A Comprehensive Review on Clay Soil Stabilization Using Rice Husk Ash and Lime Sludge

Abstract

Soil stabilization is vital in construction to enhance soil strength and durability. While conventional stabilizers like cement and lime improve soil properties, they contribute to significant carbon emissions. Given their widespread use, exploring eco-friendly alternatives is crucial. This review examines rice husk ash (RHA) and lime sludge (LS) as sustainable substitutes. Previous studies have evaluated their effectiveness in stabilizing clay soil, but a more application-focused approach, along with a detailed cost and sustainability evaluation, is needed. Standard Proctor compaction, California Bearing Ratio, and unconfined compression strength tests were analyzed from the existing literature to determine the optimal ratio of these additives for maximum soil strength. The results were compared to determine the most effective quantities of RHA and LS, either separately or combined, and inferences about their influences on clay soil attributes were drawn. Additionally, comprehensive life cycle assessment (LCA) and cost evaluation were reviewed. Finally, it was concluded that increasing the amounts of RHA and LS and combining them enhanced the strength of clay soil. Moreover, using RHA and LS for soil stabilization proved to be a cost-effective alternative to traditional methods, providing economic and environmental advantages.

1. Introduction

Soil stabilization is a process used to improve the engineering characteristics of the soil to make it more suitable for construction purposes [1]. It involves various techniques and materials applied to the soil to enhance its strength, durability, and load-bearing capacity [2]. The main goal of soil stabilization is to create a stable surface to support structures and prevent undesirable settlement or failure [3].

1.1. Stabilization History

Soil stabilization has been used for centuries, demonstrating its longstanding history. Humans have been using various techniques to enhance the strength and stability of soil for construction purposes and to prevent soil erosion [4,5]. Around 2500 BCE, the Egyptians used soil stabilization techniques to construct buildings and monuments. They mixed mud with straw to strengthen the soil properties and create more durable structures [6]. Ancient Greeks and Romans used lime and pozzolanic materials such as volcanic ash to stabilize the soil and enhance its strength and durability [7]. In ancient China, rammed earth construction was widely used, where soil was compacted in layers with wooden frames, creating durable structures like the Great Wall. They also mixed lime and clay to improve soil strength [8]. In ancient India, stabilization methods that included using lime, ash, and organic materials to strengthen soil were prevalent [9]. During the Middle Ages, lime stabilization became popular in Europe to improve the soil properties [10]. With the advent of the Industrial Revolution in the 18th century, new methods and materials for soil stabilization emerged [11]. Cement was produced on a larger scale and started to be used to stabilize soil, while lime also remained in use [12,13]. In the early 20th century, engineers began experimenting with chemical additives for soil stabilization [14,15]. Materials, including bitumen and asphalt emulsions, were deployed to improve soil properties and make them appropriate for road construction [16,17,18]. In the mid-20th century, the development of geotextiles and geosynthetics provided new opportunities for soil stabilization [19]. These materials, typically made of synthetic fibers, were utilized to reinforce soil [20]. In recent years, soil stabilization techniques have made significant strides in their development. Innovations such as using polymer additives, cementitious binders, and a range of chemical agents have emerged to improve soil properties [21,22,23]. Today, the soil stabilization process is an essential aspect of construction and civil engineering projects [24].

1.2. Stabilization Methods

The specific soil stabilization technique deployed in every project depends on the soil’s characteristics and the project’s requirements. Mechanical, chemical, and geosynthetic stabilization and soil mixing are the most common soil stabilization methods [25,26]. Different types of soils require specific soil stabilization techniques to address their unique properties and challenges. Some common soil stabilization techniques for different soil types are reviewed: clay soils can be stabilized using chemical additives such as lime, cement, or fly ash [27]. These additives react with the clay particles, reducing their plasticity, improving their strength, and increasing their stability [28]. Sandy soils can be stabilized through chemical additives, like cement, and mechanical methods, such as compaction or densification [29]. Compaction increases soil density, while techniques like vibro-compaction or dynamic compaction can enhance load-bearing capacity [30]. Also, geosynthetic materials, such as geotextiles or geogrids, can reinforce sandy soils, improving their stability, preventing erosion, and enhancing slope stability [31]. Like clay soils, silty soils can be stabilized using chemical additives like lime or cement, improving their strength and reducing their compressibility [32]. Soil mixing with additives can also be efficient for silty soils to generate a more stable and homogenous soil mixture [33]. For organic soils, preloading and surcharging can be deployed: preloading with the placement of temporary surcharges can be used to consolidate and improve strength over time. The weight of the surcharge compresses the soil and expels water, reducing settlement potential [34]. Geosynthetic materials can also be used in organic soils to provide reinforcement and prevent excessive deformations [35]. Expansive soils, which are prone to swell and shrink with moisture changes, can be treated with chemical additives like lime, cement, or fly ash to reduce their volume change potential [36]. Proper moisture control techniques, such as drainage systems or moisture barriers, can be implemented to minimize moisture fluctuations in expansive soils [37].

1.3. Conventional Stabilizers

In construction projects, Ordinary Portland Cement (OPC) is primarily utilized as a binding agent in producing concrete, mortar, and grout. It is mixed with aggregates and water to create a paste that hardens and develops strength over time. OPC-based concrete structures are widely utilized for building structural elements [38]. In the soil stabilization process, OPC can be deployed as an additive to improve the engineering properties of the soil. Adding OPC to the soil can enhance mechanical characteristics [39]. The process typically involves thoroughly mixing the OPC with the soil, forming a cementitious matrix that binds the soil particles together. This improves soil cohesion and increases its resistance to erosion [40]. The effectiveness of OPC as a soil stabilizer depends on factors such as the soil type, moisture content, and the desired outcome of the stabilization. It is commonly used for stabilizing cohesive soils such as clay or silt [41].

Calcium silicate hydrate (CSH) is the main product of the hydration of Portland cement [42]. CSH is the primary binder and the main component responsible for the strength and durability of cement-based materials [43]. It forms through a chemical reaction between cementitious materials such as Portland cement and water. CSH is responsible for developing the hardened cement paste’s strength. It reacts with water and the soil particles, improving cohesion, increasing strength, reducing permeability, and improving stability. It helps to bind the soil particles together and create a more stable and compacted soil matrix [44]. Calcium aluminum silicate hydrate (CASH) is another hydrated mineral compound formed in cementitious materials. CASH is a specific type of calcium silicate hydrate that contains additional alumina (Al₂O₃) in its structure [45]. It forms when cement containing alumina-rich materials, such as fly ash or slag, undergoes hydration. It is the primary product of the chemical reaction between cementitious materials, such as Portland cement and water, during hydration [46]. It is responsible for developing the strength and durability of hardened cement paste. CASH is an amorphous material composed of calcium, aluminum, silicon, and oxygen atoms, with water molecules trapped within its structure [47]. It forms a gel-like substance that fills the spaces between cement particles, providing cohesiveness and strength to the cement matrix. While both CSH and CASH contribute to the strength and durability of cementitious materials, CASH can exhibit different properties compared to traditional CSH due to the presence of alumina [48]. The addition of alumina can affect the structure and mechanical properties of the hydrated phases, influencing the overall performance of the cementitious material [49].

Conventional stabilizers, including cement, are widely used in construction but contribute significantly to carbon emissions, raising serious environmental concerns. As sustainability becomes a priority, finding eco-friendly alternatives is essential to reduce the environmental impact of such materials. Consequently, efforts are being made within the cement industry to reduce its carbon footprint and mitigate carbon dioxide (CO₂) emissions by:

(1)

Using alternative fuels, such as biomass or waste-derived fuels, which can help lower CO₂ emissions. Additionally, using alternative raw materials, such as industrial byproducts, can reduce the need for limestone extraction and decrease associated emissions [50,51].

(2)

Implementing energy-efficient technologies and practices can reduce the overall energy consumption and emissions from cement production. This includes optimizing kiln operations, improving heat recovery systems, and employing more efficient grinding processes [52].

(3)

Carbon capture and storage (CCS) technologies aim to capture CO₂ emissions from cement plants, store them underground, or utilize them in other industrial processes. CCS is still in the early stages of deployment in the cement industry but holds promise for significant emissions reduction [53].

(4)

Development efforts are underway to develop and promote low-carbon or carbon-neutral cement alternatives. These include cement made from supplementary cementitious materials like fly ash, blast furnace slag, or pozzolanic materials [54]. By implementing these measures, the cement industry can reduce its carbon footprint and contribute to global efforts to mitigate climate change.

1.4. Sustainable Stabilizers

Byproduct materials, such as rice husk ash (RHA) and lime sludge (LS), can play a significant role in soil stabilization applications. These materials provide several benefits, including environmental sustainability, cost-effectiveness, and improved engineering properties of the stabilized soil [55]. RHA contains high amounts of amorphous silica (SiO₂), contributing to its pozzolanic properties. RHA can be used in soil stabilization as an additive to improve soil properties, including strength, durability, and permeability. It reacts with calcium hydroxide, a byproduct of lime, to form additional calcium silicate hydrates (CSH) gel. This pozzolanic reaction improves cementitious properties, such as increased strength and reduced permeability [56]. Utilizing RHA as a sustainable utilization in soil stabilization provides an environmentally friendly solution by recycling agricultural waste material to reduce waste generation and promote sustainable practices [57].

Lime sludge (LS), also known as lime waste or lime byproduct, possesses cementitious properties that make it suitable for soil stabilization. LS is rich in calcium hydroxide (Ca(OH)₂), which is the main compound responsible for its cementitious properties [58]. Calcium hydroxide is an alkaline compound that reacts with silicates and other compounds to form cementitious products. LS exhibits pozzolanic activity, which refers to its ability to react with calcium hydroxide and water to form additional cementitious compounds. This reaction, known as the pozzolanic reaction, produces calcium silicate hydrates (CSH) gel, which contributes to the strength of the stabilized soil. LS interacts with clay minerals in soils, causing chemical and physical changes to the soil structure. This reaction decreases the plasticity of the clay minerals while enhancing their stability [59]. This reaction enhances the soil characteristics, including strength, workability, and load-bearing capacity. LS can be utilized as a stabilizing agent to enhance soil properties, mainly clayey or cohesive soils. When LS is added to these soils, it reacts with the clay minerals, resulting in improved compaction characteristics, reduced soil plasticity, increased strength, reduced swell-shrink behavior, and enhanced load-bearing capacity [60]. Lime sludge can be self-cement or self-harden when mixed with water. This property allows it to bind and stabilize soil particles, improving cohesion and strength within the stabilized soil or construction material [61]. Utilizing lime sludge in soil stabilization as a waste management solution provides a means of repurposing a waste material that might otherwise pose disposal challenges. It offers an opportunity for sustainable waste management practices and reduces the demand for traditional construction materials. When RHA and LS are used together, LS provides the calcium needed for stabilizing clay, while RHA increases the soil’s strength and durability by contributing additional pozzolanic reactions. This combination reduces soil plasticity, minimizes shrink-swell behavior, and improves compaction, making it suitable for infrastructure projects in areas with clay soils. The process involves blending appropriate amounts of LS and RHA with the soil and allowing time for curing to achieve optimal soil stabilization [62].

The produced waste of rice husk ash and lime sludge can have detrimental environmental effects [63]. Proper management and disposal of RHA and LS byproducts are essential. Inadequate disposal methods, such as open dumping or uncontrolled release, can lead to soil and water pollution, posing ecosystem risks [64]. Both RHA and LS can have environmental implications if not properly managed. RHA may contain heavy metals, such as lead, cadmium, arsenic, and mercury; if not properly managed, these metals can leach into the soil or water bodies, posing a risk to ecosystems and human health. RHA contains amorphous silica, and if fine RHA particles become airborne during handling or mixing, there is potential for inhalation. Prolonged inhalation of silica dust may pose health risks, such as respiratory issues or lung disease, caused by silica exposure. Therefore, wearing protective masks, using dust suppression techniques, and handling RHA in controlled environments can minimize health risks [65]. RHA contains a high percentage of silica, which can be released as airborne particulate matter during handling or processing. Inhalation of silica dust can cause respiratory issues, including lung damage. It may also contain organic contaminants, such as polycyclic aromatic hydrocarbons (PAHs) or volatile organic compounds (VOCs), which can harm the environment and human health [66].

LS can contain heavy metals like chromium, lead, cadmium, and nickel, so improper disposal or release of LS into the environment can contaminate soil and water bodies. Lime in LS is mildly caustic, and direct skin or eye contact can cause irritation or burns. Proper safety measures, such as wearing gloves, goggles, and protective clothing, should be in place for workers handling lime sludge. Additionally, the fine particles of LS can become airborne during handling, potentially causing respiratory irritation if inhaled. However, proper dust control and protective equipment can manage these risks effectively [67]. Lime sludge has high alkalinity due to its calcium hydroxide content. Discharging untreated lime sludge into water bodies can increase pH levels, affecting aquatic organisms and disrupting the ecosystem’s natural balance [68]. LS often contains elevated levels of nutrients, particularly phosphorus. When released into water bodies, it can contribute to eutrophication, leading to excessive algae growth, oxygen depletion, and harm to aquatic life [69].

Proper management practices should be followed to mitigate the potential environmental impact of RHA and LS. This includes appropriate storage, transportation, and disposal methods to prevent the release of impurities into the environment. Additionally, environmental regulations should be followed, and treatment techniques should be employed to minimize the impact on ecosystems and human health.

Past studies have explored the effectiveness of clay soil stabilization using eco-friendly alternative materials, including RHA and LS [70]. However, this review bridges research gaps by comprehensively analyzing laboratory test results (UCS, CBR, Mr, and Proctor tests) on RHA and LS as soil stabilizers to promote the reuse of agricultural and industrial byproducts. By identifying optimal proportions for specific applications like structural foundations and roadways, we provide engineers and researchers with precise data to enhance stabilization solutions tailored to project needs. Additionally, it provides an in-depth evaluation of sustainability and cost factors, highlighting its unique contribution to promoting sustainable practices in civil engineering to support the shift toward greener construction. By evaluating lifecycle factors, such as energy use and waste reduction, along with cost savings from locally sourced byproducts, valuable insights for policymakers and stakeholders are provided. This study provides a strong foundation for future studies by offering precise guidelines and test result analyses. It encourages further exploration of RHA and LS in combination with other additives and supports regional case studies that address local soil and waste availability, paving the way for more tailored and effective soil stabilization strategies.

To conclude, this review aims to assess the efficacy of clay soil stabilization using rice husk ash (RHA) and lime sludge (LS) to determine the optimum quantity of RHA and LS to be used based on comprehensive geotechnical tests, as well as research on cost-effectiveness and sustainability.

2. Materials

Disposing of waste materials, such as rice husk ash and lime sludge, resulting from industrial processes poses environmental concerns when deposited in landfills. However, these materials can be repurposed and utilized beneficially, particularly in geotechnical and other engineering projects [71,72]. This review focuses on applying RHA and LS in clay soil stabilization, exploring their properties, and investigating their chemical composition, as documented by various researchers who worked in the field of using green alternative materials for clay stabilization. Results of laboratory tests, including standard Proctor compaction, unconfined compression strength (UCS), and California Bearing Ratio (CBR) related to soil samples with various mixed proportions of admixtures among different papers, are discussed.

2.1. Rice Husk Ash (RHA)

Rice husk ash is a byproduct obtained from the combustion or incineration of rice husks [73]. Rice husks are the protective coverings of rice grains and are generated during the rice milling process. It constitutes around 20–23% of the total weight of the rice grain [74]. Rice husks are agricultural trash that accounts for nearly one-fifth of the world’s yearly gross rice production of 545 million metric tons [75]. Due to the high calorific power of rice husk (approximately 16,720 kJ/kg), it is increasingly used as a fuel source for heat generation in rice drying [76]. Rice husk ash is mainly produced by burning rice husks at around 450–700 °C for 1–3 h under controlled conditions [77]. During high-temperature combustion, the organic components of rice husk, including cellulose, hemicellulose, and lignin, are more effectively burned, leaving behind a higher percentage of silica-rich ash [78]. In contrast, during lower-temperature combustion, the combustion may be less efficient, and a more significant fraction of the organic matter may remain in the RHA, leading to higher carbon content [79].

Based on the requirements, the collected RHA may undergo additional processing steps, including grinding or sieving, to achieve the desired particle size distribution and remove impurities. The resulting RHA is a fine, light powder with high silica content (60 to 90%) [80]. RHA primarily consists of amorphous silica (SiO₂) and is chemically reactive. It contains a high percentage of siliceous elements, indicating its potential for exhibiting pozzolanic characteristics [81]. RHA does not possess inherent cementitious qualities; therefore, it is not used alone for soil stabilization and is commonly utilized as supplementary material in soil stabilization processes [82]. The addition of RHA can significantly influence macro aggregate production, while it has been observed that the addition of RHA decreases the fraction of micro aggregates smaller than 0.25 mm in size. The aggregation of clay particles with RHA can create larger pores, known as macropores, which help to drain the soil better, while it might not significantly impact the smallest-sized pores, known as crytopores [83]. To sum up, rice husk ash, as agricultural waste, shows remarkable potential for stabilizing clay soils.

To determine the elemental composition of rice husk ash, the results of the various chemical analyses of RHA performed by X-ray fluorescence (XRF) tests are given in

Table 1. Chemical composition of RHA obtained by XRF (in percent).

Source	CaO	SiO2	Na2O	MgO	Al2O3	Fe2O3	K2O	TiO2	LOI
[84]	0.578	96.235	0.054	0.269	0.281	1.366	0.454	-	-
[85]	0.82	80.82	0.96	-	0.25	0.38	1.25	-	12.70
[86]	0.37	87.49	0.01	0.01	0.05	0.04	0.8	0.01	11.21
[87]	0.62	89.69	1.02	0.57	1.01	0.23	0.81	4.06	-
[88]	0.82	80.02	0.01	0.74	1.81	0.79	1.01	0.43	10.62
Mean	0.6416	86.8508	0.4108	0.39725	0.6804	0.5612	0.8648	2.245	11.51
SD	0.1675	5.96	0.474	0.279	0.861	0.387	0.265	2.11	1.07

LOI = Loss on Ignition.

. Furthermore, the mean and standard deviation (SD) were calculated to determine the average chemical composition and measure the variability of the data for RHA.

Extracted results indicate that Silica (SiO₂) is the major component of RHA, ranging from 80% to 96%. Silica in RHA is highly reactive and can contribute to pozzolanic reactions in cementitious materials. Potassium oxide (K₂O) and sodium oxide (Na₂O) are found in small amounts, less than 2%. Calcium oxide (CaO) content can vary, but it has been found in small quantities, less than 1%, contributing to the overall pozzolanic activity. Iron oxide (Fe₂O₃) content can vary depending on the rice husk source and burning conditions, below 2%. These percentages can vary based on the source of the rice husk, the burning temperature, and the combustion conditions. XRF analysis quantitatively measures these elements, helping to characterize RHA for its potential use in various applications, especially in the construction industry [89,90].

2.2. Lime Sludge (LS)

Lime sludge is a byproduct produced in various industrial processes, mainly in the paper industry and water treatment plants. In the pulp and paper industry, a significant source of lime sludge, approximately 100–200 kg of LS, can be generated per ton of pulp produced. Global pulp production is around 185 million tons annually, which could imply around 18.5 to 37 million tons of lime sludge per year from this sector alone [91]. Lime sludge can vary in composition depending on the specific industrial process and the impurities in the treated lime. However, it primarily contains calcium carbonate (CaCO₃) [92]. LS can have beneficial applications as a waste material, as adopted and illustrated in Figure 1, particularly in soil stabilization and construction materials [93].

Lime sludge can be deployed as a stabilizing agent to amend the engineering properties of soil or as a raw material in the production of cementitious materials [94]. However, proper waste management of lime sludge is necessary to prevent environmental pollution. Lime sludge can be deployed to change the engineering properties of fine-grained soils and the fine-grained fractions of granular soils [95]. It can reduce the plasticity of fine-grained soils, such as clay, resulting in reduced potential for shrinkage and swelling and improved soil workability, strength, drainage, and permeability, making the soil more stable [96]. The lime sludge treatment effectively stabilizes plastic clays and other fine-grained soils with high water storage capacity. Soils with higher plasticity or water content generally require a higher lime sludge dosage for effective stabilization [97]. However, as a general guideline, a lime sludge content of 3–10 percent by dry weight is sufficient to achieve adequate stabilization [98]. The results of the chemical analysis of LS using the X-ray fluorescence (XRF) test are given in

Table 2. Chemical composition of LS obtained by XRF (in percent).

Source	CaO	SiO2	Al2O3	Fe2O3	MgO	So3	Na2O	LOI
[99]	92.25	0.06	0.56	0.45	0.03	-	0.26	6.39
[100]	95.03	0	0.13	0.08	0.25	0.02	0.05	4.33
[101]	70.85	1.19	0.69	0.12	0.53	-	0.25	26.1
[102]	40.74	5.78	0.18	0.15	0.41	0.28	2.03	-
Mean	74.72	1.76	0.39	0.20	0.31	0.15	0.65	12.27
SD	25.10	2.74	0.28	0.17	0.22	0.18	0.93	12.02

LOI = Loss on Ignition.

.

The results obtained from different studies demonstrate that calcium oxide (CaO) is a major component, reflecting the high calcium content derived from the lime used in treatment processes. The concentration can vary widely but often ranges from 40% to 95% depending on how much lime was used and how completely it reacted. Silica (SiO₂) is present in varying amounts, from 0% to 5%. Iron oxide (Fe₂O₃) is present in smaller quantities, less than 1%, and can come from various sources, including the treatment process or impurities in the lime. Magnesium oxide (MgO) is found in smaller amounts, less than 1%. It can come from the mineral composition of the lime or other additives used in the treatment process. Sodium oxide (Na₂O) is in smaller amounts, less than 3%. The exact composition of lime sludge can vary based on factors such as the source of the water being treated, the type of lime used, and the specifics of the treatment process. XRF provides a detailed elemental analysis that helps understand and manage lime sludge for various applications or disposal methods [103,104].

3. Laboratory Tests

The success or failure of enhancing the geotechnical properties of soils using different admixtures is typically determined through experimental or laboratory tests. These tests provide a means to evaluate the effectiveness of the admixtures in improving soil characteristics. The most widely used tests to evaluate the effectiveness of a soil improvement procedure include the standard Proctor compaction test, the unconfined compressive strength (UCS) test, and the California Bearing Ratio (CBR) test. In the present review, these tests are used to assess the performance of RHA and LS in clay soil improvement.

3.1. Standard Proctor Compaction Test

Assessing the compaction characteristics of soils for stabilization purposes is commonly used in geotechnical engineering. Different studies were reviewed to determine the Optimum Moisture Content (OMC) and Maximum Dry Density (MDD) of stabilized soil samples using the standard Proctor compaction test [98,105,106,107,108,109,110,111,112]. The results from the literature show that the OMC of soil samples increases and the MDD of soil samples decreases as the LS and RHA content increases in soil stabilization processes. During the pozzolanic reaction, the hydration of LS and RHA particles occurs, forming cementitious compounds such as CSH and CASH gels. This reaction consumes water, necessitating a higher water content to maintain an appropriate moisture level for the reaction to take place, leading to an increase in the OMC of the soil. Simultaneously, adding LS and RHA also influences the soil particle arrangement. The cementitious compounds formed help to fill void spaces and enhance the development of a more compacted soil structure, leading to an increase in the MDD of the soil. The analyzed results show that increasing the LS percentage in the soil sample from 5% to 10% resulted in a sudden decrease in the MDD value. Deploying RHA alone as an admixture in clay soil samples results in an increase in the OMC and a decrease in the MDD due to the lower density of rice husk ash compared to the expansive soil. Similarly, incorporating LS as a binding material with RHA and the soil sample also leads to an increase in the OMC and a decrease in the MDD value. In this context, the decrease in the MDD value can be attributed to the lower density of lime sludge compared to the expansive soil. To clarify the results obtained from the literature, the variation in OMC and MDD based on different admixtures is shown in Figure 2, Figure 3, Figure 4, Figure 5, Figure 6 and Figure 7. Methodological differences can impact the findings when comparing different results across different studies on clay soil stabilization using green alternative materials. These differences, such as clay mineralogy, soil gradation, soil preparation, curing duration, and environment, may influence the measured variations in results between studies. However, for the sake of simplicity, when comparing the results, methodological differences are not considered.

The results of the analysis of the drawn graphs from the standard Proctor compaction tests are provided below. Figure 2 demonstrates that with the increase in the percentage of RHA content added to the clay soil samples, the OMC of the soil increases. This increase in OMC with higher RHA content is due to RHA’s porous and lightweight nature, which absorbs more water and requires higher moisture for compaction. Moreover, adding 10–20% of RHA content significantly increases the OMC of the soil samples. Figure 3 shows that by increasing the amount of LS content, the OMC of the soil increases due to the water-absorbing capacity of the admixtures. Additionally, it is evident that adding 10–15% of LS content escalates the OMC value in soil samples, and by adding more than 15% of LS content, a sudden drop in OMC value is observed. This is because, beyond a certain threshold of LS content (more than 15%), the soil structure becomes overly rigid, reducing its ability to retain moisture, thereby leading to a lower OMC since less water is required for compaction. Figure 4 reveals that incorporating RHA and LS into the soil samples enhances the OMC. It is observed that OMC increases progressively with the higher amount of admixtures added due to the high water absorption capacity of both materials, which increases the overall water demand for proper compaction. Additionally, the pozzolanic reaction between RHA, LS, and clay soil minerals consumes water, further increasing OMC. Figure 5, Figure 6 and Figure 7 demonstrate a decrease in the MDD of the soil samples as the percentage of RHA and LS content increases. This reduction in MDD can be attributed to the lower specific gravity of the admixtures, which is less than the natural soil [113]. Additionally, reduced MDD indicates less strength of the matrix, which is compensated by the binding effect of the RHA and/or LS, increasing cohesion. The MDD decreases when RHA and LS are incorporated because both materials increase the void spaces within the soil matrix, making it less compact and requiring more water for lubrication, further reducing dry density. The pozzolanic reaction between RHA, LS, and clay soil also leads to the formation of cementitious compounds, which create a more open, flocculated soil structure, contributing to the reduction in MDD.

3.2. Unconfined Compressive Strength (UCS)

The unconfined compressive strength test is a laboratory test to determine improved soil strength and load-bearing capacity. The UCS test is commonly performed on cylindrical soil specimens under axial compression without lateral confinement. Different references were reviewed to present the relationships between the UCS values of different mixtures and varying amounts of RHA and LS content added to the soil samples [111,114,115,116,117,118]. The review of the extracted results is shown in Figure 8, Figure 9 and Figure 10.

The results of the analysis of the drawn graphs from the UCS tests are provided below. Figure 8 shows the variation of the UCS value with respect to the RHA content. It demonstrates that the UCS value of the soil samples shows an increase with the addition of RHA up to 10%. However, beyond this threshold, it is seen that the UCS of the clay soil starts to decline due to the reduction of cohesion and internal friction of the soil, consequently leading to a decrease in the soil’s ability to resist deformation and load. When the amount of RHA exceeds the optimal level, there might be too much binder relative to the amount of soil available for the reaction. The extra RHA particles can start to act as a filler rather than a reactive component, leading to a reduction in the overall strength of the mixture. Additionally, excess RHA can increase the porosity of the mixture due to its lightweight and relatively high specific surface area. Higher porosity can reduce the density and, consequently, the strength of the soil. Figure 9 shows the variation of the UCS value with respect to the LS content. It indicates that an increase in the LS content increases the UCS value of the soil samples due to better bonding of soil and lime. Adding LS content up to 4% can increase the UCS value. Figure 10 shows the variation of the UCS value with respect to the combined RHA and LS content. The combined use of RHA and LS can have a significant influence, leading to a notable increase in the UCS value of the soil. However, it is indicated that above 6% RHA content, the strength decreases independently of LS content, which seems to be an upper threshold for RHA. Also, it can be seen that the higher the LS, the higher the strength, which conforms with the Figure 9 analysis. Figure 8 shows an optimized value of RHA of 10%, but when mixed with LS, this threshold is clearly lower.

3.3. California Bearing Ratio (CBR)

California Bearing Ratio is a laboratory test to evaluate the strength and load-bearing capacity of soils commonly used in geotechnical engineering, particularly in road construction. The compaction process uses a specific compaction effort to achieve the desired density and moisture content representative of the in-situ conditions. Proper compaction at the OMC ensures that the soil is compacted to its optimum condition for bearing capacity and strength, which is crucial for the design and construction of roads, embankments, and other civil engineering structures. The curing period, another significant governing parameter in the CBR test, directly impacts the CBR value obtained. CBR values increase with increasing curing periods, resulting in more accurate representations of the soil performance under the applied loads [119]. The CBR values, as shown in Figure 11, were determined by analyzing the results obtained from different references [109,120,121].

The results of the analysis of the drawn graph from the CBR tests are provided below. Figure 11 illustrates the results of various studies related to the impact of RHA plus LS on the CBR parameters of soil samples. The results indicate that 4% of LS plus RHA content increases the CBR value compared to 8%. It is clear that there is a decrease in the CBR value after increasing the percentage of RHA content above 15%. However, by incorporating both RHA and LS content, the CBR value was increased overall. Higher amounts of LS aim to improve soil properties by increasing CBR, enhancing soil strength and load-bearing capacity, and reducing plasticity, but excessive use of LS can potentially lead to adverse effects if not properly managed.

The UCS and CBR are both indicators of the strength and load-bearing capacity of soils, but they measure different properties and are deployed for various purposes. CBR is usually more focused on pavement or roadway purposes, while UCS is generally more precise and relevant for clay soils, giving a more fundamental measure of compressive strength for structural purposes such as foundations or slope stability. Hence, the choice depends on the type of project.

Furthermore, the resilient modulus (M_r) is a critical parameter in road and pavement applications, as it directly reflects the elastic behavior of soil under repeated loading, which is more representative of the stresses experienced in actual pavement conditions. The CBR test indicates the soil’s strength under a static load, making it less suitable compared to Mr for evaluating materials for dynamic conditions like roads. Summarized results from various studies are provided in

Table 3. Resilient modulus (M_r) parameter for non-stabilized and stabilized clay soil.

Parameters	Mr Values	Notes	Source
Unstabilized clay	20–60 MPa	Baseline values before stabilization	[122]
Stabilized clay (RHA + Lime)	100–200 MPa (7 days curing)	Significant improvement due to pozzolanic reactions; values depend on dosage and curing time	[123]
	150–300 MPa (28 days curing)	Long-term curing increases Mr due to continued reaction	[124]
	>300 MPa (optimized mix, extended curing)	Observed in some studies with well-optimized RHA-lime mixes	[125]

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The resilient modulus of stabilized clay soils is significantly higher than non-stabilized clay due to the improved soil structure and enhanced bonding mechanisms. In non-stabilized clay, M_r values are typically low because of the soil’s high plasticity, poor particle interlocking, and susceptibility to deformation under repeated loads. Stabilization using materials like RHA and LS initiates pozzolanic reactions, forming CSH, which binds soil particles together and reduces plasticity, leading to a denser, more cohesive matrix with greater resistance to deformation under cyclic loads. Studies have shown that stabilized clay can achieve M_r values two to five times higher than non-stabilized clay, with values increasing further over time as the curing process continues. This improvement makes stabilized clay more suitable for subgrade applications in road construction.

4. Cost and Sustainability Evaluation

4.1. Cost Evaluation

Several researchers have conducted cost evaluations comparing the use of RHA and LS with traditional stabilizers, like lime and OPC, for clay soil stabilization worldwide. Conducting an extensive cost analysis for the stabilization process involves several factors, including material, transportation, application, performance, and environmental and regulatory costs. RHA and LS are often byproduct materials and can be sourced at low cost in some areas, but lime and OPC are commercially produced, with generally higher raw materials and production costs. RHA and LS may require less transport cost if sourced locally from agricultural or industrial facilities, but lime and OPC often require transport from production plants, potentially increasing costs, especially for remote sites. The application process for all stabilizers is similar, involving mixing, compaction, and curing. Labor costs are unlikely to vary significantly. RHA and LS may require higher quantities compared to lime and OPC to achieve similar performance, impacting total costs. Additionally, waste material use (RHA and LS) can reduce environmental costs and long-term maintenance. Utilizing RHA and LS supports waste recycling, reduces disposal costs, and potentially qualifies for subsidies or incentives, while OPC production has a high carbon footprint, potentially incurring environmental compliance costs. Summarized cost comparison results from various studies are provided in

Table 4. Cost evaluation for clay stabilization using RHA and LS versus traditional stabilizers.

Materials	Cost Savings	Findings	Source
RHA + Natural Lime (NL)	17% reduction in earthwork costs, 39% savings in subgrade construction costs	RHA with 2% NL effectively stabilized clay soils, offering lower costs than traditional techniques	[126]
RHA + Lime	Reduced cost compared to OPC or lime stabilization	RHA as an agricultural waste provided economic and sustainable stabilization, significantly lowering material expenses	[127]
RHA + Lime	Estimated 25% cost reduction compared to OPC	Higher performance with RHA–lime mixture, utilizing industrial waste at lower costs	[72]
RHA + Lime	Approximately 30% cheaper than using OPC	Optimal mix of 8% RHA and 5% lime reduced costs due to local availability of RHA and less lime requirement	[128]
RHA + LS	Significantly lower material costs compared to OPC or LS stabilization	RHA and LS provided sustainable alternatives with good performance characteristics at lower cost thresholds	[129]

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These studies demonstrate that using RHA and LS for clay soil stabilization can be a cost-effective alternative to traditional methods, offering both economic and environmental benefits.

4.2. Sustainability Evaluation

Life cycle assessment (LCA) is a comprehensive method to evaluate the environmental impacts of all stages of a product’s life and plays a vital role in assessing sustainability. Conducting a comprehensive LCA for stabilizing clay soils using RHA and LS involves evaluating environmental impacts across various stages, including material extraction, processing, transportation, application, and end-of-life scenarios. Existing studies provide insights into their environmental benefits compared to traditional stabilizers like OPC and lime. Summarized results from various studies are provided in

Table 5. Life cycle assessment for clay stabilization using RHA and LS.

LCA Aspect	RHA + LS Stabilization	Traditional Stabilizers (Lime/OPC)	Findings	Source
Carbon Emissions	~30–50% reduction compared to OPC stabilization	High emissions from production (~800–1000 kg CO2/ton for OPC)	RHA and LS are byproducts with negligible emissions during production, compared to OPC and lime, which involve energy-intensive processes like calcination	[72]
Energy Consumption	~60% lower than lime or OPC	High energy (~4–5 GJ/ton for OPC)	RHA and LS use energy only during minimal processing or transportation, compared to lime and OPC, which require extensive energy for raw material extraction and processing	[130]
Waste Utilization	100% reuse of agricultural (RHA) and industrial (LS) byproducts	Zero waste reuse	RHA and LS reduce waste disposal needs and landfill usage, making the stabilization process sustainable	[126]
Transportation Emissions	Low (local availability in rice/agriculture regions)	Moderate to high (dependent on production site distance)	RHA and LS sourced locally minimize transportation emissions, while OPC and lime often require transport over long distances, adding to the carbon footprint	[129]
Landfill Reduction	Significant reduction in waste sent to landfills	No landfill impact	By utilizing RHA and LS, waste that would typically go to landfills is repurposed, reducing the environmental impact of disposal and the need for landfill space	[131]
Cost of Stabilization	~25–50% lower than lime or OPC	High (cost of production and transport)	RHA and LS are low-cost or free byproducts, whereas lime and OPC involve substantial production costs that increase overall project expenses	[128]

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These studies highlight the advantages of using RHA and LS for clay soil stabilization, providing specific data on material quantities, transportation distances, energy usage, and emissions factors.

To sum up the cost and sustainability evaluation, the adoption of sustainable materials such as rice husk ash and lime sludge, despite offering substantial environmental and economic benefits, is often constrained not by their quality or performance but by shortcomings in legislative frameworks at both national and EU levels. These materials are frequently classified as waste rather than valuable raw materials, subjecting them to restrictive regulations that limit their potential for reuse and valorization. Additionally, the absence of specific standards tailored to their properties and applications creates uncertainty, making it difficult for industries to incorporate them into industrial applications confidently. Lengthy and inconsistent regulatory approval processes further compound these challenges, creating barriers to innovation and increasing adoption costs. To address these issues, transparent and harmonized end-of-waste criteria must be established, allowing rice husk ash and lime sludge to be officially recognized as resources rather than waste. Developing EU-wide standards and certification processes tailored to these materials would ensure consistent quality and streamline their integration into industrial applications. Furthermore, legislative incentives, such as subsidies, tax benefits, and grants, could encourage industries to adopt these sustainable materials, driving the transition toward a circular economy. By resolving these legislative challenges, policymakers can unlock the full potential of rice husk ash and lime sludge, reducing waste, promoting resource efficiency, and advancing sustainability goals across multiple sectors.

5. Results and Discussion

Rice husk ash and lime sludge offer unique advantages and disadvantages in clay soil stabilization. RHA, rich in amorphous silica, provides excellent pozzolanic properties, enhancing soil properties while promoting environmental sustainability by recycling agricultural waste. Similarly, LS, high in calcium hydroxide, improves soil properties and offers a cost-effective stabilization solution by repurposing industrial byproducts. However, both materials have drawbacks. RHA can release airborne silica particles, posing health risks if improperly handled, and LS may contain heavy metals or cause pH imbalances in water bodies if not managed responsibly.

This section presents the findings from the investigation into the stabilization of clay soil using RHA and LS. The results of standard Proctor compaction tests, unconfined compressive strength (UCS) tests, and California Bearing Ratio (CBR) tests are analyzed to evaluate the effects of varying proportions of RHA and LS on the clay soil characteristics. The discussion highlights the optimal proportions of RHA and LS for effective stabilization and their combined impact on improving clay soil performance.

The standard Proctor compaction test results reveal significant impacts of adding RHA and LS on the OMC and MDD of clay soils. The addition of RHA increases the OMC of the clay soil, with a notable rise when 10–20% RHA is added due to the water-absorbing capacity of RHA particles. Similarly, including LS results in increased OMC values, with optimal performance observed up to 15% LS. Beyond this threshold, the OMC declines as the clay soil matrix becomes overly rigid, reducing water retention capacity. The combined addition of RHA and LS further enhances the OMC progressively with increasing amounts of admixtures. An inverse relationship is observed between MDD and the content of RHA and LS. The reduction in MDD is linked to the lower specific gravity of the admixtures compared to natural soil, resulting in less dense compacted samples. Although the reduction in MDD could indicate decreased matrix strength, this is compensated by improved cohesion and binding effects due to RHA and LS.

The UCS tests show varied results based on the type and proportion of admixtures. Adding up to 4% LS enhances the UCS value, reflecting improved bonding between clay soil particles and lime. UCS increases with up to 10% RHA but decreases beyond this threshold. This decline is linked to a reduction in cohesion and internal friction, excess porosity, and unreactive RHA particles acting as fillers. The combination of RHA and LS has a synergistic effect, significantly improving UCS. However, the maximum UCS value is achieved with up to 6% RHA, indicating a lower optimal threshold when combined with LS. Higher LS proportions correlate with increased UCS, showcasing the stabilizing and strengthening potential of LS.

The CBR test results highlight the influence of RHA and LS on the load-bearing capacity of clay soil samples. The addition of 4% LS and RHA leads to increased CBR values, while excessive RHA (>15%) results in decreased CBR, attributed to increased porosity and reduced cohesive strength. Combining RHA and LS significantly enhances the CBR values overall, improving clay soil load-bearing capacity and reducing plasticity. While both RHA and LS improve CBR values, excessive usage (above optimal thresholds) can have adverse effects, emphasizing the need for precise mix proportions.

The extracted results underscore the effectiveness of RHA and LS as stabilizing agents for clay soils. While each admixture has its independent impact, their combined use yields enhanced mechanical properties, including higher UCS and CBR values. However, the performance depends on maintaining optimal proportions to avoid adverse effects on clay soil strength.

Regarding cost evaluation, RHA and LS are consistently more cost-effective alternatives to OPC and lime, as they are readily available byproducts from industrial and agricultural processes. Depending on the method and application, cost savings range from 17% to 39%. Moreover, their use contributes to environmental sustainability, enhancing their overall value in various applications.

Regarding LCA analysis, RHA and LS offer notable environmental benefits by significantly reducing carbon emissions, energy consumption, and waste, aligning with sustainable practices. Their cost efficiency, driven by reduced expenses and easier environmental compliance, positions them as economically viable alternatives to OPC and lime. Additionally, their local availability, particularly in agricultural and industrial regions, enhances their practicality and supports regional development.

6. Conclusions

This comprehensive review aimed to assess the efficacy of clay soil stabilization using green materials, including RHA and LS, to determine the optimum quantity of RHA and LS. Adopting alternative green materials for clay stabilization offers a win–win solution, benefiting both the environment and the construction industry. The construction sector plays a crucial role in promoting sustainability by reducing carbon emissions, preserving natural resources, and decreasing reliance on chemical stabilizers. Based on the analyzed studies, an increase in the content of RHA and LS led to an increase in the OMC of clay soil and a corresponding reduction in its MDD. Adding RHA up to 10% improved the UCS and CBR values, after which a decline was observed. Similarly, using LS content of up to 8% for structural purposes and up to 4% for roadway and pavement purposes enhanced the UCS and CBR values. Moreover, the combined use of RHA and LS resulted in a significant increase in UCS and CBR values. For optimal stabilization, a combination of 6% RHA and 7% LS is recommended for structural purposes, while 10% RHA and 4% LS are ideal for roadway and pavement applications. Finally, future studies can focus on conducting a comprehensive review of the mechanical characteristics of various soil types treated with RHA and LS, including an in-depth analysis of stabilization effectiveness, variations in strength parameters, and the long-term durability of treated soils under diverse environmental conditions. Such research can provide valuable insights into optimizing application methods, understanding soil-behavior interactions, and expanding the practical applications of RHA and LS in sustainable geotechnical engineering.