Analysis of the Mineral Compositions of Lateritic Clay in Guangxi and their Influence

In order to study the effect of mineral composition on the physical properties of lateritic clays, the mineral compositions of four kinds of lateritic clay from Guangxi province in China were quantitatively analyzed by means of DTA, XRD, SEM, XRF and total chemical element analysis. On this basis, the variation law of macroscopic physical properties of lateritic clay was analyzed from the perspective of mineralogy. The results show that kaolinite and goethite are the main minerals of lateritic clays from Guangxi in China. The mineral compositions of these four samples are quantitatively and accurately analyzed by Bogue’s method. According to the study on mineral compositions of soil samples, the physical properties of lateritic clay are related to its clay minerals and cemented substances. The liquid limit (LL) or plastic limit (PL) increases with the increase of the ability and content of clay minerals to adsorb water. The lateritic clay is dominated by kaolinite minerals with low adsorption capacity, so the observed boundary water content (LL and PL) of high value of lateritic clays can be explained by the contribution of “inert water” in soil pores. The iron-bonded cement minerals (the form of existence is goethite) in lateritic clay have great influence on the liquid and plastic limits, which decrease linearly with the increase of goethite, whereas the goethite has relatively smaller effect on shrinkage characteristics. It is believed that the shrinkage characteristics of lateritic clay may also be affected by other factors.


Introduction
Undisturbed lateritic clay has good mechanical properties but poor physical properties. It is widely distributed in Guangxi province of China [1][2][3][4][5], especially in Guilin, Liuzhou, Laibin and Wuming. In recent years, due to the extreme climate in China, engineering diseases such as slope instability, uneven settlement of foundations and cracking of embankments [6,7] have occurred frequently in lateritic clays. Due to the strong hydrophilic nature of lateritic clay, the physical-mechanical property changes caused by soilwater action are very complex, and the mineral composition as the main carrier is the material basis to explain the changes in its relevant physical-mechanical properties. In the 1950s, Norrish [8] studied the expansibility of montmorillonite crystals. Tan [9] studied the expansion and contraction mechanism of montmorillonite crystal, and discussed the relationship between the crystal plane spacing of montmorillonite and the type and concentration of electrolyte solution. Tan and Kong [10] studied the expansion and contraction laws of montmorillonite crystals, and analyzed the reasons for the di erent expansion and contraction characteristics of unsaturated montmorillonite crystals. Wang [11] rst studied the cementation of lateritic clay and discussed the in uence of free iron oxide on the engineering properties of lateritic clay. e results show that the engineering mechanical properties of lateritic clay are close to ordinary clay after removing free iron oxide. Kong et al. [12] further studied the cementation characteristics of lateritic clay, and the results show that only a part of free iron oxide really plays the role of cementation. e above research shows that the mineral composition has a significant impact on the physical and mechanical properties of soil.
For a long time, scholars at home and abroad have done a lot of research on the composition and microstructure of clay minerals and cementitious substances in soils [13][14][15][16][17]. In terms of quantitative analysis of mineral composition, Tan [18] used a simplified algorithm for quantitative analysis of clay minerals. Zhang and Fan [19] quantitatively analyzed clay minerals by X-ray diffraction phase quantitative method. Dai et al. [20] first qualitatively identified the mineral composition by X-ray diffraction, and then quantitatively analyzed the content of mineral composition according to the intensity and half height width of diffraction peak. However, the above quantitative analysis methods have their own limitations.
In this paper, the mineral composition of four kinds of lateritic clay in Guangxi province of China is qualitatively identified and quantitatively analyzed by means of differential thermal analysis(DTA), X-ray diffraction(XRD), scanning electron microscope(SEM), X-ray fluorescence analysis(XRF) and full chemical element analysis. On this basis, the influence law of mineral compositions on the physical properties of lateritic clay is revealed. is research provides a new way to further study the engineering characteristics and genesis of lateritic clay in this area.

Soil Samples
e test soil samples were taken from four typical distribution areas of lateritic clay in Guangxi province of China as follows: Guilin, Liuzhou, Laibin and Wuming, numbered as S-1, S-2, S-3, and S-4, respectively. After indoor geotechnical tests, the basic physical property indexes of the four lateritic clay specimens are shown in Table 1 [21].

Qualitative Analysis of Mineral Composition.
Clay minerals in clayey soils are mainly present in the colloidal (less than 2 μm) group, therefore, the separation/preparation of colloidal particles in soil samples needs to be carried out in advance by physicochemical methods before the experiments [22,23], and the specific experimental steps are as follows.
(i) According to the required amount of sample, weigh the air-dried sample with particle size less than 0.075 mm and place it in a small beaker for wetting and standby. (ii) Add hydrochloric acid solution with the concentration of 0.1 mol/L, fully stir to completely decompose carbonate or other soluble salts in the soil sample, and discard the upper part by decantation method after static clarification. (iii) Continue to treat the sample with dilute hydrochloric acid with pH � 4 to remove calcium and magnesium ions from the filtrate.
(iv) Place the treated sample in a constant temperature water bath at 40°C, add 30% H 2 O 2 and stir continuously to remove the organic matter in the soil sample. (v) Add sodium citrate buffer, heat it to 80°C in a constant temperature water bath, add sodium bisulfite in several times and stir fully to remove the iron cement in the sample. (vi) After adding 2% sodium carbonate solution for dispersion treatment, separate the colloidal particles of the treated sample by densitometer method, and finally dry the separated sample for standby.
3.1.1. Differential ermal Analysis (DTA) Test. Extract samples from Wuming (less than 2 μm particle size) for DTA test. Labsys synchronous thermal analyzer produced by Setaram Company of France is used for the test. e test conditions are as follows: the temperature rises from room temperature to 1000°C, the heating rate is 10°C/min, the cooling rate is 50°C/min, and the test reference sample is pure Al 2 O 3 .

X-ray Diffraction (XRD) Analysis
Test. X-ray diffraction analysis (XRD) is widely used for the identification of clay minerals and the study of their crystal structure. e principle is based on the fact that different minerals have different crystalctures; the powder crystal method was used for the qualitative analysis of the four soil samples mentioned above. e following conditions were used: tube voltage 40 kV, tube current 40 mA, initial angle 4°, termination angle 32°, scanning rate 6°/min, angle 0.0170°(2θ) per step and test temperature 25°C.
e test was carried out using an X-Pert PRO X-ray diffractometer from PANalytical B. V.

Scanning Electron Microscope (SEM) Test.
Microstructural observations were carried out on the raw and extracted samples (colloidal particles smaller than 2 μm) of the four lateritic clays mentioned above with the aid of a scanning electron microscope to analyze the morphology of the mineral constituents and the way in which the particles lap each other. Quanta-200 scanning electron microscope was used in the test. e test conditions are as follows: the resolution of dry sample under high vacuum 30 kV environments is 3.5 nm, and that of water sample under low vacuum 3 kV is 15 nm.

Quantitative Analysis of Mineralogical Composition.
In this paper, the Bogue method was used for the quantitative analysis of mineral compositions. A prerequisite for the successful application of this method is the accurate acquisition of chemical elemental results for the minerals. X-ray fluorescence analysis (XRF) and full chemical elemental analysis experiments were used to determine the elements contained in the four soil samples. 2 Advances in Materials Science and Engineering X-ray uorescence analysis (XRF) test: is is a method of qualitative and quantitative analysis of elements using X-ray excitation of the sample to produce characteristic uorescence spectral lines with high accuracy for the analysis of elements. In this paper, an Axios X-ray uorescence analyzer (PANalytical B. V.) was used. e test conditions were as follows: radiation source is Rh target; radiation tube is 2.4 kW; fused sheet method is for the sample; and atomic absorption spectrometry is for the determination of carbon.
Full chemical element analysis test: e test was completed in the material experiment center of Guilin University of technology. SiO 2 , Al 2 O 3 , Fe 2 O 3 , Cao, MgO, K 2 O, Na 2 O, MnO and other elements were measured in the sample.
e Bogue method assumes that: ①Only the phases analyzed by the X-di raction test are present in the soil sample and other trace minerals are ignored, i.e., the sum of mineral contents analyzed by X-ray di raction test is 100%; ②If a mineral contains an element, but other minerals do not contain the element, the element can be used to determine the content of the mineral in the soil sample [17,24]. Based on these two assumptions, using the molecular weight of the element, the theoretical molecular weight of the mineral and the measured elemental content, (1) can be formulated to nd the content of each mineral in the soil sample.
Where, a, b, c are the equation coe cients, respectively, whose values are the ratio of the molecular weight of the element to the theoretical molecular weight of the mineral containing the element; x, y, z are the percentage content of the mineral sought, respectively; and A, B, C are the percentage content of the measured element, respectively.

Di erential ermal Analysis
Tests. e DTA curve of Wuming lateritic clay (S-4) is shown in Figure 1. It can be seen that Wuming sample has endothermic reaction at 250-300°C, with medium intensity endothermic peak, strong endothermic peak at 500-550°C and obvious exothermic peak at about 950°C. A check of the Handbook of Di erential ermal Analysis for Mineral Identi cation [25] revealed that the rst heat-absorbing peak was characteristic for gibbsite. By analogy, kaolinite-like minerals are also present in the Wuming sample. e kaolinite-like minerals may be one or more of kaolinite/ellite/iron ellite, but ellite, iron ellite would have a clear valley of heat absorption reaction between 100 and 200°C, which is clearly inconsistent with the results in Figure 1, from which it can be inferred that the kaolinite-like minerals are kaolinite.

X-ray Di raction Test.
e X-ray di raction curves of Wuming samples with di erent particle groups are shown in Figure 2. It can be seen from Figure 2 that there are three obvious peak lines in Wuming sample, d 0 are 7.15Å, 3.58Å and 4.8Å, respectively. In fact, 7.15Å and 3.58Å are typical characteristic peaks of kaolinite [26], so it can be concluded that kaolinite exists in Wuming sample. While 4.8Å is a characteristic peak of gibbsite, which leads to the preliminary conclusion that gibbsite is present in the Wuming sample. Comparison of Figures 2(a) and 2(b) shows that the di raction curves of the original sample and the sample smaller than 75 μm are basically the same, indicating that the particles smaller than 75 μm are predominant in the Wuming sample. In addition, Figure 2(c) shows that the 4.17Å peak line is missing from the curve of the less than 2 μm sample compared to the original sample and the less than 75 μm sample. In other words, the 4.17Å peak line is also present in the Wuming sample, which is a characteristic peak for goethite. Goethite is usually red, but the sample less than 2 μm is white, i.e. no needle iron ore is present, indicating that goethite has been removed, whereas the original sample and the less than 0.075 mm soil sample are mainly red, indicating the presence of goethite in both. Comparison of the results of the di erential thermal analysis of the Wuming sample shows that the results of the   di erential thermal analysis are in general agreement with the results of the X-ray di raction analysis. Figure 3 shows the di raction curves for S-1, S-2, and S-3. e analysis process was the same as for S-4, i.e. the identi cation of mineral species was completed by comparing the results of the di erential thermal analysis tests with the measured X-ray di raction curves. Overall, the di erential thermal analysis (DTA) and X-ray di raction (XRD) tests were able to determine the main mineral composition of the soil samples from each location, and the results are summarized in Table 2.

Scanning Electron Microscope Test.
e SEM image of Wuming sample is shown in Figure 4. It can be seen that the mineral crystals in the original Wuming sample are hairy spherical and cementitious structure. Because the Wuming sample is wrapped by free iron oxide, it is di cult to see the type and connection mode of internal micro units. e micro units in the extracted samples are irregular akes, with di erent thickness and size, scattered accumulation and disorderly arrangement. e dissolved water sample expands due to water absorption, the ake particles increase, the micro unit is hexagonal, the contour is clear, the thickness is uniform, the edges are obvious, and the ake basic units are in face-to-face contact and superimposed together, which is a typical feature of kaolinite. In conclusion, the experimental results of DTA, XRD and SEM are consistent with each other.

Quantitative Analysis Results of the Bogue Method.
e results of the XRF analysis and the full chemical elemental analysis are shown in Tables 3 and 4, respectively, and the comparison shows that the results of the two chemical elemental analysis methods for the same elemental mass percentages are basically the same, indicating that the results of the chemical elemental analysis of the above four lateritic clays are reliable.
Due to space limitations, this paper only takes S-4 (Wuming lateritic clay) as an example to brie y introduce the process of quantitative analysis of mineral components in soil by Bouge method. It can be seen from Table 2 that the mineral components of Wuming lateritic clay are kaolinite, gibbsite and goethite. Table 5 shows the content and theoretical molecular weight of various elements in S-4 ( e measured element content is taken from the XRF analysis test). According to formula (1) and Table 5, the mass percentages of kaolinite, gibbsite and goethite can be calculated, respectively, as follows. Goethite: 8.53% × 79 × 2 160 9.49%. Kaolinite: 36.48% × 258 60 × 2 78.43%. Gibbsite: 1 − 78.43% − 9.49% 12.08%.
By analogy, based on the data of the elemental content measured by full chemical elemental analysis and XRF, the mineral composition content in various samples can be calculated by the Bogue method respectively, and the calculation results and error analysis are summarized in Tables 6 and 7, respectively.
As can be seen from Table 7, for the Wuming lateritic clay (S-4), the error values for the percentage mineral content solved from the results of the full chemical elemental analysis method are relatively small, and the error in the results solved by the XRF method is also small. is is due to the relatively small number and single type of major mineral components in the Wuming lateritic clay, which results in an overall small error.

In uence of Mineral Composition on the Physical
Properties of Lateritic Clay Figure 5 shows the relationship between kaolinite content (as listed in Table 6) and the Atterberg's limits (liquid limit LL and plastic limit PL, which as listed in Table 1) of the four lateritic clays mentioned above. It can be seen from Figure 5 that the correlation between kaolinite and the Atterberg's limits is very weak; However, the values of liquid limit (LL) and plastic limit (PL) are generally rather high. According to the electric double layer theory [27][28][29][30][31][32], strongly bound water is the adsorbed water layer under the action of particle surface adsorption energy, and weakly bound water is the permeable water layer between di usion layer and free liquid under osmotic pressure [33][34][35]. e more the surface charges of clay minerals, the stronger the adsorption energy, the thicker the adsorbed water layer, and the more strongly and weakly bound water. e research shows that there is an obvious correlation between strongly bound water -plastic limit (PL) and weakly bound water -liquid limit (LL) [36][37][38]. Because the lateritic clays are dominated by kaolinite minerals with low adsorption capacity, this better explains the weak correlation between kaolinite and the Atterberg's limits (PL and LL). Xue and Bian [39] pointed out that the pores in lateritic clay are mainly closed or semi closed small pores, and the water in these small pores is often    Advances in Materials Science and Engineering called "inert water", whose character is similar to weakly bound water. erefore, the authors believe that the relatively high liquid and plastic limits of lateritic clay are related to the existence of "inert water" in pores. Figure 6 shows the relationship curve between kaolinite content (as listed in Table 6) and the free swelling rate (as listed in Table 1) of the four lateritic clays. As can be seen from Figure 6, the correlation between kaolinite content in lateritic clays and its free swelling rate is rather weak. From the results of the qualitative and quantitative analysis of the mineral composition, it can be seen that the kaolinite content of the lateritic clay specimens is basically above 70%, but the kaolinite is not entirely present in a loose state of clay particles, and some of the kaolinite is wrapped by ferric substances, forming larger agglomerates of agglomerates. Scanning electron microscope (SEM) analysis shows that the encapsulated kaolinite tends not to swell or swells very weakly (see Figure 4). In addition, due to the weaker lattice substitution ability of kaolinite, the particles have relatively little surface charge and are less able to adsorb water molecules. is could better explain the relatively low free swelling of lateritic clay and the weak correlation with kaolinite content.

Influence of Goethite on Physical Properties of Lateritic
Clay. Figure 7 is the relationship curve between the goethite content (as listed in Table 6) and physical properties index (as listed in Table 1) of lateritic clay. It can be seen from Figure 7(a) that the values of liquid limit (LL), plastic limit(PL) and shrinkage limits(SL) in lateritic clay decrease with the increase of goethite content, and the relationship between goethite content and Atterberg's limits mentioned    Advances in Materials Science and Engineering above is basically linear. It can be seen from Figure 7(b) that the shrinkage coe cient increases with increase of goethite content, but the correlation is weak comparatively. Tan [40] analyzed the particularity of lateritic clay from the perspective of microstructure and found that free iron oxide has cementation e ect on clay particles. It usually exists in lateritic clay in the form of goethite, which makes the basic unit agglomerate and lls the pores between clay particles. erefore, with the increase of goethite content, the adsorption capacity of soil particles to water decreases, the water loss rate accelerates and the shrinkage coe cient increases. At the same time, the porosity of clay decreases, the content of "inert water" in pores decreases, and the liquid and plastic limits of lateritic clay decrease. However, the shrinkage characteristics of lateritic clay are not only related to goethite, but also a ected by the connection mode    Advances in Materials Science and Engineering between particles, the concentration of pore solution, initial moisture content and other factors, so the correlation between goethite content and shrinkage index is weak.

Conclusion
(1) e mineral types in lateritic clay can be preliminarily determined by di erential thermal analysis (DTA) and X-ray di raction (XDR) experiments. e experimental results show that the mineral components in lateritic clay are mainly kaolinite and goethite.
(2) By comparing and analyzing the commonly used quantitative analysis methods, the quantitative analysis method of mineral composition of Guangxi lateritic clay is determined: rstly, the content of chemical elements is determined by X-uorescence analysis or total chemical element analysis, and then the quantitative analysis of mineral composition is completed by Bogue method according to the chemical elements of soil samples, which provides a more reliable method for the quantitative analysis of mineral composition of soil samples in this area. (3) e physical properties of lateritic clay are mainly a ected by clay minerals and cementing materials: the higher the content of clay minerals which has strong adsorption capacity for water, and the greater the corresponding liquid and plastic limits. Lateritic clay is dominated by kaolinite minerals with low adsorption capacity. Due to the existence of "inert water" in soil pores, lateritic clay also has high liquid and plastic limits. e liquid and plastic limits of lateritic clay decrease linearly with the increase of goethite, but goethite has little e ect on the shrinkage characteristics. e author speculates that the shrinkage characteristics of lateritic clay may also be a ected by other factors such as the connection mode between soil particles, pore solution concentration, initial water content and so on.
Data Availability e data presented in this study are available on request from the rst author or the corresponding author.

Conflicts of Interest
e authors declare that they have no con icts of interest.    Advances in Materials Science and Engineering