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Article

Utilization of Nano Silica and Plantain Leaf Ash for Improving Strength Properties of Expansive Soil

by
Fahad Alshawmar
Department of Civil Engineering, College of Engineering, Qassim University, Buraydah 51452, Saudi Arabia
Sustainability 2024, 16(5), 2157; https://doi.org/10.3390/su16052157
Submission received: 4 January 2024 / Revised: 21 February 2024 / Accepted: 3 March 2024 / Published: 5 March 2024
(This article belongs to the Special Issue Sustainability in Construction Materials)
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Abstract

:
This study investigates the effect of nanosilica and plantain leaf ash on the sustainable stabilization of expansive soil. This study conducted various strength tests, including Unconfined Compressive Strength (UCS), direct shear, and California Bearing Ratio (CBR) tests, to analyze the enhancement of mechanical properties by adding nano silica and plantain leaf ash. Scanning Electron Microscopy (SEM) analysis was conducted to investigate the interaction mechanism between the soil and the combination of nano silica and plantain leaf ash. Three different combinations of plantain leaf ash were utilized, ranging from 5% to 15%, alongside nano silica ranging from 0.4% to 1.2%. The reinforced soil’s compressive strength, shear strength, and bearing capacity were assessed through UCS, direct shear, and CBR tests. The results demonstrated significant improvements in compressive strength, up to 4.6 times, and enhancements in cohesion and frictional angle, up to 3.3 and 1.6 times, respectively, at 28 days. Moreover, the addition of nano silica and plantain leaf ash led to increased bearing capacity and reduced soil swelling potential, contributing to the overall stability and strength improvement in expansive soil. The SEM test results demonstrate that maximum bonding and compaction occur when 1.2% nano silica and 15% plantain leaf ash are added to the soil.

1. Introduction

Expansive soils are naturally unstable and susceptible to volume changes affected by moisture content, which makes them major challenges in engineering and construction [1]. Cement and other non-renewable materials are frequently used in traditional techniques for stabilizing these soils, which reduces resources and increases environmental degradation [2]. As such, it is critical to find sustainable options to improve the strength characteristics of expansive soil.
Investigating environmentally friendly additives has recently gained popularity as a potential way to address soil stabilization while reducing environmental effects. Because of its ability to increase the strength and durability of soil, nano silica, which has been identified by its minute particle size and outstanding pozzolanic properties, has come into focus as a possible solution [3]. Plantain leaf ash, which is a byproduct of agriculture, has demonstrated potential as an additional stabilizing agent because of its high calcium concentration and natural pozzolanic reactivity [4]. In order to minimize environmental effects and promote environmentally friendly engineering techniques, sustainability is not just about improving soil qualities but also about decreasing reliance on non-renewable resources.
In recent years, scientists and engineers have been looking into new, environmentally friendly ways to reduce the adverse effects of expanding soils. Ijaz et al. [5] utilized lignosulphonate and hydrated lime for the sustainable treatment of expansive soil, and to improve the geotechnical properties of this soil. Kumar et al.’s study [6] involves the utilization of lime and fly ash in proportions of up to 10% and 20%, along with the addition of polyethylene terephthalate (PET) fibers up to 2%, for the sustainable soil stabilization of expansive soil. The results demonstrate an increase in both the Unconfined Compressive Strength (UCS) and tensile strength of the soil due to the incorporation of these additives. In another study, leaf ash was employed for the soil stabilization of expansive soil. The results indicated a noticeable increase in both the UCS and California Bearing Ratio (CBR) values of the soil upon the addition of leaf ash [7,8].
There has been an increase in interest recently in creating sustainable and economical methods for stabilizing expansive soils [9,10]. The use of nanotechnology, namely the integration of nano silica particles, in combination with waste products like ash, is one viable route in this respect. Nano silica, with its special properties, may be useful in modifying the mechanical and hydraulic properties of expansive soils [11,12], whereas waste products like ash, which are widely available from various industrial processes, offer a chance to lessen environmental waste and improve soil.
Mostafa et al. [13] utilized up to 15% silica fume and up to 3% nano silica in combination with lime for the stabilization of expansive soil. The results demonstrated that increasing the amounts of silica fume and nano silica, in conjunction with lime, led to a remarkable improvement in the California Bearing Ratio (CBR) value, which increased threefold, and the Unconfined Compressive Strength (UCS) value, which doubled. Buazar [14] utilized green nano silica and observed that, by using 1.5% nano silica, there was a 5.8 times increase in the CBR value of expansive soil. Another study demonstrated that, by utilizing nano silica in conjunction with other binding materials like cement, there is an increase in the UCS value of the expansive soil [15]. Al-Gharbawi et al.’s study [16] demonstrated that incorporating lime, cement, and nano silica, at levels of up to 9%, in expansive soil resulted in significant improvements. The results indicated a reduction in swelling pressure by up to 76% and an increase in bearing capacity by up to 82%. Another research study utilized both nano and crystalline silica to observe a reduction in the swelling and shrinkage of expansive soil [17]. Alshami et al. [18] utilized micro and nano silica as additives for expansive clay, with concentrations of up to 7%. The study aimed to analyze the increase in the Unconfined Compressive Strength (UCS) value resulting from the addition of these additives to the soil. However, Eissa et al. [19] employed cement and slag at concentrations of up to 20%, along with up to 2.4% nano silica, for enhancing expansive soil. The results demonstrated improvements in both performance and cost-effectiveness. Some recent studies utilized agricultural waste and biomass in construction [20,21]. For instance, the study conducted by Rahgozar et al. [22] utilized up to 8% rice husk ash to improve the strength properties of soil. Similarly, Sharma and Sharma [23] conducted a study on lime-stabilized soil using the same approach. Furthermore, Gidebo et al.’s [24] study utilized a variety of agricultural waste materials, such as rice, wheat, sugarcane, and bamboo ash, for the stabilization of expansive soil.
Previous research has primarily focused on the impact of nano silica on expansive soil. However, there is a noticeable gap in the literature when it comes to analyzing the combined effects of leaf ash and nano silica on soil stabilization. This study aims to bridge this gap by utilizing nano silica and plantain leaf ash, with a specific focus on achieving sustainable soil improvement in fat clay soil through the utilization of waste materials.

2. Materials and Methods

2.1. Materials

2.1.1. Expansive Soil

In this study, expansive soil was obtained from an urban area characterized by high plasticity. The physical properties, mineralogical, and chemical composition of the expansive soil used are listed in Table 1 and Table 2, respectively. The testing method described in ASTM D4318 [25] was used to determine the Atterberg limits (liquid and plastic limits) of the expansive soil used. The results showed that the liquid limit and plastic limit for the expansive soil used were 74% and 39%, respectively. According to the Unified Soil Classification System (USCS), the expansive soil used can be classified as CH (highly plastic clays).

2.1.2. Nano Silica

Nano silica powder, obtained from a local ceramic industry company, was used in this study. Typically, nano silica powder is seen as a fine, white powder. It frequently has a smooth texture [26]. When combined with liquids, the extremely dispersible powder can create solutions [27]. Nano silica particles are very small, homogeneous particles that range in size from 1 to 100 nanometers. They are frequently spherical or irregular in shape [28]. The physical properties of this nano silica are detailed in Table 3.

2.1.3. Plantain Leaf Ash

Dry plantain leaves were gathered from an agricultural area and then underwent a heating process in an oven and calcination was carried out in a makeup container below 650 °C to produce ash [29]. Afterward, they were cooled to room temperature and ground using a grinder machine to produce ash. The chemical composition of the leaf ash is presented in Table 4. A composition analysis indicated a predominant presence of calcium oxide (CaO) at 47.31% of the total leaf ash composition. Additionally, silica, calcium, and potassium were significant contributors to the composition.
Table 1. Physical properties of the expansive soil used.
Table 1. Physical properties of the expansive soil used.
ParametersValueStandard
Liquid Limit (%)74ASTM D4318 [25]
Plastic Limit (%)39ASTM D4318 [25]
Plastic Index (%)35ASTM D4318 [25]
Soil Classification (USCS)CHASTM D2487 [30]
Unconfined Compressive Strength (MPa)0.68ASTM D2166 [31]
Swelling Potential (%)72ASTM D4546 [32]
Specific Gravity (Gs)2.68ASTM D854 [33]
Optimum Moisture Content (OMC) (%)27.4ASTM D1557 [34]
Maximum Dry Density (g/cm3)1.2ASTM D1557 [34]
Table 2. Mineralogical and chemical composition of the expansive soil used.
Table 2. Mineralogical and chemical composition of the expansive soil used.
ParametersPercentage (%)
Mineralogical Composition %
Quartz46.1
Montmorillonite34.7
Kaolinite11.4
Feldspar3.1
Other4.7
Chemical Composition %
SiO259.34
Al2O323.42
FeO9.31
CaO3.07
MgO1.94
TiO21.78
K2O0.71
Na2O + P2O50.43
Table 3. Physical properties of the nano silica used.
Table 3. Physical properties of the nano silica used.
ParametersValues
ColorWhite
FormPowder
Surface Area200 (m2/g)
Refractive Index1.46 (lit.)
Average Particle Size200–300 (nm)
Density2.31 (b/cu.ft)
Table 4. Chemical composition of the plantain leaf ash used.
Table 4. Chemical composition of the plantain leaf ash used.
ParametersPercentage (%)
CaO47.31
SiO217.54
K2O16.92
Al2O34.08
Fe2O33.87
P2O53.14
SO32.69
MgO1.47
LOI0.28
Others2.70

2.2. Preparation of Soil Samples

Ten types of soil samples were prepared by varying the ratio of reinforced materials mixed with the soil. The Optimum Moisture Content (OMC) for making each sample was determined by the compaction test, as shown in Table 5. The process for preparing the reinforced soil samples, involving plantain leaf ash and nano silica, mirrored that of the unreinforced soil samples. Plantain leaf ash was used in mixtures ranging from 0 to 15%, while nano silica ranged from 0 to 1.2%. The dosage of nano silica in mixes can vary depending on its intended use, soil type, the other materials it is combined with, economic considerations, the desired soil strength achievement, and also on previous literature. When nano silica is incorporated alongside ash, a lower dosage may be sufficient [3]. This is because the ash itself often contributes to pozzolanic reactions, synergistically enhancing the properties of the mixture [35]. A study conducted by Munda et al. [36] showed that cooperating nano silica percentages up to 1.5% with fly ash show the maximum improvement in expansive soil strength. After 1.5%, the strength decreases. The study by Munawar et al. [37] analyzed the use of up to 1.2% nano silica with rice husk ash and showed the maximum improvement in expansive clay soil strength. The study by Kulanthaivel et al. [38] utilized up to 1% nano silica with PET fiber and fly ash and showed maximum improvement in expansive clay soil. However, the dosage of plantain leaf ash is also decided according to previous literature. The study by Ezema, Adinna, and Anayo [4] shows maximum improvement in soil strength by utilizing up to 10% plantain leaf ash. Another study utilized up to 10% of plantain peel ash for improving the strength properties of soil [39]. After sample preparation, strength tests were conducted to analyze how different proportions of plantain leaf ash and nano silica affected the strength properties of expansive soil. Table 5 illustrates the variations in soil samples corresponding to different ratios of plantain leaf ash and nano silica. Figure 1 displays the preparation of a reinforced soil sample by mixing reinforced materials with soil.

2.3. Test Methods

The methodology employed in this study is depicted in Figure 2. The procedures for these tests are explained in the following sections. The experimental apparatus is shown in the Supplementary Data in Figure S1.

2.3.1. Compaction Test

A soil sample weighing 5 kg was used to conduct the modified Proctor test following ASTM D1557 [34] standards. The soil was thoroughly mixed with water. Subsequently, a modified Proctor test was conducted. The mold was filled with five layers, each subjected to 25 blows with a hammer weighing 4.5 kg and dropped from a height of 45 cm. The test provided the optimum moisture content and maximum dry density. The determined optimum moisture content was utilized in preparing the UCS samples.

2.3.2. UCS Test

The Unconfined Compressive tests for both unreinforced and reinforced soil samples were conducted following ASTM D2166 [31] standards. A modified Proctor test was performed to determine the optimum moisture content, which was subsequently used in creating samples with varying ratios of plantain leaf ash and nano silica. The height and weight of the prepared samples removed from the mold were 7.6 cm and 3.7 cm. These samples underwent curing for 7, 14, 21, and 28 days before the UCS tests were conducted. To maintain moisture levels, the samples were wrapped in plastic throughout the testing period. Data collected during the UCS tests, including maximum load sustained and corresponding deformation, were meticulously recorded in compliance with ASTM standards for test methods and practices.

2.3.3. Direct Shear Test

The ASTM D3080 [40] guidelines were followed to determine the frictional angle and soil cohesiveness during a direct shear test. As per ASTM rules, a soil specimen, generally square and approximately 61 mm by 61 mm, was prepared and placed inside a shear box apparatus for the experiment. After that, in order to replicate real-world circumstances, the soil specimen was exposed to controlled stresses, usually at a constant normal stress of 100 kPa. The specimen was subjected to a regulated rate of incremental shear stress applied horizontally until failure occurred. The cohesiveness and frictional angle of the soil were determined by carefully examining the test results in accordance with ASTM guidelines. The intercept of the shear stress–displacement curve at zero displacement was used to calculate cohesiveness, which is a measure of the intrinsic strength of the soil [41,42]. In the meantime, the slope of the linear section of the curve after peak failure was used to calculate the frictional angle, which is a measure of the soil’s resistance to sliding [43].

2.3.4. CBR Test

The California Bearing Ratio (CBR) tests were conducted in accordance with ASTM D1883 and D4546 [32,44] standards to assess both unreinforced and reinforced soil samples. Unsoaked samples, measuring 15 mm in diameter and 18 cm in height, were compacted at their optimum moisture content within the mold. CBR testing was carried out at intervals of 7, 14, 21, and 28 days under a surcharge of 2500 Pa to determine the samples’ load-bearing capacity. For the soaked CBR tests, the same preparation method was followed initially. Subsequently, these samples underwent soaking in water to induce swelling. Swelling behavior was monitored using a dial gauge at various time intervals until the swelling ceased. The swell potential was quantified based on these observations.

2.3.5. SEM Test

For microstructural analysis, the soil samples underwent SEM testing in accordance with ASTM E2809 standards [45]. Fractured pieces from unreinforced soil samples, soil samples reinforced with 1.2% nano silica, and soil reinforced with 1.2% nano silica and 15% plantain leaf ash were extracted from the UCS test after 28 days. Before SEM imaging, both reinforced and unreinforced samples were dried at room temperature (35 °C). Subsequently, SEM images were captured to analyze the interaction behavior among these particles.

3. Results and Discussion

3.1. Effect of Nano Silica and Plantain Leaf Ash on Atterberg Limits

In Figure 3, the impact of increased quantities of nano silica and plantain leaf ash on the Atterberg limits of the samples is illustrated. It is evident that, as the nano silica content rises to 1.2% and the plantain leaf ash content increases up to 15%, the liquid limit of the unreinforced soil sample decreases by a factor of 1.5, while the plastic limit decreases by a factor of 1.3. The liquid and plastic limits of expansive soil can be efficiently reduced by mixing with plantain leaf ash and nano silica. Compounds in plantain leaf ash improve the binding qualities of the soil, which helps to change its structure [29]. By filling in the spaces in the soil, the ash particles lower the amount of water needed to reach the liquid limit [4]. Furthermore, because of its small particle size, nano silica reduces the flexibility of the soil by filling in the spaces between particles within the soil matrix. The filling impact lowers the plastic limit by decreasing the soil’s capacity to absorb water [46,47].

3.2. Effect of Nano Silica and Plantain Leaf Ash on UCS

Figure 4 presents the results of the unconfined compressive test conducted on soil samples reinforced with up to 1.2% nano silica and up to 15% plantain leaf ash for various curing periods (0, 7, 14, 21, and 28 days). In Figure 4a, the unconfined compressive strength (UCS) test results for both unreinforced and reinforced soil are shown, with nano silica ratios of 0.4%, 0.8%, and 1.2%, along with 5% plantain leaf ash. It is observed that a maximum improvement of approximately 3.4 times the initial unreinforced sample value is achieved by utilizing 1.2% nano silica after 28 days on the PN7 soil sample. In Figure 4b, the UCS test results for unreinforced and reinforced soil are presented with nano silica ratios of 0.4%, 0.8%, and 1.2%, and 10% plantain leaf ash. A maximum improvement of about four times the initial unreinforced sample value is observed with the utilization of 1.2% nano silica after 28 days on the PN8 soil sample. Figure 4c displays the UCS test results for unreinforced and reinforced soil, employing nano silica ratios of 0.4%, 0.8%, and 1.2%, along with 15% plantain leaf ash. It is noted that a maximum improvement of approximately 4.6 times the initial unreinforced sample value is achieved by utilizing 1.2% nano silica after 28 days on the PN9 soil sample.
Increasing the nano silica and plantain leaf ash enhances the compressive strength of the soil. Bahmani et al.’s [48] study utilized up to 1.2% nano silica and analyzed that the maximum improvement in compressive strength was observed at 28 days. Ghavami, Naseri, Jahanbakhsh, and Nejad [3] conducted another study, employing nano silica and silica fumes to analyze the enhancement in the strength properties of clayey soil. The analysis revealed that the maximum improvement in compressive strength occurred when using 1% nano silica and 15% silica fumes after 28 days. The study carried out by Bahmani et al. [49] analyzed an increase in compressive strength of up to 80% by utilizing nano silica in cement-stabilized soil. Incorporating leaf ash into soil provides stabilizing properties due to its richness in organic compounds and minerals [50]. Compressive strength is increased as a result of improved soil compaction, decreased flexibility, and increased soil particle binding capacity [51,52,53]. The tiny particle size of nano silica allows it to micro-fill in the spaces between soil particles, strengthening the structure of the soil. It improves the soil’s overall stability and compressive strength by forming a denser matrix [54,55].
The Supplementary Data provide a comparative analysis of plantain leaf ash and nano silica incorporated into the soil, as depicted in Figures S2 and S3 and Tables S1 and S2.
Figure 5 displays the failure of the unreinforced sample and the sample reinforced with 1.2% nano silica and 15% plantain leaf ash at 28 days. The sample exhibits a bulging failure on the first day; this occurs because the soil lacks internal support to withstand the applied stress uniformly, leading to lateral expansion and deformation [56], whereas the sample at 28 days reinforced with 1.2% nano silica and 15% plantain leaf ash shows shear failure in the UCS test. This phenomenon is attributed to the reinforcement’s ability to provide lateral confinement and internal support, effectively resisting bulging and promoting a more defined failure plane [57].

3.3. Effect of Nano Silica and Plantain Leaf Ash on Shear Strength Parameters

A direct shear test was performed to assess the shear strength parameters of the samples. Table 6 illustrates the enhancement in shear strength parameters due to the incorporation of nano silica and plantain leaf ash. The unreinforced soil displays a cohesion of 13.6 kPa and a frictional angle of 18.7°. With the addition of 0.4% nano silica and a 5% increment in plantain leaf ash, there is a maximum improvement in cohesion of about 2 times and in the frictional angle by approximately 1.2 times when utilizing 15% plantain leaf ash. Increasing the nano silica to 0.8% with the same 5% increment in plantain leaf ash results in a cohesion enhancement of about 3.1 times and a frictional angle improvement of around 1.3 times with 15% plantain leaf ash. At 1.2% nano silica with a 5% incremental rise in plantain leaf ash, the maximum enhancement in cohesion reaches about 3.3 times, while the frictional angle improves by about 1.6 times at 15% plantain leaf ash. Kalhor et al.’s [58] study analyzed how, by increasing the nano silica up to 3%, there was an increase in both the friction angle and cohesion of the soil. The study conducted by Changizi and Haddad [59] analyzed how, by using up to 1% nano silica, there was an increase in the shear strength parameters of cohesive soil. Another study indicated that utilizing carbon fiber and up to 3% nano silica increases the shear strength parameters of silty soil [60], while the study carried out by Inim et al. [61] analyzed how a 5% increment in bamboo leaf ash increased both the cohesion and the frictional angle value. Meanwhile, another study utilized up to 2% leaf ash with plastic and analyzed the improvement in the shear strength parameters of soil [62]. The addition of plantain leaf ash and nano silica is advantageous for expansive soil, enhancing its engineering properties [4]. Rich in minerals and silica, plantain leaf ash strengthens soil bonding and increases the soil’s frictional angle. Nano silica fills spaces between soil particles, enhancing cohesion and overall strength [63,64]. By using these procedures together, the soil’s susceptibility to changes in moisture content may be decreased.

3.4. Effect of Nano Silica and Plantain Leaf Ash on CBR

Figure 6a depicts the CBR test results for unsoaked samples reinforced with 0.4% nano silica, exhibiting a 5% increment in plantain leaf ash. It was observed that there was a maximum improvement of approximately 4.4 times the initial CBR value of unreinforced soil, which is 7%, achieved by adding 0.4% nano silica and 15% plantain leaf ash (PN3). Figure 6b displays the CBR test outcomes for unsoaked samples reinforced with 0.8% nano silica, accompanied by a 5% increment in plantain leaf ash. It was noted that a maximum improvement of about 4.8 times compared to the initial CBR value of unreinforced soil (which is 7%) was seen in PN6. Furthermore, Figure 6c exhibits the CBR test results for unsoaked samples reinforced with 1.2% nano silica, along with a 5% increment in plantain leaf ash. In this case, a maximum improvement of approximately 5.8 times the initial CBR value of unreinforced soil (which is 7%) was observed in the PN9 sample type. Patro and Sahoo [65] noted a similar observation, employing nano silica at a concentration of up to 1.5% to analyze the enhancement in the CBR of soil. Another study, conducted by Alireza et al. [66], demonstrated that utilizing the optimum combination of 5% lime and 3% nano silica significantly increased the CBR value of the weak soil. One study utilized both fly ash and nano silica and observed an improvement in the CBR value of the soil [67]. Munda, Padhi, and Mohanty’s [36] study revealed that the CBR value of unsoaked samples increased by 5.1 times when employing nano silica and fly ash in expansive soil. Eshaghzadeh et al.’s [68] study indicated that an increase in nano silica in fiber-reinforced soil has a negative impact on the CBR value of silty sand. An increase in the amount of nano silica and plantain leaf ash leads to an enhancement in the bearing capacity of the soil.
Figure 7 displays the results of the swell potential via the soaked CBR test. An increase in nano silica and plantain leaf ash resulted in a decrease in the swell potential of expansive soil. The maximum reduction in swell potential of 74% was observed in PN9 soil after 28 days. Because of the small size of its particles, nano silica can fill in the spaces between the particles in the soil, minimizing the amount of space between them and preventing water from interacting with the larger clay particles [69,70]. Because of this, the soil absorbs less water, which lowers the possibility of swell. However, the pozzolanic qualities of plantain leaf ash are well known. When combined with calcium hydroxide in the presence of water, these capabilities produce more binding compounds [29,71,72,73]. Soil is strengthened and stabilized as a result, and it is less likely to expand when it becomes moist. By improving soil microstructure, reducing water permeability, and enhancing soil stability, the combined effects of nano silica and plantain leaf ash reduce the potential for swell. Figure 8 depicts the CBR test results of soil samples, showing both the soaked and unsoaked conditions.

3.5. Effect of Nano Silica and Plantain Leaf Ash on Microstructure Analysis

The SEM (Scanning Electron Microscope) test results most likely show a loose, uneven, and unstable soil structure in Figure 9a, which depicts expansive soil without reinforcement. The expansive clay minerals in the soil are easily identified, giving rise to the soil’s tendency to expand and contract in response to variations in moisture. Comparing Figure 9b to Figure 9a, which shows expansive soil reinforced with nano silica, the SEM results indicate a more compacted and organized look. It is possible to see nano silica particles forming a network or link that strengthens the soil matrix between soil particles. The increased cohesiveness and decreased swelling potential as a result of this reinforcement produce a more stable and organized soil microstructure. A further improved soil structure is shown in Figure 9c, which shows expansive soil reinforced by a combination of nano silica and plantain leaf ash. The combined action of nano silica and plantain leaf ash may result in a more refined and interconnected network, as shown in the SEM pictures. As a supplemental element, plantain leaf ash helps to promote compaction and reinforce the soil structure. As a result, Figure 9c shows a well-organized, tightly packed soil matrix with fewer voids and greater stability.

4. Interaction Mechanism between Soil, Plantain Leaf Ash, and Nano Silica

Figure 10 shows the interaction mechanism of nano silica, plantain leaf ash, and soil. The stabilization of soil involves a physical and chemical interaction between plantain leaf ash, nano silica, and the soil. Rich in minerals like silica and potassium oxide, plantain leaf ash combines chemically with soil particles to form connections that improve soil cohesiveness when it is added to the soil [74]. Due to its high reactivity, nano silica interacts with soil constituents simultaneously to generate cementitious compounds, such as calcium silicate hydrates (C-S-H), which strengthen the soil’s structure [75]. To improve soil strength and stability, plantain leaf ash and nano silica both help to change the way soil particles are arranged, fill in gaps, and increase soil density. Furthermore, ash’s strength in increasing the soil’s cation exchange capacity enhances nutrient retention, and the combined action of these additions lessens soil swelling and plasticity [76]. When all of these activities come together, they create micro aggregates and improve soil particle adhesion, which promotes long-term stability and durability against weathering and erosion.

5. Conclusions

This study investigates the stabilization of expansive soil using a combination of nano silica and plantain leaf ash. The findings reveal that introducing up to 1.2%nano silica and up to 15% plantain leaf ash leads to a notable reduction in both the liquid and plastic limits of the soil, decreasing them by factors of 1.5 and 1.3, respectively. The observed reductions in both liquid and plastic limits signify a reduction in the soil’s propensity for volumetric changes, thereby mitigating issues related to swelling and shrinkage.
Moreover, the UCS test results demonstrate significant enhancements in the soil’s compressive strength. Specifically, incorporating 1.2% nano silica and 15% plantain leaf ash increases the compressive strength by up to 4.6 times compared to untreated soil over 28 days. The considerable increase in compressive strength highlights the enhanced load-bearing capacity of the stabilized soil.
Furthermore, improvements in the soil’s shear strength parameters are observed. The cohesion of the soil increases by a maximum of about 3.3 times with the addition of 1.2% nano silica and 15% plantain leaf ash. Additionally, the frictional angle improves by up to 1.6 times when 1.2% nano silica and 15% plantain leaf ash are added to the untreated soil. It indicates the soil’s increased resistance to shear stresses. This is particularly important in scenarios where the soil is subjected to lateral forces, such as those experienced in embankments.
In terms of the CBR test, the unsoaked condition reveals a substantial increase in bearing capacity, with the maximum improvement reaching about 5.8 times for the sample treated with 1.2% nano silica and 15% plantain leaf ash at 28 days. Furthermore, the soaked CBR test indicates a reduction in the swelling potential of up to 74% after the addition of nano silica and plantain leaf ash over the same period. The SEM analysis illustrates that the addition of nano silica and plantain leaf ash densifies the soil structure, indicating enhanced bonding within the sample. This densification suggests an improvement in the overall soil structure.
Nano silica, with its ultrafine particles, enhances the soil’s mechanical properties by filling in pore spaces and increasing cohesion, thereby reducing swelling and improving stability. Plantain leaf ash, on the other hand, acts as a natural pozzolan, reacting with calcium hydroxide in the soil to form additional binding compounds, further enhancing strength and reducing susceptibility to volume changes. Together, the composite creates a synergistic effect, providing long-term stabilization.
The limitations of this study involve the economic feasibility of this study, alongside its sustainability. Future investigations could focus on optimizing the dosage and combination of stabilizing agents, assessing long-term performance under different environmental conditions, and evaluating the feasibility of large-scale implementation in real-world engineering projects. Additionally, comparative studies with conventional stabilization methods would provide valuable insights into the cost-effectiveness and sustainability of the proposed approach.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su16052157/s1, Figure S1: Experimental apparatus; Figure S2: UCS test results of unreinforced soil and soil reinforced with 5, 10 and 15% Plantain leaf ash at 0 day; Figure S3: UCS test results of unreinforced soil and soil reinforced with 0.4, 0.8 and 1.2% Nano Silica at 0 day; Table S1: UCS test results of unreinforced soil and soil reinforced with 5, 10 and 15% Plantain leaf ash (PLA) at 0 day; Table S2: UCS test results of soil reinforced with 0.4, 0.8 and 1.2% Nano Silica (NS) at 0 day.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data present in this study are available upon request.

Acknowledgments

The researcher would like to thank the Deanship of Scientific Research, Qassim University for funding publication of this project.

Conflicts of Interest

The author declares no conflicts of interest.

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Figure 1. Preparation of reinforced soil sample.
Figure 1. Preparation of reinforced soil sample.
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Figure 2. Research methodology.
Figure 2. Research methodology.
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Figure 3. Effect of nano silica and plantain leaf ash on the Atterberg limits of soil.
Figure 3. Effect of nano silica and plantain leaf ash on the Atterberg limits of soil.
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Figure 4. UCS test results of soil samples reinforced with different percentages of nano silica and (a) 5% plantain leaf ash, (b) 10% plantain leaf ash, and (c) 15% plantain leaf ash.
Figure 4. UCS test results of soil samples reinforced with different percentages of nano silica and (a) 5% plantain leaf ash, (b) 10% plantain leaf ash, and (c) 15% plantain leaf ash.
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Figure 5. Failure of soil sample in UCS test: (a) unreinforced and (b) reinforced with 1.2% nano silica and 15% plantain leaf ash at 28 days.
Figure 5. Failure of soil sample in UCS test: (a) unreinforced and (b) reinforced with 1.2% nano silica and 15% plantain leaf ash at 28 days.
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Figure 6. CBR test results for unsoaked samples reinforced with (a) 0.4% nano silica, (b) 0.8% nano silica, and (c) 1.2% nano silica.
Figure 6. CBR test results for unsoaked samples reinforced with (a) 0.4% nano silica, (b) 0.8% nano silica, and (c) 1.2% nano silica.
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Figure 7. Comparison of swell potential in soaked CBR tests.
Figure 7. Comparison of swell potential in soaked CBR tests.
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Figure 8. CBR test for soil samples: (a) unsoaked and (b) soaked.
Figure 8. CBR test for soil samples: (a) unsoaked and (b) soaked.
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Figure 9. SEM test results (a) soil sample, (b) soil sample + 1.2% nano silica, and (c) soil sample + 1.2% nano silica + 15% plantain leaf ash.
Figure 9. SEM test results (a) soil sample, (b) soil sample + 1.2% nano silica, and (c) soil sample + 1.2% nano silica + 15% plantain leaf ash.
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Figure 10. Interaction mechanism of nano silica, plantain leaf ash, and soil.
Figure 10. Interaction mechanism of nano silica, plantain leaf ash, and soil.
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Table 5. Reinforced soil samples with different percentages of nano silica and plantain leaf ash.
Table 5. Reinforced soil samples with different percentages of nano silica and plantain leaf ash.
No.Mixed Ratios of SamplesDesignationSoil (%)Plantain Leaf Ash (%)Nano Silica (%)Total (%)OMC (%)
1Soil + 0% Plantain leaf ash + 0% Nano silicaPN01000010027.4
2Soil + 5% Plantain leaf ash + 0.4% Nano silicaPN194.650.410016.3
3Soil + 10% Plantain leaf ash + 0.4% Nano silicaPN289.6100.410017.1
4Soil + 15% Plantain leaf ash + 0.4% Nano silicaPN384.6150.410018.6
5Soil + 5% Plantain leaf ash + 0.8% Nano silicaPN494.250.810021.9
6Soil + 10% Plantain leaf ash + 0.8% Nano silicaPN589.2100.810022.4
7Soil + 15% Plantain leaf ash + 0.8% Nano silicaPN684.2150.810023.7
8Soil + 5% Plantain leaf ash + 1.2% Nano silicaPN793.851.210026.8
9Soil + 10% Plantain leaf ash + 1.2% Nano silicaPN888.8101.210028.2
10Soil + 15% Plantain leaf ash + 1.2% Nano silicaPN983.8151.210028.9
Table 6. Improvement in shear strength parameters in soil reinforced with nano silica and plantain leaf ash.
Table 6. Improvement in shear strength parameters in soil reinforced with nano silica and plantain leaf ash.
Sample DesignationNano Silica (%)Plantain Leaf Ash (%)Frictional Angle (°)Cohesion (kPa)
PN00018.713.6
PN10.4519.318.1
PN21021.124.7
PN31521.926.8
PN40.8522.632.5
PN51023.138.3
PN61524.242.9
PN71.2525.738.4
PN81028.643.7
PN91529.245.2
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Alshawmar, F. Utilization of Nano Silica and Plantain Leaf Ash for Improving Strength Properties of Expansive Soil. Sustainability 2024, 16, 2157. https://doi.org/10.3390/su16052157

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Alshawmar F. Utilization of Nano Silica and Plantain Leaf Ash for Improving Strength Properties of Expansive Soil. Sustainability. 2024; 16(5):2157. https://doi.org/10.3390/su16052157

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Alshawmar, Fahad. 2024. "Utilization of Nano Silica and Plantain Leaf Ash for Improving Strength Properties of Expansive Soil" Sustainability 16, no. 5: 2157. https://doi.org/10.3390/su16052157

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