Effects of Xanthan Gum Biopolymer on Soil Mechanical Properties

Abstract

The necessary application of sustainable engineering methodologies has been increasing as the number of environmental hazards caused by global warming is on the rise. Cement as a traditional common additive for soil improvement has several negative impacts on the environment. This led to an urge for alternative sustainable solutions. The use of biopolymers as environmentally friendly materials is one of the potential options. This study aims to investigate the effect of xanthan gum biopolymer as a sustainable solution for soil properties enhancement. The Atterberg limits, unconfined compression, CU and UU triaxial tests were performed to examine the effect of xanthan gum on the soil strength and plasticity. Additionally, the durability of biopolymer-treated and untreated soils under wetting and drying cycles and moisture susceptibility were assessed. The results showed that the compressive strength of soil increased by increasing the xanthan gum concentration and curing time and reached its peak value after a specific curing time. The addition of xanthan gum resulted in significant improvement in soil cohesion and caused a reduction in the internal friction angle of the soil. While increasing the number of wetting/drying cycles decreased the soil strength, the biopolymer-treated soil exhibited higher soil strength than the untreated soil. This study provides valuable experiences in the use of xanthan gum biopolymer in practical engineering applications.

1. Introduction

Chemical treatment is considered a soil improvement technique by adding agent stabilizers to the soil. Portland cement and lime as preferred binder materials are traditionally used to enhance the engineering properties of soil, such as soil strength, hydraulic conductivity, and erosion resistance of the weak ground.

Although cement has been widely employed for different construction purposes and ground improvements, overdependence and overuse of cement have had a major impact on the environment and causing many environmental concerns recently. After the coal and oil and gas industry, the cement industry is the third major sources of carbon dioxide (a potent greenhouse gas) emitter with annually up to 8% of global CO₂ emissions [1]. This highlights the need to use sustainable techniques in civil and geotechnical engineering practices.

Recently, an attempt has been made to use biopolymers as environmentally friendly additives in geotechnical engineering applications. Biopolymers are organic polymers produced from natural resources. Based on their source of origin, biopolymers can be classified into three main groups: plant-based, animal-based, and biopolymers produced by microorganisms [2]. Guar [3], lignin [4,5], agar [6], beta-glucan [7], and alginate [8] are some examples of plant-based biopolymers whose applications in geotechnical engineering were previously studied. Regarding animal-based biopolymers, chitosan which is produced from the waste crustacean shells, and caisin, a protein-based biopolymer extracted from milk and dairy products, are two animal-based biopolymers whose applications in geotechnical engineering were investigated [9,10]. Microorganism-based biopolymers such as xanthan gum, gellan gum, and dextran are produced from the fermentation process. Their potential for soil properties improvement has been reported in previous studies [11,12,13,14].

The enhancement of soil strength through biopolymer additive agents has been investigated previously. Soldo et al. [8] examined the effect of different biopolymers, including xanthan gum, beta-glucans, guar gum, chitosan, and alginate on the improvement of soil strength by conducting various laboratory experiments. They reported substantial increases in biopolymer-treated soil strengths over a longer period. Lignin, a by-product of paper industries and sugarcane mills, has shown its effectiveness in improving soil strength [15,16]. Through conducting UCS tests, the effect of wetting and drying cycles on the guar gum and xanthan gum-treated silty sand was assessed by Soldo et al. [3]. They confirmed the effectiveness of a 0.5% xanthan gum additive in lowering the loss of soil strength reduction after each wetting and drying cycle.

Among all applicable biopolymers in geotechnical engineering, xanthan gum has been shown to be one of the best cost-effective materials and was selected for use in the current study.

To improve the efficacy of biopolymer treatment, in-situ influencing elements must be taken into account. Even though xanthan gum proved to increase the compressive and shear strength of soil [8,13], previous research mainly concentrated on investigating fundamental strengthening behavior and validating feasibility. In particular, inadequate consideration of in situ three-dimensional stress situations has been made. Besides, in previous studies, the shearing behavior of xanthan gum-treated soil has been mostly investigated through conducting shear box tests for dry samples which may not properly represent the underground conditions. Additionally, a few studies investigated the effect of wetting and drying cycles on xanthan gum-treated soil.

To address the mentioned gaps, this study aims to investigate the shear strength properties of xanthan gum (XG) under different confining pressures and biopolymer concentrations. Moreover, extensive laboratory experiments including Atterberg limits tests, unconfined compressive strength (UCS) tests, and triaxial tests for the XG-treated soil samples were performed to obtain the effect of a given biopolymer agent on the soil mechanical properties under various conditions. The durability and strength of biopolymer-treated soils under wetting and drying cycles and moisture susceptibility were examined. Finally, scanning electron microscopy (SEM) analyses were conducted to evaluate the microstructure mechanism of such treatment approach.

2. Materials and Methods

2.1. Materials

Soil. The soil was collected from the Gold Coast area, Australia. The grain size distribution of soil was obtained using hydrometer and sieve analysis tests according to the ASTM D422-63 [17] and shown in Figure 1a. Based on the ASTM D4318-17 [18], the plastic and liquid limits and plasticity index for the given soil were 26.9, 38, and 11.1, respectively. The soil was classified as low plasticity silt according to the Unified Soil Classification System, ASTM D2487-17 [19]. The specific gravity of the soil was 2.77 according to the ASTM D854-14 [20]. The standard proctor compaction test in accordance with ASTM D698-12 [21] was performed, and a maximum dry density of 1.72 g/cm³ and corresponding optimum water content of 21.7% were obtained. The mineral compositions of the soil were obtained employing the X-ray diffraction (XRD) measurements using GBC Scientific, MMA diffractometer equipped with a Cu-Kα anode operated at 28.5 kV and 35 mA employing a 2D array detector to minimize fluorescent backgrounds and an automated knife edge to allow simultaneous measurements at low and high angles. Diffraction patterns were recorded by continuous scans from 10 to 80° 2θ, with a step size of 0.02° at a scan rate of 0.560 sec per step and a scan time of 35 min, as shown in Figure 1b. The XRD measurements revealed that soil samples were mainly composed of quartz with additional inclusions of kaolinite and calcite.

The energy dispersive X-ray spectroscopy (EDS) measurements acquired using TESCAN MIRA scanning electron microscope (SEM) equipped with an 80 mm² ThermoFisher Scientific UltraDry EDS detector were performed to obtain the elemental composition of the soil. The EDS elemental analysis was performed using Pathfinder Microanalysis^® software using the point mapping function, and the EDS patterns are shown in Figure 1c, confirming the silicon and oxygen as the main elements of soil particles.

Biopolymer. Xanthan gum (XG) was obtained from Glentham Life Sciences, UK. The used XG was a powder with pH in water (6–8) and viscosity of 1000–1800 mPa.s (1% in 1% KCl).

2.2. Specimen Preparation

The soil was first oven-dried followed by crushing and sieving to 2.36 mm to remove the gravel. With respect to the dry mass of soil, three concentrations of biopolymer agent, including (0.5, 1, and 2%) of XG by dry soil weight, were used. In the current study, the dry mixing technique followed by Soldo et al. [8] was adopted. The XG powder was first uniformly mixed with dry soil for about 15 min. Then, the desired mass of water, corresponding to the optimum water content obtained from the compaction test, was sprayed into the soil mixture and thoroughly mixed. To avoid the formation of aggregations and ensure a homogenous mixture of biopolymer with soil particles, the resulting mixtures were sealed with double-layer plastic wrap and placed at a controlled room temperature for 24 h. To make a soil sample, the prepared soil mixtures were placed into a cylindrical metal mold and evenly compacted in 5 layers. The mold had a diameter of 50 mm and a length of 150 mm, and the specimens with the same diameter and length of around 110 mm were extruded from the mold after each compaction set. The dry density of each specimen was ensured to be above 95% of the maximum dry density.

2.3. Experimental Program

Atterberg limits tests. To assess the effect of biopolymer additives on the soil plasticity, the Atterberg limits tests for the given concentrations of XG-treated soil and the untreated soil were carried out.

UCS tests. To define the optimum curing time, the prepared specimens were placed in a controlled temperature room at various times, including (0, 1, 4, 7, 10, 14, 28, and 35 days). The specimens without curing time were taken as wet samples, and the rest were considered dry specimens. The UCS tests in accordance with ASTM D2166-06 [22] were performed for the all-prepared specimens. Three replicated specimens were prepared and examined to minimize experiment errors. With regard to the results of the UCS test, the optimum curing time for the stabilized samples was selected for the following experiments.

Triaxial tests. The effect of saturation conditions on the strength of biopolymer-treated soil was investigated by conducting triaxial tests. The consolidated undrained (CU) triaxial tests for the saturated samples and unconsolidated undrained (UU) triaxial tests for the dried specimens were performed at three different confining pressures (50, 100, and 200 kPa). The sample preparation was consistent with the UCS tests under optimum curing times. Once the samples were cured accordingly, the CU and UU triaxial tests were conducted. For the UU tests, no saturation was carried out on the given specimen, and the dry specimen was first subjected to the considered confining pressure, and then the shearing was directly applied.

Wetting and drying cycles tests. To determine the effect of wetting and drying cycles on the soil strength, six wetting and drying cycles with respect to the results of curing time were considered. To conduct the part of experiments, PVC molds (inner diameter of 51 mm and length of 130 mm) with bottom caps were first constructed. This extra space allowed the specimen to expand once it got saturated. Each cycle was initiated with one wetting cycle in which the cured dry sample was placed into a PVC mold, and then the mold was mounted in water for 24 h. The drying cycle was then started by expelling the specimen out of the mold and letting it dry under room temperature conditions for the given optimum curing time. Following the completion of each cycle, the UCS test was performed to achieve the compressive strength of each treated and untreated soil sample.

Soaking test. The soaking test for the given concentrations of XG-treated soil was conducted to examine the effect of such biopolymer treatment on the moisture stability of soil mixtures. All prepared treated and untreated specimens were submerged in water for over five days, and the degree of disintegration was visually assessed.

SEM analysis. An untreated sample and a specimen treated with 2% XG were prepared and cured accordingly, and SEM imaging was conducted for the samples. Imaging was done on the TESCAN MIRA 3 scanning electron microscope, using 5 kV voltage with magnifications of 2500 and 15,000 times for low and high magnification images, respectively. The samples were platinum sputter coated (ca. 5 nm) immediately prior to SEM image collections.

3. Results and Discussions

3.1. Atterberg Limits

The results of Atterberg limits tests are given in

Table 1. Results of Atterberg limits tests.

Soil Reference	Liquid Limit, LL (%)	Plastic Limit, PL (%)	Plasticity Index, PI (%)
Pure soil	38.0	26.9	11.1
0.5% XG	59.6	28.5	31.1
1% XG	62.2	29.6	32.6
2% XG	64.7	30.7	34.0

.

Regardless of the percentages of XG in the soil, the plastic and liquid limits for the XG-treated soil were higher than pure soil. Significant increases in LL (64.7 in 2% XG additive-treated soil versus 38 in pure soil) and slight gains in PL were seen. A considerable increase (about two times) in the plasticity index occurred when adding 2% XG. This arises from the gel type and hydrophilic nature of XG, which results in absorbing more water and increases the plasticity index of soil. Additionally, the concentration of XG affects the Atterberg limits. As shown in Table 1, soil treated with 0.5% XG had lower PL and LL compared to 1% and 2% XG-treated soil. Higher soil plasticity as a result of the XG additive can be a practical implication as it is expected to reduce soil hydraulic conductivity [23,24].

3.2. Unconfined Compressive Strength (UCS)

To determine the optimum curing time and examine the compressive strength of the stabilized specimens, a series of UCS tests for the given percentages of XG-treated soil was carried out. Figure 2a shows the values of UCS for the XG-treated soil with curing time.

For all XG-treated specimens, the compressive strength of soil significantly increased with an increasing curing time, especially after 7 days of curing. The XG-treated specimens reached their peak soil strength values after 10 days, whereas untreated soil reached its maximum strength after 7 days of curing. The soil compressive strength increased with the increasing content of XG. Under 10 days of curing, the 2% XG-treated specimen shows almost 2.5 times higher UCS than the untreated sample. The increase is attributed to the bonding of soil with the XG additive. As moisture content plays a significant role in soil behavior, the moisture content of each specimen at the end of the UCS test was measured and summarized in Figure 2b. The moisture content of all samples, regardless of the XG percentage, was drastically decreased, and the moisture content of 10 days of cured specimens corresponds to the maximum soil strength in UCS tests. As the water content reduces, the biopolymer converts to a stiff material, which corresponds to an increase in soil strength. A similar relationship between water content and soil strength was also reported in previous studies [8,25].

The secant stiffness of the XG-treated specimens is given in Figure 3. Adding XG may enhance soil stiffness. The stiffness of both treated and untreated soil specimens gained with increasing the curing time and after 10 days remained almost unchanged.

The obtained curing time corresponding to the maximum UCS value was considered the optimum curing and was taken for the following experiments (Figure 4). The results were consistent with earlier studies [25,26,27]. Chang et al. [25] examined the effect of XG on the compressive strength of sand, mixture of sand with silt, and high-plasticity clay. They found that adding XG provides the highest strength for the mixture of sand with silt.

3.3. Shear Strength

3.3.1. Saturated Sample (CU Test)

The variations in deviator stresses and corresponding strains and shearing-induced pore water pressures for the pure soil and XG-treated specimens under 50 kPa confining pressure are shown in Figure 5.

As seen in Figure 5, the addition of biopolymer reduces the brittleness of soil in all XG-treated specimens. While there is no well-defined peak of shear stress for biopolymer-treated specimens, the pure soil experienced brittle behavior, and peak deviator stress is clearly determined.

The soil strength was significantly reduced by adding XG biopolymer (Figure 5a). Soil treated with 1% and 2% XG experienced a considerable 40% and 60% reduction in maximum deviatoric stress at 200 kPa confining pressure. The reason arises from the hydrophilic feature of gel-type biopolymers such as XG, which leads to absorbing water during the initial phase of the CU test. This water fills the pores between soil particles, causing a considerable decrease in soil strength. Corresponding behavior from generated pore water pressure during shearing was also observed (Figure 5b). The absolute values of induced pore water pressures in pure soil were much higher than in the treated soil with XG.

The soil shear parameters for the untreated and XG-treated soil were obtained through the plotted failure envelope curves (Figure 6).

The XG biopolymer caused a reduction in both soil cohesion and friction angle (Figure 6). A higher concentration of XG significantly reduced soil cohesion and friction angle of soil. As mentioned, this is ascribed to the higher absorbed water due to the gel-type feature of XG, which resulted in occupying the voids with water.

Typically soil friction angle depends on the angularity of soil particles, soil gradation, normal stress, etc. Adding biopolymer provides a coating over soil particles leading to a reduction in soil grain angularity and surface roughness. This causes a reduction in internal friction angle after adding biopolymer.

Table 2. Effective friction angle and cohesion of pure soil and XG-treated soil.

Soil Reference	Effective Cohesion, c′ (kPa)	Effective Internal Friction Angle, ϕ′ (°)
Pure soil	24	28.9
1% XG	17	21.8
2% XG	1	18.0

provides the calculated shear parameters from the CU triaxial tests.

3.3.2. Dry Sample (UU Test)

The deviatoric stress–axial strain curves obtained from UU triaxial tests for the dried untreated and biopolymer-treated specimens are shown in Figure 7.

Soil strength improvement is seen due to the biopolymer additive. Adding higher percentages of XG shows a larger enhancement in soil strength. Regardless of XG content, all specimens showed higher strength under higher confining pressures. Specimens treated with 1% XG caused a 46% increase in maximum deviatoric stresses. This shows that adding only 1% biopolymer can significantly enhance soil strength.

Once the XG-treated soil dried, the gel converted to a firm plastic material leading to the strong bonding between soil particles. This brings a higher resistance against the shearing of soil.

Previous studies from direct shear tests showed that changes in shear parameters depend on biopolymer and soil type. Cho et al. [28] investigated the effect of gellan gum on the soil cohesion and friction angle. They used a mixture of sand and clay and reported that adding gellan gum increased the cohesion of pure sand. At the same time, its friction angle remained relatively constant with increasing gellan gum content. In terms of pure clay, adding gellan gum to the soil caused an increase in both cohesion and friction angle. Khatami et al. [29] reported that even though adding agar and starch biopolymers to sand may effectively increase the cohesion of the soil, it leads to a reduction of soil friction angle when compared with untreated soil. By conducting the direct shear test, Ayeldeen et al. [30] showed that xanthan gum and guar gum significantly increase soil cohesion but slightly reduce soil friction angle. Soldo et al. [8] investigated the shear behavior of silty sand treated with xanthan gum, guar gum, and beta-glucan biopolymers by performing a direct shear test. Similarly, they reported adding biopolymers increases soil cohesion but causes soil friction angle reduction.

In the current study, the results of UU triaxial tests for the given XG concentrations under various confining pressures were used to obtain the shear parameters.

The failure envelope curves were plotted (Figure 8), and the corresponding shear parameters were determined (

Table 3. Results of UU triaxial tests; internal friction angle and cohesion of pure soil and XG-treated soil.

Soil Reference	Cohesion, c (kPa)	Internal Friction Angle, ϕ (°)
Pure soil	280	41.0
1% XG	580	36.6
2% XG	970	25.9

).

The addition of XG resulted in a drastic improvement in soil cohesion, as adding only 1% XG doubled the soil cohesion. A similar trend can be seen for the 2% XG-treated soil with the above threefold soil cohesion improvement. The main reason is that adding XG acts like glue and supplies strong binding between soil particles, causing a significant increase in soil cohesion. While XG covers the soil particle surfaces, causing a reduction in grain angularity and thereby leading to decreases in internal friction angle. The obtained results have consistency with the reported outcomes from previous studies [8,30].

3.4. Wetting and Drying Cycles

A series of tests with wetting and drying cycles for the untreated, 1%, and 2% XG-treated specimens were conducted. Figure 9 represents the results of the wetting and drying cycle tests. The UCS at cycle zero represents the compressive strength of each sample under optimum curing time.

The UCS of pure soil gradually decreased with increasing wetting/drying cycles. The biopolymer concentration has a significant impact on soil compressive strength during wetting/drying cycles. Due to biopolymer degradation, the 1% XG-treated sample experienced a sharp UCS reduction at the second cycle, and then the soil strength slightly decreased by increasing the wetting/drying cycles. A relatively significant deterioration in soil strength with increasing cycles was seen for higher concentrations of XG. Despite UCS reduction in all specimens, the biopolymer-treated samples exhibited higher soil strength than the pure soil samples and leveled off after a certain number of cycles. The plotted lines are the fitting curves for each soil reference.

The obtained results prove the effectiveness of XG biopolymer in reducing soil strength loss during cyclic wetting/drying.

3.5. Moisture Susceptibility

Figure 10 shows the untreated and XG-treated specimens submerged in water for over five days of the soaking test. The pure soil sample started disintegration immediately after immersing in water, and within 4 h was fully disintegrated. As a result of absorbing water, the size of all XG-treated specimens increased but kept their general shapes. Specimens with 0.5% XG and 1% XG experienced slight disintegration from the top and perimeter while still holding their cylinder shapes after two days and completely disintegrated after three days of the experiment. The general integrity of the 2% XG-treated sample was noticeable even after 5 days of submerging in the water, and its diameter was increased by almost 50%. The soaking test proves the capability of XG in maintaining soil integrity and enhancing soil resistance to water.

3.6. SEM Analysis

SEM images were used to investigate the effect of XG on the soil microstructures (Figure 11).

The untreated soil has a separate, flaky, and discontinuous structure. Larger voids with more conspicuous clumps of soil particles are observed (Figure 11a,b).

The soil particles are coated and bonded with XG biopolymer. The pore fills with XG making an attachment of soil particles via the hard and firm biopolymer monomers indicating strong soil–biopolymer interactions (Figure 11c,d).

4. Conclusions

The primary goal of this study was to investigate the effect of XG biopolymer on the improvement of soil mechanical properties. UCS, CU, and UU triaxial tests were carried out to examine the soil strength. The durability of biopolymer-treated soil during cyclic processes of wetting and drying and susceptibility to water were also investigated. The following conclusions can be drawn:

Adding XG caused substantial increases in LL and the plasticity index of soil. The UCS values of soil substantially increased with increasing the XG concentration and reached their peak value after a specific curing time depending on biopolymer concentration. The compressive strength of the soil increased 2.5 times after adding 2% XG. As a result of absorbing water during CU triaxial tests, the XG additive caused a significant reduction in soil strength and shearing parameters, including soil cohesion and friction angle. Results of UU triaxial tests for dried samples showed significant improvement in soil strength and soil cohesion due to the XG additive indicating promising results. Although during the wetting/drying cycles, the UCS values in all specimens were reduced by increasing the number of cycles, the biopolymer-treated samples exhibited higher soil strength than the pure soil samples. The XG-amended samples demonstrated strong resistance to the loss of compressive strength caused by repeated wetting and drying cycles. While the pure soil sample was fully disintegrated within 4 h of immersing in water, all XG-treated samples remained intact after 24 h and kept their shapes after two days of submerging in water. This proves the capability of biopolymer additives to keep soil structure once submerged in water.

With respect to the overall results of conducted experiments, the XG additive has shown promising results in enhancing soil mechanical properties and can be an alternative sustainable solution for soil stabilization. The authors would suggest that for future studies, large-scale field experiments and in situ tests need to be conducted to obtain broader insight into the practical application of XG for soil stabilization. The suggestion would be to mix small amounts of XG with soil on the ground surface, followed by water application.