3.3.3. Microstructural Study on Shear Strength Development

Advanced microstructural analysis techniques were employed to comprehensively understand shear strength development in the reinforced and treated soil composite. Key among these were SEM and EDS (JEOL Ltd., Tokyo, Japan) SEM imaging provided high-resolution, three-dimensional visualizations of the soil composite at the micrometer scale, revealing detailed surface morphology and structural changes induced by the stabilizing additives [32,33]. By utilizing low-energy secondary and backscattered electrons, SEM effectively captured the micro-level alterations in the composite material [34,35,36]. Complementing this, EDS was used to analyze the chemical composition and crystalline structures within the treated soil. This technique offered critical insights into integrating chemical stabilizing agents and the chemical transformations driven by pozzolanic reactions [37,38]. SEM and EDS enabled a thorough examination of the stabilized soil’s physical and chemical modifications, providing a deeper understanding of its enhanced mechanical properties.

4. Results and Discussion

4.1. Influence of Stabilizers on Compaction Behavior

Figure 5 compares untreated soil’s OMC and MDD values with those of stabilized mixtures. All compaction curves remained below full saturation (Sr = 100%), confirming the results aligned with theoretical expectations. The OMC increased from 21% for untreated soil to 22–24% for mixtures containing 6% cement, 6% RHA, or their combinations. For example, most RHA-cement blends had OMC values of 23–24%, except for the 2%RHA2%C mixture, which had a slightly lower OMC of 22.5%. Meanwhile, MDD decreased modestly, from 1.55 g/cm3 in the 2%RHA2%C and 2%RHA4%C mixtures to 1.50 g/cm3 in the 4%RHA4%C and 4%RHA6%C mixtures, and further to 1.46 g/cm3 in the 6%RHA and 6%RHA6%C mixtures. This decline in MDD is attributed to the porous nature of RHA, which increases water absorption, and the exothermic hydration of cement, which raises moisture demand. In addition, the lower specific gravity of RHA and cement compared to native soil particles leads to higher void ratios and reduced MDD. This lower density implies that the stabilized blends require less compaction energy, potentially reducing field application costs. These findings are consistent with previous studies on RHA–cement-stabilized soils [34,35,36].

4.2. The Axial Strain and Deviatoric Stress Relationship Curves for Different Types of Soil–RHA–Cement Combinations

The relationship between axial strain and several specimen combinations for the top, bottom, and dual layers is shown in Figure 7, Figure 8 and Figure 9. These mixtures contained different proportions of cement and RHA: 2, 4, and 6%. Axial strain varied between 5% and 7% for the top layer and 5% and 6% for the bottom and dual layers when 2% cement was added. The axial strain was reduced to 6% for the top layer and 5% for the dual layer when the cement percentage was raised to 4%. However, the axial strain was almost the same at 5% to 6% for all subgrade layers when using 6% cement.
This observed trend indicates that increasing the cement content enhances soil rigidity and strength, reducing axial strain. This occurrence is supported by research findings comparable to those published by [39]. This behavior is characteristic of soil stabilization, where an optimal additive content exists beyond which additional improvements are marginal. For specimens containing only RHA, the axial strains increased across all sub-grade layers because the porous structure of RHA is collapsible. However, when 2% to 6% cement was added, the axial strains were reduced to around 4% across all layers due to cementitious compounds forming within the soil structure. RHA alone increased axial strain due to its porous nature and lack of binding capacity. RHA with cement significantly reduced axial strain due to cementitious compound production, which improves soil hardness and density. The synergistic effect of RHA and cement created a well-bonded and less compressible soil structure, resulting in uniformly lower axial strains across all layers, a phenomena supported by research comparable to what [16,40] reported. In summary, adding 2% to 6% cement improves soil stabilization by forming cementitious compounds that counteract the compressibility of RHA, leading to reduced axial strains.
The link between different sample configurations in the top, bottom, and dual layers and deviatoric stress is shown in Figure 10, Figure 11 and Figure 12. The findings demonstrate a distinct pattern of rising deviatoric stress as confining pressures rise from 50 kPa to 150 kPa. The former showed the highest deviatoric stresses when comparing dual-layer samples to bottom-and top-layer samples. For example, the sample with a blend of 2%RHA6%C accomplished peak deviatoric stress of 446 kN/m2 in the dual layer, whereas the top and bottom layers recorded rates of 361 kN/m2 and 371 kN/m2, respectively, as depicted in Figure 10 and Figure 11. This significant rise in deviatoric stress is due to RHA particles filling the voids between soil grains, which can help increase the soil mix’s overall density and inter-particle friction. The increased resistance to deformation is because of the pozzolanic reactions among rice husk ash and cement. RHA’s pozzolanic activity stems from its amorphous silica content, which combines with calcium hydroxide produced during cement hydration. Other cementitious substances, such as calcium silicate hydrate (C-S-H), are created by this reaction [41]. This reaction can enhance the binding within the soil structure, increasing resilience and stiffness and, consequently, higher deviatoric stresses and stress resistance [42,43]. With RHA, the combination of improved particle packing and pozzolanic reactions results in a soil structure that can resist higher deviatoric stresses than control specimens and those with only cement. This is due to the enhanced inter-particle cohesion and reduced porosity, matching results similar to those published in previous research [16,29,44]. Adding 2%RHA to all sample compositions significantly improved deviatoric stress in all subgrade layers, outperforming control and cement-only samples. However, when the RHA concentration rose from 2% to 6%, there was a minor decrease in deviatoric stress, particularly in the dual layer. In conclusion, the highest deviatoric stress was achieved in the dual-layer specimens with 2%RHA6%C, as highlighted in Figure 12. This outcome underscores the effectiveness of combining RHA and cement in enhancing soil stabilization, particularly in a dual layer.
As shown in Figure 13, we examined photographic data that revealed failure patterns during testing to better understand the mechanical response to the shear behavior of unreinforced and rice husk ash-cement-reinforced samples. Figure 13a shows simple shear in the soil specimen, resulting in well-defined cracks along the failure plane. Upon reaching the maximal deviatoric forces, the fragility of the soil caused this cracking. Interestingly, including rice husk ash and cement in the subgrade soil was crucial in determining where failure cracks began. The starting points of these cracks were influenced mainly by the distribution of the RHA–cement blends and the number of reinforcement layers. For instance, when a single layer of 2%RHA6%C or 4%RHA6%C was placed in the top and bottom sections, cracks tended to form in the top area and spread toward the middle, as illustrated in Figure 13b,c.
On the other hand, when dual layers of 2%RHA6%C or 4%RHA6%C were applied, the reinforcement in the top part proved inadequate, leading to cracks originating in the weaker top zone and progressing into the intermediate soil layers, as shown in Figure 13d. In contrast, incorporating dual layers of 6%RHA6%C resulted in a robustly stiffened zone. Additionally, when 6%RHA6%C was embedded, as seen in Figure 13b,c, there was a discernible decrease in the top and bottom layers’ prominent vertical axes of fragility (cracks). This reduction highlights the sample’s increased elastic properties, which is in line with the results from other studies [16,20,45,46,47], and confirmed the effectiveness of RHA–cement composites in reducing cracks in RHA–cement-composite soil.

4.3. Shear Strength of Soil Combination with Rice Husk Ash Percentages

In this study, the shear properties of the treated soil were evaluated using the Mohr–Coulomb failure criterion, which describes the relationship between shear strength and effective normal stress. The results, including the internal friction angle, are presented in Figure 14, Figure 15 and Figure 16. A significant improvement in the internal friction angle was observed across all tested samples, highlighting the considerable influence of cement and RHA incorporation compared to the untreated soil. These findings are consistent with previous research [16,29], demonstrating the effectiveness of such additives in enhancing soil shear properties, particularly in the samples with 6%RHA6%C across all sub-grade layers (top, bottom, and dual layers). The dual-layer configuration with 6%RHA6%C achieved the highest internal friction angles, as shown in Figure 16. Several causes contributed to this progress, including microstructural improvement, particle packing, cementitious binding, and pozzolanic reactions.
Nevertheless, minor fluctuations were noted in the internal friction angle, particularly in the bottom layer, as shown in Figure 15. These variations can be explained by the specimen’s subgrade layer’s location, where the shear plane was placed well above it. Other researchers have also observed this occurrence [48,49]. The porous nature of RHA particles aids in filling voids between soil grains, thereby improving packing density and increasing inter-particle friction [50]. Additionally, calcium hydroxide interacts with the silica in RHA from cement hydration to form secondary C-S-H, which binds soil particles together [51], further strengthening the soil structure by creating more binding material, reducing relative movement and thus increasing shear strength and internal friction angle [52]. This combination enhances soil cohesion and frictional properties [53], creating a more stabilized and solidified soil matrix less prone to deformation and more resistant to shearing forces. Consequently, the soil’s overall porosity and void ratio are significantly reduced [54], resulting in a denser matrix. A denser matrix means particles are in closer contact, increasing frictional resistance during shear [55] and raising the internal friction angle.

4.4. Effects on Different Layers

The top layer, directly exposed to external loads and environmental conditions, benefits from the enhanced internal friction angle due to the combination of RHA and cement. This enhancement helps resist surface shear stresses and reduces susceptibility to erosion and deformation under loads [56]. A higher internal friction angle improves the top layer’s load distribution capacity, making it more stable and durable. The bottom layer, serving as the foundation for the top layers, also shows significant improvements. Its improved internal friction angle provides a more substantial base [57], reducing deformation under load [58]. With a higher internal friction angle, the bottom layer can more effectively transfer loads from the top layers to the subgrade, enhancing the overall stability of the soil. The synergistic effects of RHA and cement are maximized in the double-layer configuration [59]. The interactions between the layers lead to a uniformly enhanced soil structure with improved frictional properties throughout. The combination of RHA and cement ensures that both layers contribute to the overall stability, leading to consistent performance under load [60]. The internal friction angle is uniformly high, providing balanced resistance to shear across the entire depth of the subgrade. These improvements ensure that each top, bottom, and dual layer benefits from enhanced stability, leading to a more durable and resilient soil–RHA–cement composite. In essence, this research aimed to analyze and measure the shear strength of treated soil comprehensively. The resulting data, summarized in Figure 14, Figure 15 and Figure 16, showcase the impact of incorporating cement and rice husk ash on the internal friction angle across different subgrade layers.
The shear strength variables are also displayed in Figure 17, Figure 18 and Figure 19. Regarding the cohesive strength seen in the dual and bottom layers, the addition of cement and RHA induced slight variations in the cohesive strength compared to the soil-only sample. Samples containing 2%RHA6%C at the dual layers attained the maximum cohesiveness levels, exhibiting cohesion values of 115 kN/m2, as shown in Figure 19. This demonstrates notable advancements in the double subgrade layers. As illustrated in Figure 17, a minor decrease in cohesion values was observed for all sample configurations. This trend indicates that the top subgrade layer exhibited the smallest cohesion, reflecting weaker particle bonding and a less stable composition. Notably, this layer was positioned along the shear plane, making it the first point of contact with the applied axial load during testing. This direct exposure likely contributed to the observed reduction in cohesion. The slight variations observed in cohesive strength might be due to the inconsistent distribution of RHA and cement within the soil matrix. The overall impact on cohesion might be minor, as the improvements from cement and RHA would be incremental rather than transformative. The combination of RHA and cement leads to slight variations in cohesive strength compared to the control, mainly due to the improved particle binding and microstructural enhancements provided by the cementitious compounds and the pozzolanic activity of RHA. However, these effects are incremental, leading to slight rather than drastic changes in cohesion.

4.5. SEM Analysis

Integrating rice husk ash (RHA) and cement into soil matrices markedly influenced shear-induced failure patterns. A thorough microstructural evaluation was undertaken to elucidate the stabilization mechanisms and assess their efficacy in mitigating weak zone formation within composite systems. First, the baseline structural properties of untreated soil samples were evaluated, followed by a comparative analysis of the composite system after integrating RHA and cement. This approach revealed that the additives altered the material’s resistance to shear stresses and minimized structural vulnerabilities. Untreated soil samples exhibited frequent micro-cracking, with a porous structure and visible micro-fractures that suggested a heightened vulnerability to shear failure under stress, as evidenced in Figure 20 for the unreinforced soil (S Control).
In contrast, specimens stabilized with 2%RHA6%C and 4%RHA6%C, as depicted in Figure 21 and Figure 22, displayed minimal micro-cracking and formed a denser, more interconnected matrix, enhancing inter-particle binding. Increasing the RHA content to 6% alongside 6%C (6%RHA6%C) further amplified the cementation process, resulting in fewer micro-pores and cracks, as shown in Figure 23. Quantitative image analysis indicated that the reduction in micro-crack density and pore size was directly associated with measurable increases in cohesion and internal friction angle in mechanical tests. These findings corroborate similar observations in other studies [15,38,61,62], thereby linking the microstructural densification to enhanced macroscopic shear strength.

4.6. The Strengthening Mechanisms of RHA and Cement

Incorporating RHA and cement into untreated soils in this investigation markedly altered their shear-induced failure behavior. A comprehensive assessment of the stabilization processes was essential to evaluate their efficacy in mitigating structural vulnerabilities, such as weak zones within the composite matrix. Advanced SEM and EDS techniques were employed to investigate these mechanisms. These methods provided key insights into the structural and chemical modifications resulting from the RHA–cement integration, highlighting how the additives enhanced cohesion and reduced microstructural defects.
SEM imagery illustrated the progressive development of interfacial bonding and structural cohesion within the reinforced specimens. Initially, unreinforced soil samples displayed micro-cracks and a porous morphology, indicating increased susceptibility to shear failure. However, interfacial connections between the cementitious chemicals and the soil particles occurred after introducing RHA and cement. For instance, specimens containing (2%RHA + 6%C) and (4%RHA + 6%C) demonstrated a discernible decrease in the development of microcracks and the appearance of a more unified composite structure. The enhancement in microstructural integrity was further amplified with higher RHA and cement content (6%RHA + 6%C), leading to a denser and more cemented soil matrix. RHA, cement, and soil underwent pozzolanic reactions, producing calcium-silicate-hydrate (C-S-H) and calcium-alumino-silicate-hydrate (C-A-S-H) gels, and forming solid and interlocking bonds. These existence of mechanisms are also supported by [17,20,63,64,65]. These reactions significantly increased the frictional resistance and cohesion within the composite, restricting particle movement and enhancing shear strength metrics such as internal friction angle, cohesion, axial strain, and deviatoric stress.
EDS analysis provided complementary data, indicating increased SiO2 levels corresponding to higher RHA content and the formation of C-S-H gels due to pozzolanic reaction. Aluminum ions [30,51] within the C-S-H gel further strengthened the composite structure, increasing mechanical properties. These microstructural developments ensured efficient load transfer within the composite material, effectively preventing the propagation of cracks and making the strengthened soil more stable and long-lasting. The combined effects of improved particle packing, cementitious binding, and pozzolanic reactions highlight the efficacy of RHA and cement as reinforcing agents, resulting in a resilient and durable composite material suitable for sustainable ground engineering projects in geotechnical engineering.

4.7. EDS Analysis

To further investigate the elemental composition and relative abundance of various constituents in the specimens, EDS analysis was performed on both untreated and stabilized soils (2%RHA + 6%C, 4%RHA + 6%C, 6%RHA + 6%C), as shown in Figure 24, Figure 25, Figure 26 and Figure 27. The inclusion of RHA and cement markedly altered the chemical profiles of the samples, with higher RHA proportions correlating with increased silicon dioxide (SiO2) levels, a reflection of RHA’s silica-rich nature. This trend was consistent with the observed progressive rise in SiO2 content as RHA levels increased from 2% to 6%.
Moreover, the synergistic interaction of RHA and cement promoted significant formation of calcium-silicate-hydrate (C-S-H) gels through pozzolanic reactions among RHA, cement, and soil constituents. The integration of aluminum ions into the gel matrix further enhanced this reaction [51,66,67], leading to the formation of robust calcium-alumino-silicate-hydrate (C-A-S-H) bonds, as identified by the formula [(CaO)(Al2O3)(SiO2)6·4(H2O)]. These chemical changes were quantitatively linked to improvements in macroscopic mechanical properties; for example, the increased formation of C-S-H and C-A-S-H phases corresponded with measurable gains in shear strength and overall stability. The reduction in porosity and enhanced inter-particle bonding, as evidenced by the EDS spectra, directly contributed to the improved mechanical performance observed in triaxial tests. Similar outcomes have been documented in other pertinent research studies [20,67,68,69].
The SEM and EDS analyses underscore the benefits of incorporating cement and RHA into the soil, highlighting how microstructural integrity and chemical modifications directly influence and enhance the macroscopic shear resistance of stabilized soils.

5. Future Research Directions

While our study focused on the mechanical performance of soils stabilized with rice husk ash (RHA), future research should comprehensively explore the benefits of this layering stabilization method by investigating long-term stability under various environmental conditions, assessing changes in permeability and hydraulic conductivity crucial for water-resistant applications, evaluating environmental impacts including the sustainability of RHA sourcing and processing, conducting microstructural analyses to elucidate the mechanisms behind observed property improvements, and performing large-scale field trials to validate laboratory findings and assess the practicality of RHA stabilization in real-world scenarios. Additionally, we will conduct detailed quantitative analyses to correlate microstructural characteristics with the observed mechanical performance in our future studies. Addressing these aspects will enhance the understanding and application of RHA in sustainable ground engineering practices.

6. Conclusions

This study evaluated the stabilization of clay subgrade soils using rice husk ash and cement blends through consolidated drained (CD) triaxial tests on sixteen specimens with varying compositions. Key findings demonstrated that higher RHA–cement ratios significantly elevated the internal friction angle, particularly in multi-layered configurations, especially 6%RHA6%C. The findings revealed that integrating RHA and cement significantly enhanced the soil’s mechanical performance and microstructural integrity, primarily due to improved particle interlocking and the formation of cementitious gels (C–S–H and C–A–S–H) as confirmed by SEM and EDS analyses.
These results underscore the potential of RHA–cement blends as innovative and eco-friendly subgrade materials, offering practical solutions for ground improvement in sustainable construction. However, certain limitations should be acknowledged. The study focused on a specific soil type, which may not fully represent the behavior of different soil compositions encountered in various geotechnical settings. Moreover, variations in RHA properties, environmental conditions, and construction practices and factors not exhaustively explored in this investigation may influence the observed performance. Future research should address these limitations by examining a broader range of soil types, incorporating variability in RHA characteristics, and refining experimental methodologies to enhance the robustness and generalizability of the findings.
Overall, this work advances our understanding of stabilized soil behavior and lays the groundwork for developing resilient, cost-effective, and sustainable geotechnical solutions.

Author Contributions

A.A.: Conceptualization, Methodology, Data curation, Writing—Original draft preparation. Z.H.: Supervision and Reviewing. All authors have read and agreed to the published version of the manuscript.

Funding

The authors would like to acknowledge the JST SPRING, Japan Grant Number JPMJSP2137, for providing financial support through Mie University, where this study was conducted. This research was also supported by Make Integrated Technology Limited (MIT), which provided experimental materials.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
RHARice Husk Ash
SSoil
CCement
SEMScanning Electron Microscopy
EDSEnergy-Dispersive X-ray Spectroscopy
OMCOptimum Moisture Content
MDDMaximum Dry Density

References

  1. Galvin, J. Critical role of risk management in ground engineering and opportunities for improvement. Int. J. Min. Sci. Technol. 2017, 27, 725–731. [Google Scholar] [CrossRef]
  2. Basu, D.; Misra, A.; Puppala, A.J. Sustainability and geotechnical engineering: Perspectives and review. Can. Geotech. J. 2015, 52, 96–113. [Google Scholar] [CrossRef]
  3. Singh, S.; Ru, J. Goals of sustainable infrastructure, industry, and innovation: A review and future agenda for research. Environ. Sci. Pollut. Res. 2023, 30, 28446–28458. [Google Scholar] [CrossRef]
  4. Medina-Martinez, C.J.; Sandoval-Herazo, L.C.; Zamora-Castro, S.A.; Vivar-Ocampo, R.; Reyes-Gonzalez, D. Natural fibers: An alternative for the reinforcement of expansive soils. Sustainability 2022, 14, 9275. [Google Scholar] [CrossRef]
  5. Afrin, H. A review on different types soil stabilization techniques. Int. J. Transp. Eng. Technol. 2017, 3, 19–24. [Google Scholar] [CrossRef]
  6. Zhang, C.Y.; Han, R.; Yu, B.; Wei, Y.M. Accounting process-related CO2 emissions from global cement production under Shared Socioeconomic Pathways. J. Clean. Prod. 2018, 184, 451–465. [Google Scholar] [CrossRef]
  7. Imbabi, M.S.; Carrigan, C.; McKenna, S. Trends and developments in green cement and concrete technology. Int. J. Sustain. Built Environ. 2012, 1, 194–216. [Google Scholar] [CrossRef]
  8. Jittin, V.; Bahurudeen, A.; Ajinkya, S.D. Utilisation of rice husk ash for cleaner production of different construction products. J. Clean. Prod. 2020, 263, 121578. [Google Scholar] [CrossRef]
  9. Nzereogu, P.U.; Omah, A.D.; Ezema, F.I.; Iwuoha, E.I.; Nwanya, A.C. Silica extraction from rice husk: Comprehensive review and applications. Hybrid Adv. 2023, 4, 100111. [Google Scholar] [CrossRef]
  10. Al-Saffar, F.Y.; Wong, L.S.; Paul, S.C. An Elucidative Review of the Nanomaterial Effect on the Durability and Calcium-Silicate-Hydrate (CSH) Gel Development of Concrete. Gels 2023, 9, 613. [Google Scholar] [CrossRef]
  11. Li, N.; Pang, Y.; Wang, W.; Wu, E.; Fan, M.; Jiang, P.; Mei, G. Enhancement effect of calcium carbide residue and rice husk ash on soft soil: Small-strain property and micro mechanism. J. Mater. Res. Technol. 2024, 32, 774–791. [Google Scholar] [CrossRef]
  12. Malpani, S.K.; Goyal, D. Synthesis, analysis, and multi-faceted applications of solid wastes-derived silica nanoparticles: A comprehensive review (2010–2022). Environ. Sci. Pollut. Res. 2023, 30, 28321–28343. [Google Scholar] [CrossRef] [PubMed]
  13. Abolhasani, A.; Samali, B.; Aslani, F. Rice husk ash incorporation in calcium aluminate cement concrete: Life cycle assessment, hydration and strength development. Sustainability 2022, 14, 1012. [Google Scholar] [CrossRef]
  14. Pode, R. Potential applications of rice husk ash waste from rice husk biomass power plant. Renew. Sustain. Energy Rev. 2016, 53, 1468–1485. [Google Scholar] [CrossRef]
  15. Endale, S.A.; Taffese, W.Z.; Vo, D.H.; Yehualaw, M.D. Rice husk ash in concrete. Sustainability 2022, 15, 137. [Google Scholar] [CrossRef]
  16. Shehata, A.A.A.; Owino, A.O.; Islam, M.Y.; Hossain, Z. Shear strength of soil by using rice husk ash waste for sustainable ground improvement. Discov. Sustain. 2024, 5, 64. [Google Scholar] [CrossRef]
  17. Owino, A.O.; Hossain, Z. Correlation between one-dimensional consolidation coefficients and different basalt fiber lengths and RHA-cement contents in fiber-reinforced stabilized expansive soils. Soils Found. 2023, 63, 101351. [Google Scholar] [CrossRef]
  18. Saad, A.H.; Nahazanan, H.; Yusuf, B.; Toha, S.F.; Alnuaim, A.; El-Mouchi, A.; Elseknidy, M.; Mohammed, A.A. A systematic review of machine learning techniques and applications in soil improvement using green materials. Sustainability 2023, 15, 9738. [Google Scholar] [CrossRef]
  19. Gunasekaran, V.; Sathiparan, N. Combine the use of rice husk ash and quarry waste for sustainable masonry blocks: Mechanical characteristics, durability and eco-benefits. J. Build. Eng. 2024, 98, 111194. [Google Scholar] [CrossRef]
  20. Atef, A.; Hossain, Z. Sustainable Soil Reinforcement by Maximizing Geotechnical Performance with Rice Husk Ash in Subgrade Layers. Materials 2025, 18, 873. [Google Scholar] [CrossRef]
  21. Hao, H.; Bi, K.; Chen, W.; Pham, T.M.; Li, J. Towards next generation design of sustainable, durable, multi-hazard resistant, resilient, and smart civil engineering structures. Eng. Struct. 2023, 277, 115477. [Google Scholar] [CrossRef]
  22. Khan, M.; McNally, C. A holistic review on the contribution of civil engineers for driving sustainable concrete construction in the built environment. Dev. Built Environ. 2023, 16, 100273. [Google Scholar] [CrossRef]
  23. Okalebo, J.R.; Gathua, K.W.; Woomer, P.L. Laboratory methods of soil and plant analysis: A working manual second edition. Sacred Afr. Nairobi 2002, 21, 25–26. [Google Scholar]
  24. Obalum, S.E.; Chibuike, G.U. Air-drying effect on soil reaction and phosphorus extractability from upland-lowland tropical soils as related to their colloidal stability. Appl. Ecol. Environ. Res. 2017, 15, 525–540. [Google Scholar] [CrossRef]
  25. Gee, G.W.; Or, D. 2.4 Particle-size analysis. In Methods of Soil Analysis: Part 4 Physical Methods; Soil Science Society of America, Inc.: Madison, WI, USA, 2002; Volume 5, pp. 255–293. [Google Scholar]
  26. Erich, M.S.; Hoskins, B.R. Effects of Soil drying on soil pH and nutrient extractability. Commun. Soil Sci. Plant Anal. 2011, 42, 1167–1176. [Google Scholar] [CrossRef]
  27. Haluschak, P. Laboratory methods of soil analysis. Can.-Manit. Soil Surv. 2006, 3, 133. [Google Scholar]
  28. JIS A 1210; Test Method for Soil Compaction Using a Rammer. Japanese Industrial Standard, Guidance and Basic—Soil Test. The Japanese Geotechnical Society: Tokyo, Japan, 2010; pp. 71–78. (In Japanese)
  29. Owino, A.O.; Hossain, Z. The influence of basalt fiber filament length on shear strength development of chemically stabilized soils for ground improvement. Constr. Build. Mater. 2023, 374, 130930. [Google Scholar] [CrossRef]
  30. Nahar, N.; Hossain, Z.; Tamaki, N. Optimum utilization of rice husk ash waste for ground improvement. Int. Agric. Eng. J. 2021, 30, 1–10. [Google Scholar]
  31. JGS 0524; Method for Consolidated-Drained Triaxial Compression Test on Soils. The Japanese Geotechnical Society (Dositsu Shiken Hou Dositsu Kogakkai): Tokyo, Japan, 2001; pp. 233–243. (In Japanese)
  32. Mazumder, M.; Ahmed, R.; Ali, A.W.; Lee, S.J. SEM and ESEM techniques used for analysis of asphalt binder and mixture: A state of the art review. Constr. Build. Mater. 2018, 186, 313–329. [Google Scholar] [CrossRef]
  33. Wang, G.; Zhang, X.; Liu, X.; Gao, H.; Pan, Y. Stabilization of micaceous residual Soil with industrial and agricultural by-products: Perspectives from hydrophobicity, water stability, and durability enhancement. Constr. Build. Mater. 2024, 430, 136450. [Google Scholar] [CrossRef]
  34. Sun, X.; Zhang, B.; Dai, Q.; Yu, X. Investigation of internal curing effects on microstructure and permeability of interface transition zones in cement mortar with SEM imaging, transport simulation and hydration modeling techniques. Constr. Build. Mater. 2015, 76, 366–379. [Google Scholar] [CrossRef]
  35. Polo-Mendoza, R.; Martinez-Arguelles, G.; Walubita, L.F.; Moreno-Navarro, F.; Giustozzi, F.; Fuentes, L.; Navarro-Donado, T. Ultraviolet ageing of bituminous materials: A comprehensive literature review from 2011 to 2022. Constr. Build. Mater 2022, 350, 128889. [Google Scholar] [CrossRef]
  36. Cheraghian, G.; Wistuba, M. Ultraviolet aging study on bitumen modified by a composite of clay and fumed silica nanoparticles. Sci. Rep. 2020, 10, 11216. [Google Scholar] [CrossRef]
  37. Ahmed, A. Compressive strength and microstructure of soft clay soil stabilized with recycled basanite. Appl. Clay Sci. 2015, 104, 27–35. [Google Scholar] [CrossRef]
  38. Azad, N.M.; Samarakoon, S.M.S.M.K. Utilization of industrial by-products/waste to manufacture geopolymer cement/concrete. Sustainability 2021, 13, 873. [Google Scholar] [CrossRef]
  39. Sahoo, S.; Singh, S.P. Strength and durability properties of expansive soil treated with geopolymer and conventional stabilizers. Constr. Build. Mater. 2022, 328, 127078. [Google Scholar] [CrossRef]
  40. Haeri, S.M.; Khosravi, A.; Garakani, A.A.; Ghazizadeh, S. Effect of soil structure and disturbance on hydromechanical behavior of collapsible loessial soils. Int. J. Geomech. 2017, 17, 04016021. [Google Scholar] [CrossRef]
  41. Maddalena, R.; Li, K.; Chater, P.A.; Michalik, S.; Hamilton, A. Direct synthesis of a solid calcium-silicate-hydrate (CSH). Constr. Build. Mater. 2019, 223, 554–565. [Google Scholar] [CrossRef]
  42. Choobbasti, A.J.; Samakoosh, M.A.; Kutanaei, S.S. Mechanical properties soil stabilized with nano calcium carbonate and reinforced with carpet waste fibers. Constr. Build. Mater. 2019, 211, 1094–1104. [Google Scholar] [CrossRef]
  43. Hasanzadeh, A.; Shooshpasha, I. PET fiber reinforcement efficiency in the mechanical and microstructural characteristics of cemented sand modified with silica fume. Constr. Build. Mater. 2023, 397, 132363. [Google Scholar] [CrossRef]
  44. Feng, Y.; Zhao, H.; Liu, J.; Song, Z.; Che, W.; Ma, K.; Wang, Y. Study on mechanical properties and microstructure of porous organic polymer reinforced low-grade sand under wetting–drying cycles. Constr. Build. Mater. 2023, 409, 134042. [Google Scholar] [CrossRef]
  45. Wei, J.; Meyer, C. Utilization of rice husk ash in green natural fiber-reinforced cement composites: Mitigating degradation of sisal fiber. Cem. Concr. Res. 2016, 81, 94–111. [Google Scholar] [CrossRef]
  46. Muthukrishnan, S.; Gupta, S.; Kua, H.W. Application of rice husk biochar and thermally treated low silica rice husk ash to improve physical properties of cement mortar. Theor. Appl. Fract. Mech. 2019, 104, 102376. [Google Scholar] [CrossRef]
  47. Tang, Z.; Li, G.; Lu, S.; Wang, J.; Chi, L. Enhance mechanical damping behavior of RHA-cement mortar with bionic inorganic-organic laminated structures. Constr. Build. Mater. 2022, 323, 126521. [Google Scholar] [CrossRef]
  48. Goud, G.N.; Umashankar, B. Interface shear strength properties of gravel bases and subgrades with various reinforcements. Int. J. Geosynth. Ground Eng. 2018, 4, 7. [Google Scholar] [CrossRef]
  49. Ikechukwu, A.F.; Hassan, M.M. Assessing the extent of pavement deterioration caused by subgrade volumetric movement through moisture infiltration. Int. J. Pavement Res. Technol. 2022, 15, 676–692. [Google Scholar] [CrossRef]
  50. Abhishek, A.; Guharay, A.; Raghuram, A.S.S.; Hata, T. A state-of-the-art review on suitability of rice husk ash as a sustainable additive for geotechnical applications. Indian Geotech. J. 2024, 54, 910–944. [Google Scholar] [CrossRef]
  51. Othman, R.; Putra Jaya, R.; Duraisamy, Y.; Sulaiman, M.A.; Chong, B.W.; Ghamari, A. Efficiency of waste as cement replacement in foamed concrete—A review. Sustainability 2023, 15, 5163. [Google Scholar] [CrossRef]
  52. Cui, H.; Jin, Z.; Bao, X.; Tang, W.; Dong, B. Effect of carbon fiber and nanosilica on shear properties of silty soil and the mechanisms. Constr. Build. Mater. 2018, 189, 286–295. [Google Scholar] [CrossRef]
  53. Munirwan, R.P.; Taha, M.R.; Mohd Taib, A.; Munirwansyah, M. Shear strength improvement of clay soil stabilized by coffee husk ash. Appl. Sci. 2022, 12, 5542. [Google Scholar] [CrossRef]
  54. Yin, K.; Fauchille, A.L.; Di Filippo, E.; Kotronis, P.; Sciarra, G. A review of sand–clay mixture and soil–structure interface direct shear test. Geotechnics 2021, 1, 260–306. [Google Scholar] [CrossRef]
  55. Ratnaweera, P.; Meegoda, J.N. Shear strength and stress-strain behavior of contaminated soils. Geotech. Test. J. 2006, 29, 133–140. [Google Scholar] [CrossRef]
  56. Kumar, S.; Roy, L.B. Rainfall induced geotextile reinforced model slope embankment subjected to surcharge loading: A review study. Arch. Comput. Methods Eng. 2022, 29, 3203–3221. [Google Scholar] [CrossRef]
  57. Indraratna, B.; Nguyen, T.T.; Atapattu, S.; Ngo, T.; Rujikiatkamjorn, C. Subgrade soil response to rail loading: Instability mechanisms, causative factors, and preventive measures. Transp. Geotech. 2024, 46, 101267. [Google Scholar] [CrossRef]
  58. Chen, J.; Zhou, Y. Dynamic vertical displacement for ballastless track-subgrade system under high-speed train moving loads. Soil Dyn. Earthq. Eng. 2020, 129, 105911. [Google Scholar] [CrossRef]
  59. Maheepala MM AL, N.; Nasvi MC, M.; Robert, D.J.; Gunasekara, C.; Kurukulasuriya, L.C. A comprehensive review on geotechnical properties of alkali activated binder treated expansive soil. J. Clean. Prod. 2022, 363, 132488. [Google Scholar] [CrossRef]
  60. Huang, J.; Kogbara, R.B.; Hariharan, N.; Masad, E.A.; Little, D.N. A state-of-the-art review of polymers used in soil stabilization. Constr. Build. Mater. 2021, 305, 124685. [Google Scholar] [CrossRef]
  61. Najm, H.M.; Ahmad, S.; Khan, R.A. Mechanical and microstructural analysis of waste ceramic optimal concrete reinforced by hybrid fibers materials: A comprehensive study. J. Archit. Environ. Struct. Eng. Res. 2022, 5, 11–33. [Google Scholar] [CrossRef]
  62. Ramesh, H.N.; Manjunatha, B.V. Justification of strength properties of microstructural changes in the black cotton soil stabilized with rice husk ash and carbide lime in the presence of sodium salts. SN Appl. Sci. 2020, 2, 457. [Google Scholar] [CrossRef]
  63. Ramadan, M.; Kohail, M.; Abadel, A.A.; Alharbi, Y.R.; Soliman, A.M.; Mohsen, A. Exploration of mechanical performance, porous structure, and self-cleaning behavior for hydrothermally cured sustainable cementitious composites containing de-aluminated metakaolin waste and TiO2 nanoparticles. J. Mater. Res. Technol. 2023, 25, 3998–4019. [Google Scholar] [CrossRef]
  64. Lin, Y.; Alengaram, U.J.; Ibrahim, Z. Effect of treated and untreated rice husk ash, palm oil fuel ash, and sugarcane bagasse ash on the mechanical, durability, and microstructure characteristics of blended concrete–A comprehensive review. J. Build. Eng. 2023, 78, 107500. [Google Scholar] [CrossRef]
  65. Zajac, M.; Song, J.; Ullrich, P.; Skocek, J.; Haha, M.B.; Skibsted, J. High early pozzolanic reactivity of alumina-silica gel: A study of the hydration of composite cements with carbonated recycled concrete paste. Cem. Concr. Res. 2024, 175, 107345. [Google Scholar] [CrossRef]
  66. Pradhan, S.S.; Mishra, U.; Biswal, S.K. Mechanical and microstructural study of slag based alkali activated concrete incorporating RHA. Constr. Build. Mater. 2023, 400, 132685. [Google Scholar] [CrossRef]
  67. Zheng, H.; Duan, Y.; Lu, X.; Li, M.; Chen, J.; Jin, Z.; Hou, D.; Hong, S.; Wang, P.; Dong, B. Molecular-scale insights into the leaching of Ca ions and carbonate deposition behavior in CSH under carbonation. J. Build. Eng. 2024, 98, 111358. [Google Scholar] [CrossRef]
  68. Yang, J.; Ding, Q.; Zhang, G.; Hou, D.; Zhao, M.; Cao, J. Effect of sulfate attack on the composition and micro-mechanical properties of CASH gel in cement-slag paste: A combined study of nanoindentation and SEM-EDS. Constr. Build. Mater. 2022, 345, 128275. [Google Scholar] [CrossRef]
  69. Baldovino, J.A.; Nuñez de la Rosa, Y.E.; Namdar, A. Sustainable cement stabilization of plastic clay using ground municipal solid waste: Enhancing soil properties for geotechnical applications. Sustainability 2024, 16, 5195. [Google Scholar] [CrossRef]
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Figure 1. Distribution curve of particle sizes for cement, RHA, and soil.
Figure 1. Distribution curve of particle sizes for cement, RHA, and soil.
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Figure 2. Experimental materials and samples preparation. (a) Materials: soil, RHA, and cement. (b) Mixing the dry materials. (c) Addition of water and moisture content for every blend. (d) Preparation for every layer of soil mixture. (e) Preparation of samples for testing.
Figure 2. Experimental materials and samples preparation. (a) Materials: soil, RHA, and cement. (b) Mixing the dry materials. (c) Addition of water and moisture content for every blend. (d) Preparation for every layer of soil mixture. (e) Preparation of samples for testing.
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Figure 3. Specimen preparation for different layers.
Figure 3. Specimen preparation for different layers.
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Figure 4. Laboratory specimen preparation for (a) top, (b) bottom, and (c) dual layers.
Figure 4. Laboratory specimen preparation for (a) top, (b) bottom, and (c) dual layers.
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Figure 5. Impacts of cement and RHA on compaction properties.
Figure 5. Impacts of cement and RHA on compaction properties.
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Figure 6. Equipment for triaxial compression testing.
Figure 6. Equipment for triaxial compression testing.
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Figure 7. The correlation between the top layer sample combination and axial strain (ε).
Figure 7. The correlation between the top layer sample combination and axial strain (ε).
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Figure 8. The correlation between the bottom layer sample combination and axial strain (ε).
Figure 8. The correlation between the bottom layer sample combination and axial strain (ε).
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Figure 9. The correlation between the dual layer sample combination and axial strain (ε).
Figure 9. The correlation between the dual layer sample combination and axial strain (ε).
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Figure 10. The correlation between the top layer sample combination and deviatoric stress (Δσ).
Figure 10. The correlation between the top layer sample combination and deviatoric stress (Δσ).
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Figure 11. The correlation between the bottom layer sample combination and deviatoric stress (Δσ).
Figure 11. The correlation between the bottom layer sample combination and deviatoric stress (Δσ).
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Figure 12. The correlation between the dual layer sample combination and deviatoric stress (Δσ).
Figure 12. The correlation between the dual layer sample combination and deviatoric stress (Δσ).
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Figure 13. Failure modes of the samples at a restrictive pressure of 150 kPa. (a) Control, (b) top layers, (c) bottom layers, and (d) dual layers.
Figure 13. Failure modes of the samples at a restrictive pressure of 150 kPa. (a) Control, (b) top layers, (c) bottom layers, and (d) dual layers.
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Figure 14. The correlation between the top layer sample combination and internal friction angle.
Figure 14. The correlation between the top layer sample combination and internal friction angle.
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Figure 15. The correlation between the bottom layer sample combination and internal friction angle.
Figure 15. The correlation between the bottom layer sample combination and internal friction angle.
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Figure 16. The correlation between the dual layer sample combination and internal friction angle.
Figure 16. The correlation between the dual layer sample combination and internal friction angle.
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Figure 17. The correlation between the top layer sample combination and cohesion.
Figure 17. The correlation between the top layer sample combination and cohesion.
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Figure 18. The correlation between the bottom layer sample combination and cohesion.
Figure 18. The correlation between the bottom layer sample combination and cohesion.
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Figure 19. The correlation between the dual layer sample combination and cohesion.
Figure 19. The correlation between the dual layer sample combination and cohesion.
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Figure 20. SEM examination at 500× for soil control.
Figure 20. SEM examination at 500× for soil control.
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Figure 21. SEM examination at 500× for the 2%RHA6%C sample.
Figure 21. SEM examination at 500× for the 2%RHA6%C sample.
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Figure 22. SEM examination at 500× for the 4%RHA6%C sample.
Figure 22. SEM examination at 500× for the 4%RHA6%C sample.
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Figure 23. SEM examination at 500× for the 6%RHA6%C sample.
Figure 23. SEM examination at 500× for the 6%RHA6%C sample.
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Figure 24. EDS examination for soil control.
Figure 24. EDS examination for soil control.
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Figure 25. EDS examination for 2%RHA6%C.
Figure 25. EDS examination for 2%RHA6%C.
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Figure 26. EDS examination for 4%RHA6%C.
Figure 26. EDS examination for 4%RHA6%C.
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Figure 27. EDS examination for 6%RHA6%.
Figure 27. EDS examination for 6%RHA6%.
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Table 1. Basic characteristics of soil.
Table 1. Basic characteristics of soil.
DataBasic Characteristics of SoilWorth
Basic Characteristics of Soilsand (75 μm–2 mm), %0.02
silt (5–75 μm), %57.28
clay < 5 μm, %42.7
liquid limit, LL, %48.5
plastic limit, PL, %28.5
plasticity Index, PI, %19.9
maximum dry density, g/cm31.58
AASHTO classificationA-7–6(5)
Table 2. Basic characteristics of RHA.
Table 2. Basic characteristics of RHA.
DataBasic Characteristics of RHAWorth
Basic Characteristics of RHAaverage particle size, mm0.001 to 0.3
loss of ignition, %4–6
specific gravity, g/cm32.12
burning temperature, °C650–700
burning time, hour27
silica (SiO2), %91.10
carbon dioxide (CO2) %4.35
potassium oxide (K2O), %2.40
calcium oxide (CaO), %0.57
iron oxide (Fe2O3), %0.05
alumina (Al2O3), %0.03
others, %1.50
Table 3. Specific mix combinations.
Table 3. Specific mix combinations.
CombinationsSoilRHACement
control10000
soil + 2RHA9802
soil + 4RHA9604
soil + 6RHA9406
soil + 2C9820
soil + 4C9640
soil + 6C9460
soil + 2RHA + 2C9622
soil + 2RHA + 4C9424
soil + 2RHA + 6C9226
soil + 4RHA + 2C9442
soil + 4RHA + 4C9244
soil + 4RHA + 6C9046
soil + 6RHA + 2C9262
soil + 6RHA + 4C9064
soil + 6RHA + 6C8866
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Atef, A.; Hossain, Z. Assessing the Impact of Rice Husk Ash on Soil Strength in Subgrade Layers: A Novel Approach to Sustainable Ground Engineering. Sustainability 2025, 17, 5457. https://doi.org/10.3390/su17125457

AMA Style

Atef A, Hossain Z. Assessing the Impact of Rice Husk Ash on Soil Strength in Subgrade Layers: A Novel Approach to Sustainable Ground Engineering. Sustainability. 2025; 17(12):5457. https://doi.org/10.3390/su17125457

Chicago/Turabian Style

Atef, Abdelmageed, and Zakaria Hossain. 2025. "Assessing the Impact of Rice Husk Ash on Soil Strength in Subgrade Layers: A Novel Approach to Sustainable Ground Engineering" Sustainability 17, no. 12: 5457. https://doi.org/10.3390/su17125457

APA Style

Atef, A., & Hossain, Z. (2025). Assessing the Impact of Rice Husk Ash on Soil Strength in Subgrade Layers: A Novel Approach to Sustainable Ground Engineering. Sustainability, 17(12), 5457. https://doi.org/10.3390/su17125457

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