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Technical Note

Effect of Curing Condition and Solvent Content on Mechanical Properties of Zein-Biopolymer-Treated Soil

by
Quadri Olakunle Babatunde
1,
Dong Geon Son
1,
Sang Yeob Kim
2 and
Yong-Hoon Byun
1,*
1
School of Agricultural Civil & Bio-Industrial Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 41566, Republic of Korea
2
Department of Fire and Disaster Prevention, Konkuk University, 268, Chungwon-daero, Chungju 27478, Republic of Korea
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(15), 12048; https://doi.org/10.3390/su151512048
Submission received: 21 June 2023 / Revised: 1 August 2023 / Accepted: 5 August 2023 / Published: 7 August 2023
(This article belongs to the Section Sustainable Materials)
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Abstract

:
The curing condition and solvent composition of biopolymer binders may impact their efficacy for soil stabilization. This study introduces a novel hydrophobic biopolymer, zein, and investigates the effects of solvent and curing conditions on the mechanical properties of zein-treated soils. The zein biopolymer is used to prepare cohesionless soil with various ethanol contents. Unconfined compressive strength and microscopic tests are used to investigate the treated specimens under two different curing conditions. The mechanical properties of the treated specimens are evaluated in terms of compressive strength and the secant elastic modulus. The experimental results show that the compressive strength and elastic modulus increase with the curing period under both curing conditions. Higher curing temperature improves the compressive strength of biopolymer-treated specimens. The linear relationship between compressive strength and the elastic modulus of zein-treated soils shows higher strength and a lower elastic modulus compared to rock. Furthermore, the zein biopolymer shows significant strength improvement compared to the existing biopolymers, including casein and lignin. Thus, the effects of solvent and curing conditions on the mechanical properties of zein-treated soil should be considered for its application to soil stabilization.

1. Introduction

Low-strength and high-compressibility properties of cohesionless soils result in loss of bearing capacity, differential settlement, and lateral movement upon shear loading [1]. Chemical and mechanical stabilization techniques have been employed to enhance the mechanical properties of unstable soils. For soil modification, conventional binders, such as ordinary Portland cement and lime, are mainly used to increase the compressive strength and erosion resistance of soils [2,3]. However, the emission of the principal greenhouse gas, carbon dioxide, into the atmosphere while producing conventional binders has been a global challenge. Accordingly, environmentally friendly bio-cementation, such as microbially induced calcium precipitate (MICP) and enzyme-induced calcium precipitate (EICP), has been recently introduced. MICP and EICP utilize a living organism, specifically, bacteria with sizes ranging from 0.5 to 0.3 µm, to precipitate calcium carbonates into soils [4]. The induced calcium carbonates produce cementation between soil particles, enhancing the mechanical properties of unstable soils. The MICP and EICP for soil stabilization have been widely used to improve the mechanical strength and erosion resistance of soils [5]. Nevertheless, the inability to apply MICP and EICP in fine-grained soils due to bacteria sizes has been a major drawback of these techniques [6].
Biopolymers are natural polymers induced by living organisms that are covalently bonded [7]. The application of biopolymer binder contributes to eco-friendly construction practices, aligning with global efforts to combat climate change and promote sustainable development. Moreover, unlike traditional cementitious materials, such as Portland cement, biopolymer cementation does not involve the formation of calcium carbonate due to different chemical reactions and mechanisms [8]. The cementation mechanism combines electrostatic interactions, hydrogen bonding, and chemical crosslinking, which leads to the aggregation and bonding of soil particles. Biopolymer binders utilize high-molecular-weight polymers, such as lignin, Xanthan gum, Beta-glucan, and chitosan, to enhance the strength of unstable soils through the surface coating of soil particles [9]. A variety of biopolymer binders was used to improve the slope stability of soils [10], mitigate soil erosion [11], and increase the compressive strength of soils [12,13]. Most of the biopolymer binders suggested in previous studies are hydrophilic-based, which breaks the hydrogen bond between the hydroxyl groups of the polymers when saturated, resulting in a significant strength loss [14]. Recently, Chang et al. [14] and Gao et al. [15] utilized a hydrophobic-based biopolymer binder to improve the mechanical strength of problematic soils. The biopolymers significantly enhanced the compressive strength of the soils in dry and wet conditions, compared to the hydrophilic-based biopolymer binders reported previously.
A hydrophobic-based biopolymer, zein, is a protein biopolymer extracted from the endosperm of maize using benzene solution [16]. Zein biopolymers are categorized into four different groups: α-zein, β-zein, γ-zein, and δ-zein, based on their hydrophobic molecular sizes [17,18]. Among the four molecular groups of zein, α-zein is generally used due to its hydrophobic characteristics and higher molecular weight, which ranges from 22 to 24 kDa [17,19]. The molecular structure of zein is dominated by hydrophobic amino acids with long alkyl side chains. These chains interact with each other through protein interactions, reducing the solubility of zein in water. The zein biopolymer is insoluble in water due to polar amino acids, proline and leucine (Figure 1), which control its hydrophobic properties [18]. The presence of charged side chains in the polar amino acids contributes to ion pairing and further decreases solubility. However, the hydrophilic amino acid, glutamine, facilitates interaction with water molecules; thus, promoting the cohesion and adhesion of soil cementation. Zein biopolymer can be dispersed using a polar solvent, such as ethanol, organic acid, and aliphatic polyether polyol [20,21]. The binary solvent consisting of ethanol and water is commonly used to dissolve zein biopolymer due to the high degree of solubility and improved cementation [22]. Zein biopolymer has been widely used as a binder in various fields due to its hydrophobicity and rapid cementation effect [23,24]. Babatunde and Byun [25] recently applied the zein biopolymer in the field of geotechnical engineering to improve the strength of sandy soils. Focusing on the short-term curing period of one day, Babatunde et al. [26] analyzed the rheological characteristics of the zein biopolymer and evaluated the shear wave velocity of zein-treated specimens under three different ethanol contents and two curing conditions. However, understanding the chemical reactions involved in biopolymer soil cementation is crucial for ensuring optimal, eco-friendly soil stabilization techniques that are economically viable and suitable for a wide range of geotechnical applications. Therefore, it is necessary to investigate the effects of solvent concentrations and curing conditions on the long-term mechanical properties of zein-treated specimens. The novelty of this study lies in the selection of optimum solvent and curing conditions, which leads to a significant improvement in strength and stiffness.
This study presents the effects of solvent and curing conditions on compressive strength and stiffness properties of soils treated with a novel zein biopolymer. In addition, the study highlights the potential of the newly suggested biopolymer binder as a sustainable alternative to traditional cementitious materials. First, the eco-friendly zein biopolymer is introduced, and the gradation and compaction properties of soils are then evaluated from sieve analysis and standard proctor tests. Subsequently, the experimental conditions and procedures for the unconfined compressive test and microscopic investigation are described. Afterward, the variation in compressive strengths and stiffness with the curing period and ethanol content is analyzed. Finally, the effects of ethanol content, curing condition, and curing period on the mechanical properties of zein-biopolymer-treated specimens are discussed.

2. Materials

2.1. Zein Biopolymer

Zein is extracted from the cytoplasm of maize endosperm between starch particles [16]. Generally, benzene or ether solution is used to extract commercial zein biopolymer from maize and to remove the starch particles and oil substances, as shown in Figure 2a. Zein is an amphiphilic substance that consists of hydrophilic and hydrophobic protein groups that are soluble and insoluble in water, respectively. The molecular structure of zein is dominated by hydrophobic amino groups. The hydrophobicity of zein biopolymer can be attributed to the nonpolar amino groups forming a weak hydrogen bonding [16]. α-zein is a hydrophobic amino group with an apparent molecular weight of 22–24 kDa and accounts for 70–85% of the molecular structure of zein biopolymer, as summarized in Table 1 [17,26]. Moreover, β-, γ-, and δ-zein have moderate solubility and can interact with water molecules, making them less suitable for cementation. The reaction of the hydrophobic amino group and a polar solvent induces the cementation of the zein biopolymer. The evaporation of the solvent leads to the formation of a stable zein molecule matrix which is covalently bonded, forming a three-dimensional protein network (Figure 2b). The chemical reaction involved in the polymerization of zein occurs through the condensation of reactive functional groups, such as amide (-NH), methyl (CH3) and hydroxyl (OH) groups, between adjacent zein molecules. This process results in the formation of covalent bonds, leading to the development of a three-dimensional polymer network [17]. Zein biopolymer has viscoelastic properties during its cementation, which show viscous and elastic behavior when subjected to shear forces [27]. Furthermore, zein biopolymers are biocompatible, which means they are non-toxic to living organisms. Thus, the zein biopolymer is considered an eco-friendly binder due to its unique properties, such as non-toxic and improved stabilization.

2.2. Soils

Silica sand was considered for the preparation of the zein-biopolymer-treated soils. The sieve analysis of the sand was carried out according to ASTM D6913 [28], and the sand was poorly graded with a mean diameter (D50) of 0.64 mm. The index properties of the sand are summarized in Table 2. The standard proctor tests were conducted according to ASTM D698 [29] by preparing the soil specimens at different moisture contents using a standard compactive effort. The maximum dry unit weight and optimum moisture content for the sand are 19.0 kN/m3 and 9.0%, respectively.

2.3. Solvent

Solvents are versatile liquids that can dissolve a variety of chemical compounds. The reaction of solvent with chemical or biological substances disintegrates the molecular structure of substances. The molecular structure and physiochemical characteristics of a solvent determine the degree of solubility of a solute. Solvents can be categorized as polar or non-polar based on their affinity for hydrogen ions [30]. The interaction of biopolymer molecules with the appropriate solvent leads to a polymerization process which results in biocementation. The solubility and hydrogen ion affinity of solvent can be affected by various parameters, such as temperature, chemical composition, and polarity [31]. In this study, zein biopolymer is dissolved in a mixture of polar and non-polar solvents of different chemical compositions. Ethanol is considered a polar solvent, while water acts as a non-polar solvent. The zein biopolymer chemically reacts with ethanol to expose the hydrophobic protein network, resulting in biocementation with the hydrophilic group [26,32]. Generally, ethanol evaporates faster than water. However, the evaporation rate of ethanol can be affected by diffusion rate, humidity, concentration, and temperature. At a relative humidity of less than 40% and room temperature, the evaporation of water can be faster than ethanol [33]. Thus, based on the evaporation rate of the solvent, the effects of curing temperature and ethanol content on the biocementation process of zein biopolymer were analyzed.

2.4. Sample Preparation

Dry soil was mixed with 3% zein biopolymer by weight for 5 min to achieve a homogenous treated specimen. Solvents were prepared by mixing the ethanol with water to form the desired contents of 50%, 70%, and 90% using a magnetic stirrer. Subsequently, the soil specimens were mixed with each solvent content. The moist specimens were then prepared to investigate the compressive strength, elastic modulus, and microstructure, under different solvent and curing conditions.

3. Experimental Study

3.1. Unconfined Compressive Test

The treated soils were prepared in a cylindrical split mold with a height of 100 mm and an internal diameter of 50 mm. The joints of the split mold were lubricated with vacuum grease to prevent water leakage through the joints. The split molds were tightly supported with two steel cable ties to maintain the dimension of the mold during the compaction. Using a hammer with a weight of 5.4 N, the soils were compacted by applying 25 blow counts to three uniform layers of each specimen. To investigate the effect of curing conditions, the specimens were cured under the controlled chamber and atmospheric conditions. The temperature and humidity of chamber condition were 50 °C and 30%, respectively, whereas those of atmospheric condition were 23 ± 2 °C and 20 ± 1%, respectively.
Unconfined compressive tests were performed to evaluate the compressive strength of zein-biopolymer-treated sands after curing for 3, 7, 14, and 28 days under two different controlled conditions. For the unconfined compressive tests, cylindrical specimens were prepared and subjected to axial loading at a displacement rate of 1 mm/min and up to a 5% axial strain according to ASTM D2166 [34]. The compressive strength and secant modulus of each specimen were determined from the stress and strain curves as suggested in previous studies [35,36].

3.2. Microscopic Investigation

The microscopic investigation of treated specimens can provide insights into the cementation of zein biopolymer in soil particles according to different ethanol contents and curing conditions. The microstructure analysis of the treated specimens was performed according to ASTM E986 [37]. The specimens were prepared and cured under the same conditions for the unconfined compressive tests. After curing, each specimen was cut into small square shapes of 10 mm by 10 mm, and the specimen surface was polished to a high degree of smoothness. The polished specimens were mounted in a scanning electron microscope (SEM). An evaporator was used to deposit the conductive coating material which produced a thin layer of metal and reduced the electrical charging on the specimen surface to improve the quality of the SEM images. Afterward, high-resolution images of treated soil specimens cured for 3 and 28 days were captured.

4. Results and Discussion

4.1. Compressive Strength and Elastic Modulus

The compressive strength and elastic modulus of biopolymer-treated soils are examined based on the curing period and ethanol content. Generally, the unconfined compressive strength increases with the curing period in both conditions, as shown in Figure 3. The increasing compressive strength with the curing period can be considered as the biocementation process of the treated specimens [38]. The biocementation process can be attributed to the polymerization of amino groups, which bind soil particles together, resulting in the strengthening and stabilization of soil. Under atmospheric conditions, the compressive strength of treated soils significantly increases after curing for 14 days, as shown in Figure 3a. The compressive strength under atmospheric conditions ranges from 0.04 to 8.2 MPa. The biocementation improvement before 14 days of curing can be attributed to the slow polymerization of the zein protein network induced at low curing temperature. Moreover, the low evaporation of the solvent under low temperatures leads to incomplete hydration of the protein networks, resulting in the incomplete crosslinking of the polymeric chains and less development of compressive strength of treated specimens. Before 28 days of curing, the specimens with 70 and 90% ethanol contents have lower compressive strength than those with an ethanol content of 50%. Nonthanum [27] also reported the rapid gelation of zein at lower ethanol content under lower curing temperatures. After 28 days under atmospheric conditions, the specimens with an ethanol content of 90% show the highest compressive strength among other ethanol contents, and the 28-days compressive strengths of the treated specimens significantly increase with increasing ethanol contents, which shows continuous evaporation of solvent, resulting in additional strength of zein biopolymer gel in the soil matrix. The presence of undissolved zein moieties results in the initial compressive strength observed at 50% solvent content. In the early curing period, the undissolved zein moieties act as nucleation regions for further polymerization reactions. Moreover, the micro aggregates induce the cementation of zein molecules from the dissolved phase, initiating crosslinking and polymerization reactions. However, after 28 days of curing, the specimens with 90% and 70% solvent content showed the highest compressive strength. This is attributable to the progressive evaporation of the solvent and improved particle packing, which resulted in a more cohesive and denser biopolymer–soil matrix.
The compressive strengths of zein-biopolymer-treated specimens at higher curing temperatures are greater than those at lower curing temperatures, regardless of ethanol contents, as shown in Figure 3b. Under chamber conditions, the compressive strengths for all curing periods increase with increased ethanol content. In addition, the compressive strengths of all ethanol contents increase with the curing period. Especially for 3 days, the rapid evaporation of the solvent under chamber conditions may accelerate the polymerization of the zein protein network, which simultaneously enhances the cementation of treated specimens. The increasing soil strength with ethanol content at higher curing temperatures can be attributed to the increase in viscosity induced by the biopolymer gel [26]. The results are in good agreement with the findings reported by Fatehi et al. [39], that higher temperature enhances the interlocking of soil particles cured in a chamber, compared to atmospheric conditions. All specimens after 28 days of curing show the highest compressive strength among the different curing periods.
The solvent emitted was determined by dividing the weight difference of specimens during the curing periods by the initial weight. Figure 4 shows the relationship between the evaporated solvent and compressive strength. The solvent emitted reduces with increasing ethanol content due to the lighter density of ethanol, compared to water. The ethanol content of 90% shows the strongest coefficient of determination, and the relationship demonstrates that the continuous evaporation of solvent at the ethanol content of 90% can increase the molecular surface area of the zein biopolymer in the soil matrix, which leads to the polymerization of the protein network within the zein molecule [40]. Thus, developing a strong crosslinking induced by a covalent bond between the mobilized biopolymer molecules may result in improved cementation.
The variation in the secant elastic modulus of zein-treated specimens with ethanol content is shown in Figure 5. Similar to the compressive strength trend, the secant modulus generally increases with the curing time. Overall, the secant elastic modulus under chamber conditions was greater than those under atmospheric conditions. For 3 and 28 days, the secant moduli under both conditions increase with an increase in ethanol content. The results are in good agreement with the previous finding that the interaction between the zein gels and sand particles enhances the stiffness properties of treated specimens [25]. However, the variation of the elastic modulus between the 7- and 14-day curing periods shows the low reactivity and bonding of the zein biopolymer at higher ethanol content, disrupting the proper distribution of biopolymer crosslinking and weakening the internal structure [41]. The simultaneous dehydration of the external and internal parts of the treated specimen at higher curing temperatures can lead to a more tightly packed soil structure. The improved secant elastic modulus indicates a strong biocementation of the zein biopolymer and soil particles.

4.2. Comparison

Understanding the elastic behavior of sandy soils is crucial in geotechnical engineering, to examine the potential deformations and settlements of structures. The relationships between the secant elastic modulus and unconfined compressive strength of zein-biopolymer-treated soils are shown in Figure 6, compared with rocks investigated in a previous study [42]. Generally, the secant elastic modulus linearly increases with the compressive strength of all treated specimens. The linear relationship between the secant modulus (E50) and unconfined compressive strength (UCS) of the materials can be expressed as follows:
E50 = α·UCS
where α is the slope of the linear relationship between the secant modulus and unconfined compressive strength, and the values of α for the rocks reported in previously are summarized in Table 3. Even at a binder content of 3%, the zein-biopolymer-treated soils show significant strength improvement and lower secant elastic moduli. The enhanced interparticle bonding of soil and zein biopolymer shows significant biocementation of the treated specimens.
Figure 7a shows the strength evolution of zein-biopolymer-treated soils, compared with other suggested biopolymers used in previous studies for soil stabilization [43,44,45,46,47]. The zein biopolymer content of 3% and ethanol content of 90% under atmospheric and chamber conditions were selected for comparison. Both protein- and polysaccharide-based biopolymers were included. Note that casein and zein are protein-based biopolymers, while lignin, beta-glucan, guar gum, and xanthan gum are polysaccharide-based biopolymers. The compressive strength of each treated specimen increases with the curing periods. Until the curing period of 7 days, the compressive strengths of zein-treated specimens under atmospheric conditions were lower than those of the previously suggested biopolymers. After curing for 28 days, the compressive strengths of zein-treated specimens under atmospheric conditions were greater than those of the previously suggested biopolymers. In contrast, the specimens cured in the chamber showed the highest compressive strength throughout the whole curing period, compared with the previously suggested biopolymers. The 28-day compressive strengths of several biopolymer-treated sandy soils are compared in Figure 7b. Note that the biopolymer contents of previously reported binders range from 3 to 15%, and the other biopolymers used in the previous studies were cured under atmospheric conditions. Compared to previous polysaccharide binders, protein-based biopolymers caused significant strength improvement in soils. The significant increases in the compressive strength of the soil can be attributed to the hydrophobic intermolecular crosslinking of protein-based biopolymers, such as zein and casein. Although the biocementation in both casein and zein biopolymers is induced by hydrophobic protein networks, the zein-treated specimens cured in similar conditions show a higher strength improvement. Compared to casein, the higher molecular weight and hydrophobicity of zein biopolymer can also enhance their biocementation with soils [48]. Moreover, zein has a more rigid and compact molecular structure compared to casein, which allows it to form a more stable interfacial gel and resist deformation under shear forces [49].

4.3. Microstructure

The SEM images of biopolymer-treated soils after curing for 3 and 28 days are shown in Figure 8. The zein biopolymer coats the surfaces of the soil particles, forming hydrophobic bridges between the particles. The SEM images indicate that the compressive strength and stiffness of soil specimens depend on the zein biopolymer gel network in the soil matrix. Figure 8a shows the microstructure of the specimen cured for 3 days with a presence of a solvent that retards the compressive strength at an early age. With continuous biocementation, the zein biopolymer gel fills the soil voids and develops a network of interlocking particles, as shown in Figure 8b, which improves the compressive strength and overall stability of the specimens. Figure 8c also shows zein biopolymer bridging of the soil particles after curing for 28 days, which may enhance the strength and stiffness of the soil. Considering the relationship between the compressive strength and solvent emitted, the cementation of zein biopolymer in the treated specimens strongly depends on solvent evaporation, which is influenced by the curing periods and temperature. The induced hydrophobic protein network formed a direct interaction between the soil particle and the zein biopolymer. The micro-interaction of zein biopolymer links the soil particles, improving strength and stiffness. Thus, the cementation mechanism of zein-biopolymer-treated specimens can be attributed to a hydrophobic protein network between zein biopolymer binder and sandy soils.

5. Conclusions

Biopolymer-based binders have been used due to their eco-friendliness and stabilization efficacy. This study investigated the impact of curing condition and ethanol content on the mechanical properties of soils treated with a novel biopolymer, zein. The specimens were prepared with sandy soils at various ethanol contents and cured under two different conditions. The unconfined compressive test was conducted to evaluate the compressive strength and stiffness of zein-treated specimens. The micro-interaction of zein-biopolymer-treated soils was assessed using scanning electron microscopy. The significant findings from the study are:
  • Compressive strength and elastic modulus of zein-treated soils increased with the curing period. Strength and stiffness variation occurred under different ethanol contents and curing conditions.
  • Hydrophobic cementation of zein biopolymers formed under atmospheric conditions. The rate of strength increase during curing was influenced by ethanol content.
  • After 28 days, zein biocementation reached peak compressive strength and stiffness. Ethanol contents of 50% and 90% yielded the lowest and highest compressive strengths, respectively.
  • Specimens cured in a chamber outperformed those cured under atmospheric conditions.
  • Zein-treated soils demonstrated a linear relationship between compressive strength and elastic modulus, exhibiting a higher strength and lower elastic modulus compared to rock.
  • Zein biopolymers showed higher long-term compressive strength with lower binder contents, compared to other biopolymers.
This study highlights the vital role of ethanol content and curing conditions on the compressive strength and stiffness of soils treated with eco-friendly zein biopolymers, offering strategic insights for their optimal use in geotechnical engineering. However, future studies should focus on durability assessment, field-scale testing, economic analysis, environmental impact assessment, and compatibility with different soils to further validate and enhance the applicability of zein biopolymers.

Author Contributions

The study was initiated by Q.O.B. and Y.-H.B. Y.-H.B. and S.Y.K. designed the scope of the study. The experiment was performed by D.G.S. and Q.O.B. and supervised by S.Y.K. and Y.-H.B. Q.O.B. wrote the manuscript draft, and Y.-H.B. reviewed the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2020R1C1C1008925; NRF-2021R1A5A1032433).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets used and analyzed during the current study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Molecular structure of zein biopolymer. The red spot indicates the overlap of two methyl groups, leading to ‘steric interactions. These refer to the repulsive forces that occur between atoms or groups within a molecule when they come into close proximity in three-dimensional space.
Figure 1. Molecular structure of zein biopolymer. The red spot indicates the overlap of two methyl groups, leading to ‘steric interactions. These refer to the repulsive forces that occur between atoms or groups within a molecule when they come into close proximity in three-dimensional space.
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Figure 2. Zein biopolymer: (a) extraction process; (b) polymerization at microscale.
Figure 2. Zein biopolymer: (a) extraction process; (b) polymerization at microscale.
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Figure 3. Variation in unconfined compressive strength with curing period in two conditions: (a) atmospheric; (b) chamber. EC indicates the ethanol content.
Figure 3. Variation in unconfined compressive strength with curing period in two conditions: (a) atmospheric; (b) chamber. EC indicates the ethanol content.
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Figure 4. Unconfined compressive strength versus solvent emitted.
Figure 4. Unconfined compressive strength versus solvent emitted.
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Figure 5. Variation in secant modulus with curing period in two different curing conditions: (a) atmospheric; (b) chamber. EC indicates the ethanol content.
Figure 5. Variation in secant modulus with curing period in two different curing conditions: (a) atmospheric; (b) chamber. EC indicates the ethanol content.
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Figure 6. Secant elastic modulus versus unconfined compressive strengths of zein-biopolymer-treated soils and rocks (data from Elhakim [42]).
Figure 6. Secant elastic modulus versus unconfined compressive strengths of zein-biopolymer-treated soils and rocks (data from Elhakim [42]).
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Figure 7. Comparison of unconfined compressive strengths of various biopolymer-treated specimens: (a) variation with curing period (data from Soldo et al. [46] and Zhang et al. [47]); (b) after 28 days of curing (data from Chang et al. [43]).
Figure 7. Comparison of unconfined compressive strengths of various biopolymer-treated specimens: (a) variation with curing period (data from Soldo et al. [46] and Zhang et al. [47]); (b) after 28 days of curing (data from Chang et al. [43]).
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Figure 8. SEM images of zein-biopolymer-treated soil at two curing periods: (a) 3 days; (b) 28 days (scale bar: 10 μm); (c) 28 days (scale bar: 50 μm).
Figure 8. SEM images of zein-biopolymer-treated soil at two curing periods: (a) 3 days; (b) 28 days (scale bar: 10 μm); (c) 28 days (scale bar: 50 μm).
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Table 1. Classification of zein biopolymer based on solubility.
Table 1. Classification of zein biopolymer based on solubility.
TypesMolecular Weight [kDa] Total Mass [%]
α-zein22–2470–85
β-zein14–1720
γ-zein14–165
δ-zein10–12<1
Table 2. Index properties of soil.
Table 2. Index properties of soil.
Particle Sizes Corresponding to Percent Finer [mm]Coefficient of Uniformity
Cu		Coefficient of Curvature
Cc		Specific Gravity
Gs		Unified Soil Classification System
D10D30D50D60
0.130.190.641.078.40.262.61SP
Table 3. Slope and coefficient of determination of the relationship between secant elastic modulus and unconfined compressive strength for zein-biopolymer-treated soils and rocks.
Table 3. Slope and coefficient of determination of the relationship between secant elastic modulus and unconfined compressive strength for zein-biopolymer-treated soils and rocks.
ReferenceMaterial TypeSlope, αCoefficient of Determination, R2
Elhakim [42]Rock127.50.850
This studyZeinbinder-treated soil15.00.917
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Babatunde, Q.O.; Son, D.G.; Kim, S.Y.; Byun, Y.-H. Effect of Curing Condition and Solvent Content on Mechanical Properties of Zein-Biopolymer-Treated Soil. Sustainability 2023, 15, 12048. https://doi.org/10.3390/su151512048

AMA Style

Babatunde QO, Son DG, Kim SY, Byun Y-H. Effect of Curing Condition and Solvent Content on Mechanical Properties of Zein-Biopolymer-Treated Soil. Sustainability. 2023; 15(15):12048. https://doi.org/10.3390/su151512048

Chicago/Turabian Style

Babatunde, Quadri Olakunle, Dong Geon Son, Sang Yeob Kim, and Yong-Hoon Byun. 2023. "Effect of Curing Condition and Solvent Content on Mechanical Properties of Zein-Biopolymer-Treated Soil" Sustainability 15, no. 15: 12048. https://doi.org/10.3390/su151512048

APA Style

Babatunde, Q. O., Son, D. G., Kim, S. Y., & Byun, Y.-H. (2023). Effect of Curing Condition and Solvent Content on Mechanical Properties of Zein-Biopolymer-Treated Soil. Sustainability, 15(15), 12048. https://doi.org/10.3390/su151512048

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