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Article

Analysis of the Shear Strength of Iron Oxide-Kaolinite Cementing Materials in Granite Red Soil

by
Hualing Yan
,
Nanbo Cen
,
Qinmin Zheng
,
Jinshi Lin
,
Fangshi Jiang
,
Yanhe Huang
and
Yue Zhang
*
Jinshan Soil and Water Conservation Research Center, Fujian Agriculture and Forestry University, Fuzhou 350002, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(1), 16; https://doi.org/10.3390/min15010016
Submission received: 9 November 2024 / Revised: 15 December 2024 / Accepted: 25 December 2024 / Published: 27 December 2024
(This article belongs to the Section Clays and Engineered Mineral Materials)
Browse Figures
Versions Notes

Abstract

:
Shear strength is the key index to determine the stability of a soil slope, and cementation between iron oxide and clay minerals is one of the internal factors affecting soil shear strength; however, the effects of the form of iron oxide on the shear strength of granite-weathered red soil are still unclear. Kaolinite, which is the main clay mineral of granite red soil, was selected as the research object, and the effects of three different forms of iron oxide (hematite: HT, goethite: GT, and amorphous iron oxide: AIO) on the soil microstructure, microscopic quantitative parameters, cohesion, internal friction angle, and shear strength were analyzed by scanning electron microscopy, X-ray diffraction, and the shear strength test. The results revealed that the iron oxide promoted the cementation of soil particles, and the cementation characteristics differed with the different forms of iron oxide. Hematite mainly showed flocculent cementation, poor cementation, and simple soil microstructures. Goethite mainly exhibited acicular cementation and the best cementation effect. The degree of aggregation of the soil particles was increased by the coatings, thus forming larger aggregate particles. The cementation effect of amorphous iron oxide was between those of hematite and goethite but included both the flocculation cementation of hematite and acicular cementation of goethite. Amorphous iron oxide and goethite effectively increased the contact area and friction degree between soil particles, while hematite had the opposite effect. The addition of three kinds of ferric oxide reduced the fractal dimension of soil, increased the apparent porosity, and promoted the irregularity of particles to a certain extent, among which hematite had the most significant growth on the long and short axes of the particles. At a content of 10 g kg−1, the addition of AIO and GT increased the soil cohesion and internal friction angle, and therefore increased the soil shear strength, and it was mainly determined by the soil microstructure: the contact area, apparent porosity, and particle short axis. These results indicated that GT and AIO are the main cementing materials affecting soil mechanical properties, and the transformation of iron oxide should be paid attention to when predicting soil slope stability.

1. Introduction

Granite-weathered red soil is extensively distributed in tropical and subtropical regions, and it results from the physical and chemical weathering of granite and consists of coarse particles [1]. Granite-weathered red soil shows heterogeneity, anisotropy, high porosity, shrinkage, low compressibility, and susceptibility to softening and disintegration when exposed to water [1,2,3]. The parent rock of granite-weathered red soil may produce joint structural planes during geological activity, and the structural characteristics are retained in the soil to some extent [4,5,6]. Furthermore, the volume shrinkage deformation of granite-weathered red soil is severe under the frequent dry–wet alternations and causes cracks on the soil surface [4,7]. These structural changes lead to a decrease in soil mechanical strength, especially in shear strength, which accelerates soil erosion and induces natural disasters, such as landslides, debris flows, reservoir silting, and vegetation destruction [8,9]. A collapsing gully is a kind of soil slope collapse phenomenon in the hilly area of granite red soil, and its occurrence process is closely related to soil mechanics characteristics. Some studies have pointed out that there are differences in the physical and chemical properties of soil in different regions, resulting in the spatial heterogeneity of the soil mechanical properties, and the cementation material in a collapsing gully-occurring area is significantly lower than that in a non-collapsing gully-occurring area [10]. Therefore, it is very important to study the relationship between the cementing material and the mechanical characteristics of granite soil for predicting and analyzing the occurrence process of a collapsing gully.
As an important cementing material existing in soil, iron oxide is a key factor affecting the soil mechanical properties [11,12,13,14]. Iron oxide and its hydrates have been reported to enhance the mechanical strength of soil [15,16,17]. In general, free iron oxide cements clay minerals in the form of “bridges”, membranes, or single-crystal particles, thus promoting the formation of soil aggregates [18,19]. Cementation is essentially an electrostatic adsorption process in which positively charged iron oxides adsorb onto clay minerals with negative charges, thus inhibiting the dispersion of soil particles and enhancing the stability of the soil structure [19]. Zhang et al. [19] studied the microstructure characteristics of free iron oxide in Zhanjiang clay soil and Longmen red soil and reported that the removal of free iron oxide caused damage to the water-stable soil aggregate structure, leading to a significant increase in the particle dispersion and clay content. Bai et al. [20] studied the soil strength and deformation characteristics of red soil and reported that the deformation resistance of soil decreases with decreasing free iron oxide content. Zhang et al. [17] studied the physicochemical and mechanical properties of natural clay soil and reported that the removal of free iron oxide reduced the shear strength of the soil. The same results were reported by Chen et al. [10], who studied the physicochemical properties of noneroded and eroded granite-weathered red soil and reported that compared with eroded soil, noneroded soil had a high total iron content and thus superior physicochemical properties, such as greater cohesion and shear strength. Therefore, iron oxide has a significant positive effect on the physicochemical properties of soil. Granite-weathered red soil is rich in iron oxides, and cementation of kaolinite (the main clay mineral) with iron oxide is prevalent [21]. However, cementation of kaolinite with iron oxide is affected by physical and chemical factors, resulting in the destruction, transformation, or migration of iron oxide [22]. In addition, there are many types of iron oxide in soil, which can be divided into amorphous iron oxides and crystalline iron oxides according to their crystal morphology, where crystalline iron oxides mainly include hematite and goethite [23,24,25]. However, the relevant research has focused mainly on the total content of iron oxides, and few studies have clarified the mechanism of influence of different forms of iron oxide on the soil mechanical strength at the microscopic scale.
To study the influence of different forms of free iron oxide on the mechanical properties of granite-weathered red soil, kaolinite, which is the main clay mineral of granite red soil, was selected as the research object. Three different forms of iron oxide (hematite, goethite, and amorphous iron oxide) were added to pure kaolinite, and the changes in the soil microstructure, microscopic quantitative parameters, cohesion, internal friction angle, and shear strength were analyzed to explore the effects of different forms of iron oxide on the mechanical properties of granite red soil. The results provide theoretical support for the prevention of slope instability in granite-weathered red soil.

2. Materials and Methods

2.1. Methods

Experimental materials: Kaolinite (KT) (Shanghai Macklin Biochemical Technology Co., Ltd. Shanghai, China; chemically pure, density: 2.6 g·cm−3, type: k812209, calcined at a high temperature, and kaolinite content ≥ 99%), hematite (Shanghai Macklin Biochemical Technology Co., Ltd. Shanghai, China), goethite (Shanghai Macklin Biochemical Technology Co., Ltd. Shanghai, China), and amorphous iron oxide (162.5 g ferric chloride (FeCl3·H2O) and 120 g sodium hydroxide (NaOH)) were rapidly mixed under intense agitation in cold deionized water, washed with warm deionized water to remove soluble salt, and then centrifuged and freeze-dried to obtain amorphous iron oxide. X-ray diffraction showed that the sample has no significant characteristic peak in the 2θ range (Figure 1), which is consistent with the distinction of amorphous iron oxide. The microstructural characteristics of the different forms of iron oxide are shown in Figure 2.
Experimental steps: A total of 990 g of kaolinite was weighed and placed in a bucket and then 10 g of iron oxide (amorphous iron oxide (AIO), hematite (HT), or goethite (GT) was added, and they were recorded as KT-AIO-10, KT-HT-10, and KT-GT-10, respectively. After uniform mixing, 450 g of deionized water was added to achieve a moisture content of 45%, and the sample was then passed through a 2 mm sieve into a sealing bag. After 24 h, the mixed soil sample was prepared into a cutting-ring sample with a bulk density of 1.35 g/cm3, a diameter of 6.18 cm, and a height of 2 cm. The moisture content of each sample was increased to 55% with a dropper according to a weighing method. The sample was subsequently packed into a sealed pot, separated by a cross, sealed with a sealing film to prevent water evaporation, and stored in an incubator for 3 days at 25 °C. Then, the shear strength tests and microstructure scanning were carried out. In order to further explore the effect of iron oxide on soil mechanical strength, the effect of iron oxide with a 3 g kg−1 concentration on the soil shear strength was compared in the shearing test, and they were recorded as KT-AIO-3, KT-HT-3, and KT-GT-3, respectively.
Soil shearing test: The shear strength test was carried out with a quadruple direct shear apparatus (LH-DDS-4, Nanjing TKA Technology Co., Ltd., Nanjing, China). The water content of the mixed soil sample and pure kaolinite was adjusted to 15%, and a cutting-ring sample with a bulk weight of 1.35 g·cm−3 and a volume of 60 cm was prepared. The samples were subjected to undrained fast shear tests under vertical pressures σ of 50, 100, 200, and 300 kPa; a shear rate of 0.8 mm·min−1; and a maximum shear displacement of 6 mm.
Soil microstructure analysis of mixed samples of pure kaolinite and iron oxide of different forms: Glue was dropped onto the surface of each freeze-dried cutting-ring sample. After the glue dried, the fresh section of the sample was slowly removed with tweezers, the disturbed particles were subsequently blown away, and a sample with the original structure was obtained. The sample was fixed on the preparation table with a conductive adhesive and sprayed with gold to increase the electrical conductivity. Images of the mixed soil samples were obtained via scanning electron microscopy (Fhenom Pro-X, Eindhoven, The Netherlands), and a 10 kV acceleration voltage and backscatter mode was applied to capture SEM images of the soil sample. Image-Pro Plus 6.0 processing software was used for image preprocessing (noise reduction, intensity enhancement, etc.). The subsequent image processing was divided into two steps: Image segmentation: The threshold of the structure of the segmentation object was determined. Eigenvalue acquisition: After the threshold was determined, the SEM image was converted into a binary image according to the threshold, and then, the microscopic quantification parameters of the mixed soil sample were obtained. The detailed information of the method for calculating the microscopic parameters is given in the literature by Wang et al. [26] and Zhang et al. [27].

2.2. Data Analysis

2.2.1. Data Calculation

The shear strength parameters were calculated according to the different axial normal stresses, and the Coulomb equation was used to calculate the shear strength index. The equation is shown in (1):
τ = c + σ tanφ
where τ is the soil shear strength (kPa); c is the soil cohesion (kPa); σ is the normal stress on the soil (kPa); and φ is the internal friction angle (°).

2.2.2. Statistical Analysis

SPSS 26 software was used to perform the analysis of variance and regression analysis. Multiple comparisons were made using Duncan’s test at the 95% significance level (p < 0.05) to compare the differences among the treatments. Excel 2016 was used for chart drawing and data processing.

3. Results

3.1. Changes in the Soil Microstructure After the Addition of Different Forms of Iron Oxide

The microstructure of the pure kaolinite sample without iron oxide was relatively loose at 5000 times magnification (Figure 3a). The soil particles were mainly composed of sheets superimposed on each other, while a few particles had irregular shapes. The contact form was mainly face-to-surface contact, and the microscopic pore structure of the soil was not obvious. A layer of a flocculent white material was clearly attached to the surface of the soil particles after the addition of hematite (Figure 3b), resulting in soil particles that were superimposed on each other and tightly cemented, thus forming larger soil particles, and some particles even “huddled” together into large particles. The soil microcosmic image was more structured, and the soil particle size was larger than that for pure kaolinite. After the addition of goethite (Figure 3c), the cementation of the soil particles became obvious, and the aggregated soil particles were larger than those in the soil with hematite. In addition, many acicular substances were interspersed inside the aggregated soil particles or attached to the surface of the particles. Compared with that of pure kaolinite, the soil microstructure clearly changed. Notably, the soil microstructure also significantly changed after the addition of amorphous iron oxide (Figure 3d). However, the cementation and agglomeration abilities of the amorphous iron oxide group were between those of hematite and goethite. The addition of amorphous iron oxide led to a soil microstructure with flocculation cementation, similar to that obtained with hematite, and a large degree of particle aggregation, similar to that obtained with goethite. The soil microstructure was superior to that of pure kaolinite.

3.2. Quantitative Analysis of Microstructural Parameters of Ferric Oxide–Kaolin Complex

The influence of the three forms of iron oxide on the contact of kaolin particles is different (Table 1). After adding amorphous iron oxide and goethite, the contact area of the soil particles increased slightly, which increased by 3.79% and 3.48%, respectively, compared with the pure kaolin group, but the difference was not significant. When hematite was added, the contact area of the soil particles was significantly reduced by 58.6% compared with that of the pure kaolin group. From the results of the fractal dimension comparison, the soil fractal dimension of the hematite group is significantly different from that of the amorphous ferric oxide group, goethite group, and pure kaolin group. But the difference in the fractal dimension among the amorphous iron oxide group, goethite group, and pure kaolin group is not significant. Among the four groups of soil samples, the fractal dimension of the pure kaolin group is the largest, with a value of 1.22. The amorphous iron oxide group and goethite group followed, with a value of 1.21. The hematite group is the smallest, with a value of 1.20. The apparent porosity of soil in the amorphous iron oxide group, hematite group, and goethite group is higher than that in the pure kaolin group, and the apparent porosity of soil in the hematite group is the largest (52%). The amorphous iron oxide group and goethite group followed, with an apparent porosity of 51%. The pure kaolin group is the smallest (50%). From the results of the microstructural quantification parameters such as the roundness, abundance, particle length axis, and particle length axis, the addition of three different forms of iron oxide did not cause significant changes in the soil particle morphology. However, from the change in the quantization parameter value of the microstructure, the addition of different forms of iron oxide caused certain changes in the soil particle morphology. Among the four groups of soil samples, the roundness of the pure kaolin group is the largest, with a value of 5.86. The amorphous iron oxide group is the second group; the value is slightly lower than the pure kaolin group, and the value is 5.82. Then, for the goethite group, the value is 5.37. The hematite group has the smallest value (5.22). On the contrary, the abundance of the four soil samples was the highest in the hematite group and goethite group, both of which were 0.62. The amorphous iron oxide group and pure kaolin group followed, with values of 0.60 and 0.61, respectively. The change trend of the long axis and short axis of the particles is consistent, and the hematite group, goethite group, amorphous iron oxide group, and pure kaolin group decrease successively. Among them, the long axis values of the particles decreased in the following order: 2.59, 2.46, 2.33, and 2.27. The short axis value of the particles decreased in the following order: 1.60, 1.52, 1.39, and 1.38. Therefore, the above results show that the addition of three different forms of iron oxide will affect the soil microstructure. The addition of iron oxide changes the shape of the soil particles to a certain extent, making them “irregular”, which affects the connection between the particles and then leads to changes in the soil micropores. Among the three forms of iron oxide, hematite has the greatest effect on the soil micropore structure, followed by goethite and finally amorphous iron oxide.

3.3. Changes in Cohesion, Internal Friction Angle, and Soil Shear Strength After the Addition of Different Forms of Iron Oxide

The addition of different forms of iron oxide to pure kaolinite resulted in significant changes in the soil cohesion (Figure 4). Compared with KT, the cohesion of the KT-HT-3 decreased by 11.03%, while the KT-GT-3 and KT-AIO-3 increased the soil cohesion by 2.92% and 5.80%, respectively. The cohesion of the KT-HT-10 increased by 22.01%, that with goethite increased by 25.10%, and that with amorphous iron oxide significantly increased by 37.45%. Notably, there were no significant differences in the internal friction angle. Compared with that of pure kaolinite, the internal friction angle of the KT-HT-10 was reduced by 5.56%, while the internal friction angles of the soil with goethite and amorphous iron oxide were slightly greater, increasing by 3.06% and 1.26%, respectively. These results indicated that the influence of different forms of iron oxide on the cohesion was greater than that of the internal friction angle, and the increase in the iron oxide concentration mainly increased the cohesion of the soil.
Under different kinds of iron oxide, the shear strength of the soil with a high concentration was higher than that with a low concentration (Figure 5). The shear strengths of the samples with the addition of 10 g kg−1 of the three different iron oxides were significantly greater than that of pure kaolinite under a vertical pressure of 50 kPa; however, there were no significant differences among the three different iron oxide groups. Compared with the KT, the shear strengths of the KT-HT-10, KT-GT-10, and KT-AIO-10 increased by 11.16%, 18.19%, and 20.44%, respectively. When the vertical pressure increased to 100 kPa, the difference in the shear strengths of the three different iron oxide samples with the addition of 10 g kg−1 gradually became obvious; the increase rates slightly decreased, but the shear strengths were still greater than that of the KT. The KT-AIO-10 had the highest shear strength, which was 19.30% greater than that of the KT, followed by goethite and hematite, with shear strengths 12.33% and 7.37% greater than that of the KT, respectively. As the vertical pressure increased to 200 kPa, the shear strengths of the three different iron oxide samples with the addition of 10 g kg−1 were still greater than that of the KT. But the shear strength of the KT-HT-10 was not significantly different from that of pure kaolinite, increasing by only 1.48%. When the vertical pressure increased to 300 kPa, the difference in the shear strengths between the hematite and pure kaolinite groups was the smallest, and the shear strength of the KT-HT-10 was 0.38% lower than that of pure kaolinite. However, the amorphous iron oxide and goethite samples still showed higher shear strengths than that of pure kaolinite, in which the strength of the KT-AIO-10 increased by 7.9%, whereas that of the KT-GT-10 increased by 8.3%. The results showed that the addition of amorphous iron oxide, goethite, and hematite caused changes in the cohesion and internal friction angle, resulting in a change in the shear strength. The addition of goethite can increase the shear strength of soil at different concentrations, and the addition of amorphous iron oxide can improve the shear strength of soil at high concentrations, but the addition of hematite has a limited effect on the improvement in soil shear strength in this study.

3.4. Correlation Analysis of Soil Microscopic Parameters and Shear Strength Parameters

A linear regression analysis (enter) was performed on the microscopic parameters and shear strength parameters under the addition of 10 g kg−1 and found that the contact area, apparent porosity, and particle short axis were the three main factors that affected the soil shear strength. The functional relationships among the microscopic parameters and shear strength parameters were as follows:
c = −822.723 + 0.099 × contact area + 1764.349 × apparent porosity − 17.584 × particle short axis
φ = −32.402 + 0.022 × contact area + 131.587 × apparent porosity − 3.281 × particle short axis

4. Discussion

4.1. Effects of Different Forms of Iron Oxide on the Soil Microstructure

According to the SEM results, when hematite, goethite, and amorphous iron oxide were added to pure kaolinite, the amount of white cementing material significantly increased, and a pore structure formed, which produced a good coating effect compared with pure kaolinite. The main reason for this phenomenon was that white cementing substances (iron oxide colloids) could be regarded as an “adhesive” with a strong adhesiveness, which could attach to the surface of the soil particles, greatly enhance the connection between particles, and promote the formation of soil aggregates and microscopic pore structures. These results were similar to those reported by Ng et al. [11] and Zhang et al. [19]. Cementation is essentially an electrostatic adsorption process in which positively charged iron oxide and negatively charged kaolinite can adsorb onto each other through electrostatic interactions to form a stable iron oxide-kaolinite aggregate [28].
Iron oxide is adsorbed onto the surface of kaolinite (silica tetrahedron) under normal conditions, and the kaolinite and metal oxide can form compounds. Some iron oxides may replace the aluminum-oxygen octahedra, and some may be embedded within the oxide inside kaolinite and form “iron-oxygen octahedra”, thus promoting the stable structure of the iron oxide and kaolinite compound [29,30]. Therefore, the effect of iron oxide on the soil structure was achieved mainly through the covering and bonding of the iron adhesive film, which formed more stable kaolinite compounds [31].
The soil particle coating and particle aggregation effects showed the following order: HT < AIO < GT, in which hematite had the weakest cementation effect on the soil particles. Compared with the KT, the KT-HT-10 samples had a large amount of cementing material and an aggregate structure, but the conditions for the formation of complexes in the KT-HT samples were severe: a temperature of 80 °C (pH = 6) was conducive to accelerating the formation of stable hematite–kaolinite complexes [24]. However, the temperature of the experiment was 25 °C in this study, which was not conducive to the formation of stable hematite–kaolinite complexes. Compared with hematite, the conditions needed to form stable goethite–kaolinite complexes are milder. Goethite can adsorb on the surface of kaolinite via electrostatic interactions; moreover, goethite α-FeO(OH) has a special octahedral structure, and the surface of goethite α-FeO(OH) is composed of amphoteric iron hydroxyl groups (≡Fe-OH) and edge-coordinated unsaturated Fe atoms with a positive charge. It easily forms hydrogen bonds with ≡X-O or ≡X-O- and Si-O... Fe, Al-O... Fe coordination bonds on the surface of kaolinite [30]. The structure of the formed goethite–kaolinite complex was superior to that of the KT-HT. Intuitively, although the cementation and agglomeration of AIO on soil particles are inferior to those of goethite, amorphous iron oxide, also known as activated iron, has a higher activity among iron oxides and a larger surface area, which can increase the roughness after combining with particles. Moreover, the chemical properties of the particle surface can be “passivated” by amorphous iron oxide, thus forming a “shielding film” on the surface of the particle, which can stabilize the structure of the particle [15,25]. Therefore, the microstructure indicates that amorphous iron oxide exhibits both the flocculation cementation of hematite and the coating aggregation of goethite. These two factors are beneficial to the formation of a pore structure and soil aggregates. Moreover, amorphous iron oxides are metastable and can be converted to crystalline iron oxides (including goethite and hematite) via dissolution or reprecipitation [32], and the structure of the formed complexes is stronger than that of complexes obtained via “mechanical mixing”.

4.2. Effects of Different Forms of Iron Oxide on the Cohesion, Internal Friction Angle, and Shear Strength of Soil

Iron oxides are among the important factors affecting the soil mechanical strength, and free iron oxide plays the most obvious role among the iron oxides [14,33]. Iron oxides, including crystalline iron oxides (hematite and goethite) and noncrystalline iron oxides (amorphous iron oxide), have a strong ability to undergo cementation with clay minerals to form an aggregate structure, thus reducing the dispersion and weakening the expansion and shrinkage of particles; therefore, there is a strong viscosity between particles [14,18,22]. The study results revealed that compared with that of pure kaolinite, the cohesion of soil containing different forms of iron oxide significantly increased, and the effect is more obvious in a high concentration. The cohesion of the amorphous iron oxide-kaolinite sample was the highest, followed by the goethite-kaolinite sample, the hematite-kaolinite sample, and the pure kaolinite sample. The effects of different forms of iron oxide on the soil mechanical properties differed. Amorphous iron oxide had the highest activity, was easily transformed under different water contents, had better “wedgeness” than kaolinite, and had a significant effect on cohesion [34]. The effect of hematite on cohesion was not as significant as that of goethite. The main reason was that goethite easily formed a goethite–kaolinite complex with kaolinite. Although hematite could also form complexes with kaolinite, the formation conditions were severe, and the coating effect of hematite was weaker than that of goethite; therefore, the mechanical properties of soil containing goethite were generally better than those of soil containing hematite [30].
In this study, there were no significant differences in the internal friction angle when different forms of iron oxide were added; however, the addition of amorphous iron oxide and goethite with high concentrations increased the internal friction angle to a certain extent. In contrast, the internal friction angle after the addition of hematite was slightly lower than that of pure kaolinite. The contact area of the hematite-kaolinite sample was significantly lower than that of the other three samples; moreover, the fractal dimension of pure kaolinite was the highest, followed by the amorphous iron oxide-kaolinite sample and the goethite-kaolinite sample, and that of the hematite-kaolinite sample was the lowest and significantly different from those of the other three samples. The fractal dimension represents the complexity of the soil physical structure, and the soil structure becomes simple as the fractal dimension decreases [9]. Therefore, according to the microstructure image results, hematite promoted particle cementation, but the complexity of the microstructure and roughness of the particles were lower than those of the KT-AIO, KT-GT, and KT, resulting in a slight reduction in the internal friction angle. In addition, different forms of iron oxide could influence the soil particle morphology to a certain degree, although the influence was not significant. The roundness of the mixed samples containing different forms of iron oxide was lower than that of the KT, whereas the abundance was slightly greater than that of pure kaolinite, resulting in an “irregularity” of the particle morphology. Moreover, the iron oxide coating on the soil particles increased after the addition of iron oxide, which promoted the “growth” of the long and short axes of the particles, leading to larger soil particles than those in pure kaolinite. The formation of the irregular-shaped and large-size particles was caused by the cementation of iron oxide with clay minerals, which indicated that the cementation of the iron oxide coating was obvious. Compared with the other three soil samples, pure kaolinite had more fine particles and greater dispersion due to the lack of iron oxide cement. The results revealed that at a concentration of 10 g kg−1, the contact area of the AIO-KT sample and GT-KT sample were both larger than that of the KT sample. However, the contact area of HT-KT is smaller than that of the KT samples. Therefore, the internal friction angle of the AIO-KT sample and GT-KT sample is slightly greater than the KT.
The cohesion and internal friction angle are important indicators of the soil shear strength [35,36]. The cohesion and internal friction angle are affected by the soil microstructure [17,37]. The results revealed that the amorphous iron oxide-kaolinite samples presented the highest soil shear strength under different vertical pressures, followed by the goethite-kaolinite samples, hematite-kaolinite samples, and pure kaolinite samples, in accordance with the effects of the different forms of iron oxide on the soil microstructure discussed above. Amorphous iron oxide had a high number of active sites and a high specific surface area and exhibited the flocculation cementation of hematite and the coating agglomeration of goethite, so it could cement with soil particles and increase the roughness of the particles, thus increasing the cohesion, internal friction angle, and shear strength of the soil [24,25,30].
Goethite could be interspersed inside the aggregated soil particles or attached to their surface because of its acicular cementation characteristics and produced greater cohesion and internal friction angle than hematite with flocculation cementation. In addition, goethite combined with kaolinite was looser than hematite combined with kaolinite [30]. Therefore, the shear strength of the KT-GT was higher than that of the KT-HT samples. The KT-HT-10 had greater cohesion than the KT because of the flocculation property of hematite; however, the microstructure of the samples was relatively simple, and the contact area of the particles was significantly lower than that of the other three soil samples, leading to a lower particle roughness and internal friction angle. Therefore, the soil shear strength of the hematite-kaolinite samples was lower than that of the amorphous iron oxide-kaolinite samples and goethite-kaolinite samples. In summary, the effects of amorphous iron oxide, goethite, and hematite on the soil mechanical strength differed. Amorphous iron oxide had the most significant effect on the soil mechanical strength, followed by goethite, whereas hematite had a slight effect on the soil mechanical strength.

5. Conclusions

The effects of different forms and concentrations of iron oxide on the soil microstructure and shear strength were investigated in this study. The results revealed that the soil microstructure and shear strength clearly changed after the addition of amorphous iron oxide, hematite, or goethite. The addition of different forms of iron oxide could promote the cementation of soil particles, which could change the contact area of the particles and increase the number of micropores and particle sizes, leading to changes in the cohesion, internal friction angle, and shear strength of the soil. The cementation effects of the three kinds of iron oxide on the soil particles were different. Hematite mainly showed flocculent cementation and a poor cementation effect. Goethite mainly exhibited acicular cementation and the best cementation effect. The cementation effect of amorphous iron oxide was between those of hematite and goethite but included both the flocculation cementation of hematite and acicular cementation of goethite. The shear strength decreased in the following order: KT-AIO- > KT-GT- > KT-HT- > KT. In conclusion, iron oxide is an important cementing material in soil, among which amorphous iron oxide and goethite are the main iron oxide forms that promote the formation of the soil structure and increase the shear strength of soil. In the future, we will further study the effects of different forms of iron oxide on other mechanical properties of soil, including expansion and contraction, the liquid-plastic limit, filtration resistance, etc., and explore the possibility of using iron oxide to improve soil mechanical properties.

Author Contributions

Conceptualization, Y.Z. and H.Y.; methodology, N.C.; software, N.C.; validation, H.Y., F.J. and Q.Z.; formal analysis, N.C. and H.Y.; investigation, Q.Z.; resources, Y.Z.; data curation, Q.Z.; writing—original draft preparation, H.Y. and Q.Z.; writing—review and editing, Y.Z. and J.L.; supervision, Y.Z.; project administration, Y.H.; funding acquisition, Y.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Significant Science and Technology Project of the Ministry of Water Resources, grant number: SKS-2022073, and the Water Conservancy Science and Technology Project of Fujian Province, grant numbers: KJG21009A and MSK202217.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Minerals 15 00016 g001
Figure 1. X-ray diffraction for the amorphous iron oxide sample.
Figure 1. X-ray diffraction for the amorphous iron oxide sample.
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Figure 2. Microstructural characteristics of different forms of iron oxide. (a) Hematite; (b) goethite; and (c) amorphous iron oxide.
Figure 2. Microstructural characteristics of different forms of iron oxide. (a) Hematite; (b) goethite; and (c) amorphous iron oxide.
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Figure 3. Microstructural characteristics of the soil after the addition of different forms of iron oxide. (a) KT; (b) KT-HT-10; (c) KT-GT-10; and (d) KT-AIO-10.
Figure 3. Microstructural characteristics of the soil after the addition of different forms of iron oxide. (a) KT; (b) KT-HT-10; (c) KT-GT-10; and (d) KT-AIO-10.
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Figure 4. Changes in the soil shear strength parameters after the addition of different forms of iron oxide. Bars with different letters are significantly different at p = 0.05 probability level.
Figure 4. Changes in the soil shear strength parameters after the addition of different forms of iron oxide. Bars with different letters are significantly different at p = 0.05 probability level.
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Figure 5. Changes in the soil shear strength after the addition of different forms of iron oxide. (a) Changes in the shear strength of KT with different forms of iron oxides at 10 g/kg concentration, and (b–d) changes in the soil shear strength at different concentrations (3–10 g/kg) of HT, GT, and AIO, respectively.
Figure 5. Changes in the soil shear strength after the addition of different forms of iron oxide. (a) Changes in the shear strength of KT with different forms of iron oxides at 10 g/kg concentration, and (b–d) changes in the soil shear strength at different concentrations (3–10 g/kg) of HT, GT, and AIO, respectively.
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Table 1. Quantitative analysis of microstructural parameters of ferric oxide–kaolin complex.
Table 1. Quantitative analysis of microstructural parameters of ferric oxide–kaolin complex.
Contact Area/μm2Fractal DimensionApparent PorosityRoundnessAbundanceParticle Long Axis/μmParticle Short Axis/μm
KT296.10 ± 18.39 a1.22 ± 0.00 a0.50 ± 0.01 a5.86 ± 0.24 a0.61 ± 0.01 a2.27 ± 0.02 a1.38 ± 0.00 a
KT-HT-10122.59 ± 16.81 b1.20 ± 0.00 b0.52 ± 0.01 a5.22 ± 0.21 a0.62 ± 0.01 a2.59 ± 0.09 a1.60 ± 0.08 a
KT-GT-10306.41 ± 18.77 a1.21 ± 0.00 ab0.51 ± 0.01 a5.37 ± 0.12 a0.62 ± 0.00 a2.46 ± 0.02 a1.52 ± 0.02 a
KT-AIO-10307.32 ± 32.62 a1.21 ± 0.00 ab0.51 ± 0.01 a5.82 ± 0.46 a0.60 ± 0.02 a2.33 ± 0.26 a1.39 ± 0.19 a
Note: Different lowercase letters indicate significant differences (p < 0.05).
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Yan, H.; Cen, N.; Zheng, Q.; Lin, J.; Jiang, F.; Huang, Y.; Zhang, Y. Analysis of the Shear Strength of Iron Oxide-Kaolinite Cementing Materials in Granite Red Soil. Minerals 2025, 15, 16. https://doi.org/10.3390/min15010016

AMA Style

Yan H, Cen N, Zheng Q, Lin J, Jiang F, Huang Y, Zhang Y. Analysis of the Shear Strength of Iron Oxide-Kaolinite Cementing Materials in Granite Red Soil. Minerals. 2025; 15(1):16. https://doi.org/10.3390/min15010016

Chicago/Turabian Style

Yan, Hualing, Nanbo Cen, Qinmin Zheng, Jinshi Lin, Fangshi Jiang, Yanhe Huang, and Yue Zhang. 2025. "Analysis of the Shear Strength of Iron Oxide-Kaolinite Cementing Materials in Granite Red Soil" Minerals 15, no. 1: 16. https://doi.org/10.3390/min15010016

APA Style

Yan, H., Cen, N., Zheng, Q., Lin, J., Jiang, F., Huang, Y., & Zhang, Y. (2025). Analysis of the Shear Strength of Iron Oxide-Kaolinite Cementing Materials in Granite Red Soil. Minerals, 15(1), 16. https://doi.org/10.3390/min15010016

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