# The Interplay of Clay Behavior and Particle Structuring: A Multiscale Investigation

Mengyu Ma, Fares Bennai, Mahdia Hattab, Pierre-Yves Hicher, François Nicot

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Mengyu Ma, Fares Bennai, Mahdia Hattab, Pierre-Yves Hicher, François Nicot. The Interplay of Clay Behavior and Particle Structuring: A Multiscale Investigation. IOP Conference Series: Earth and Environmental Science, 2025, 1480 (1), pp.012096. 10.1088/1755-1315/1480/1/012096. hal-05064262

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Submitted on 19 May 2025

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# The Interplay of Clay Behavior and Particle Structuring: A Multiscale Investigation

To cite this article: M. Ma et al 2025 IOP Conf. Ser.: Earth Environ. Sci. 1480 012096

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# THE INTERPLAY OF CLAY BEHAVIOR AND PARTICLE STRUCTURING: A MULTI-SCALE INVESTIGATION

M. Ma<fnref n="1" />, F. Bennai<fnref n="1" />✉, M. Hattab<fnref n="1" />, P. -Y. Hicher<fnref n="2" />, F. Nicot<fnref n="3" />

## Abstract:

The effects of different particle associations on the mechanical behavior of kaolinite are investigated by coupling the experimental mechanical results with microscopic analyses. Two types of particle structures are generated through sedimentation tests conducted in solutions with different pH values, in which flocculation and dispersion of particles take place. It is shown that the compressibility of flocculated samples with an edge-to-face particle structure is higher than that of dispersed samples with a face-to-face particle structure. The particle associations are strongly influenced by the pH of the surrounding fluid. Mercury Intrusion Porosimetry (MIP) results showed that the dominant pore size of flocculated samples is higher than that of dispersed samples. Scanning Electron Microscope (SEM) observations on the vertical plane of clay samples provided a visualization of the particle arrangement after mechanical load.

## 1. Introduction

Understanding the micromechanical behavior of clay is important to better explain microscopic phenomena and their relationship with the overall mechanical behavior in order to construct relevant multiscale models for clayey materials [1]. Clay behavior is influenced by its microscopic structure, which refers to the clay fabric constructed by different particle associations [2]. The kaolinite particle has a flat hexagonal plate shape and is negatively charged on its surface due to isomorphic substitutions in the silica layer. The charges present at the particle edges are sensitive to the pH of the surrounding solution. These charges are positive in low pH environments and could turn to negative in high pH environments [3]. This effect leads to different configurations of particle assembly, which can be observed during sedimentation processes: flocculation and dispersion [4]. In an acidic environment, the positive charges at the particle edges are attracted to the negative charges on the surface, forming an edge-to-face structure. This results in the aggregation of clay particles as flocs, which then sediment. The accumulation of the aggregated particles forms a structure that resembles a card house or a honeycomb-like clay fabric [5]. Otherwise, when the positive edge charges are neutralized and then converted to negative in an alkaline environment, repulsive forces dominate the interaction between clay particles and the particles sediment individually. Consequently, the particles organize in a parallel way and form a face-to-face structure [6]. The mercury intrusion porosimetry (MIP) technique allows for the analysis of the porosity and the pore size distribution of the clay samples, which enables to establish a connection between the macroscopic mechanical behavior of clay and its microscopic structures [6]. In addition to MIP, scanning electron microscopy (SEM) observation offers a more intuitive understanding of the clay's microstructure [1] [7]. SEM provides high-resolution images that allow for a detailed examination

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<footnote n="1">¹Université de Lorraine, CNRS, Arts et Métiers ParisTech, LEM3, 57000 Metz, France</footnote>

<footnote n="2">²Ecole Centrale de Nantes, CNRS, Research Institute of Civil Engineering and Mechanics (GeM), 44300 Nantes, France</footnote>

<footnote n="3">³Université Savoie Mont-Blanc, ISTerre, 73376 Le Bourget-du-Lac, France</footnote>

<footnote>✉fares.bennai@univ-lorraine.fr</footnote>
of the particle configuration, pore space and the evolution of the microstructure under different
mechanical conditions. This study presents an experimental investigation into the clay behavior coupled
with microstructural analysis provided by MIP and SEM, aiming to relate initial particle structure to the
mechanical behavior of the kaolin clay.

## 2. Material and experiment method

Kaolin powder type K13 which is composed of over 90% of kaolinite and which has a liquid limit W_L = 0.40 and a plastic limit W_P = 0.20 was used in this study. The grain size distribution showed that the mean dimensions of K13 are in the range of 2.5 to 20µm and the density of the solid grains is 2.63 g/cm³ [7]. In order to create different particle structures, two types of sedimentation tests were carried out in two different fluid environments, in which the flocculation and dispersion of the clay particles are generated. Different solutions were prepared by adding different salts in distilled water: 0.01 mol/L of CaCl₂ and NaOH were chosen as the reactive agents to adjust the pH values of the solutions and modify the interaction between clay particles consequently. The measured pH values were 6 and 12 for flocculation and dispersion, respectively. 25g/L of kaolinite powder was mixed with adequate solutions and well stirred. As the particles began to sediment, a separation line between the clay mixture and the supernatant occurred and was followed throughout the sedimentation test. The sedimentation was maintained for at least 7 days until the sediment height no longer evolved. The supernatant was then removed and the sedimented clay slurry was collected in a container and stirred to homogeneity. The homogeneous sedimented clay slurry was subjected to oedometric loading at the following levels of effective vertical stress: 120 and 1000 kPa for the compressibility behavior study. Microscale analysis and observation were conducted on the samples after the mechanical tests, using mercury intrusion porosimetry technique and scanning electron microscopy. The samples subjected to oedometric loading were divided into smaller samples and were frozen in liquid nitrogen. Then samples were dehydrated using the lyophilization technique, which allows the frozen pore fluid to sublime without passing through the liquid phase to avoid disturbing the particle structures. Part of small dehydrated samples was used to MIP analysis, which allows to determine the volume of voids and the pore size distribution (PSD) of the samples. Another part of small samples was gold coated for gathering conductivity and then observed by scanning electron microscope.

## 3. Results and discussion

Classic flocculated free settling and dispersed free settling [4] were observed in flocculation and
dispersion samples, respectively. The final sediment height recorded during the sedimentation process
allowed to calculate the sediment's volume and, in turn, the void ratio of the different sediments. The
results showed that although the flocculated and dispersed samples performed differently during the
sedimentation test, they reached a similar final sediment height, which indicates that the clay sediments
have similar void ratios. This similarity between the two different samples may be attributed to the effect
of the pH values of the surrounding fluid. The pH value of the point of zero charge for kaolin is
approximately 5 [7], while the pH values generated in this study were 6 and 12. Therefore, it can be
inferred that the higher pH medium may have favored the repulsion forces between the particles during
dispersion, resulting in an increased distance between particles.
Table 1 presents the mechanical parameters and results of the oedometric loading tests. The dispersed samples present a higher final void ratio at each effective stress level. The compressibility index Cc of the flocculated samples determined by the slope of the loading path, is higher than that of the dispersed samples. The higher compressibility suggests that the edge-to-face structure of the flocculated samples has a higher void volume and that the structure may change under mechanical load. The similarity of the swelling index Cs of the specimens indicates a similar microstructure of different clay samples after loading. The compressibility and swelling indexes are both intrinsic parameters of the material.

<table><caption>Table 1. Results of oedometric loading tests of different samples.</caption><thead><tr><th>Initial state</th><th>Particle structure</th><th>pH</th><th>Vertical stress (kPa)</th><th>Initial void ratio e<sub>0</sub></th><th>Final void ratio e<sub>f</sub></th><th>Cc</th><th>Cs</th></tr></thead><tbody><tr><td rowspan="4">Slurry</td><td rowspan="2">Flocculated</td><td rowspan="2">6</td><td>120</td><td>1.85</td><td>1.21</td><td rowspan="2">0.56</td><td rowspan="2">0.13</td></tr><tr><td>1000</td><td>1.96</td><td>1.05</td></tr><tr><td rowspan="2">Dispersed</td><td rowspan="2">12</td><td>120</td><td>1.59</td><td>1.27</td><td rowspan="2">0.40</td><td rowspan="2">0.12</td></tr><tr><td>1000</td><td>1.78</td><td>1.19</td></tr></tbody></table>

The PSD results, obtained by MIP, are presented in Figure 1. Flocculated samples consistently exhibit larger dominant pores compared to dispersed samples at each stress level. The final void ratios of the samples were also calculated using the total volume of mercury introduced in the small samples, and the results were consistent with those obtained in the mechanical tests, where the flocculated samples exhibited a lower void volume than the dispersed samples. However, the difference between these two types of samples was relatively minor, suggesting that they may have similar microstructures at this stage. Indeed, from a certain level of stress, mechanical loading induces a reorganization of the initial microstructures (flocculated and dispersed) and leads to similar structures.

<figure><img src="image_8.png" /><figcaption>Figure 1. Pore size distribution of different samples at different stress levels.</figcaption></figure>

The vertical planes of small samples (i.e., the plane perpendicular to the surface normal to the vertical stress) were observed by SEM. Figure 2 shows the SEM images of two types of samples under a 120 kPa vertical load. Both samples showed a clear preferred orientation of particles that perpendicular to the stress axis. Some edge-to-face structures remained in the flocculated sample, while the clay fabric
exhibited a predominantly horizontal orientation. Upon examination of hundreds of SEM images, we observed that at a stress level of 120 kPa, the majority of edge-to-face structure had been destroyed, and particles were rearranged in parallel. This explains the similarity of the microstructure properties present in both flocculated and dispersed samples.

<figure><img src="image_9.png" /><figcaption>Figure 2. SEM images of flocculated sample (left) and dispersed sample (right) under 120kPa stress.</figcaption></figure>

## 4. Conclusion

The mechanical response and microscopic properties of flocculated and dispersed kaolinite samples were investigated. It was found that the pH value of the sedimentation environment affected not only the particle association mode but also the magnitude of the interparticle forces, which lead to different clay fabric and void ratio. The compressibility of kaolin K13 clay was found to be related to the arrangement of particles. Microscopic observations by SEM showed that particles rearranged under mechanical loading, with the majority of the edge-to-face particle structures being destroyed under a 120 kPa vertical stress. MIP analysis revealed that larger pores remained in the flocculated samples after the mechanical loading, but with a lower total void space than that in the dispersed samples. This coupling of MIP and SEM results provided a comprehensive understanding of the clay structural characteristics, opening the door toward further micromechanically-based modeling.

## References

[1] Ding, Y., Bennai, F., Jrad, M., Guyon, J., & Hattab, M. (2023). Three-dimensional pore network of kaolin using FIB-SEM imaging. *Particulate Science and Technology*, 1-17.

[2] Van Olphen, H. (1977). *An Introduction to Clay Colloid Chemistry: For Clay Technologists, Geologists, and Soil Scientists*. 2e edition. New York (N.Y.): Wiley, Print.

[3] Mitchell, J. K. & Soga, K. (2005). *Fundamentals of soil behavior (Vol. 3, p. USA)*. New York: John Wiley & Sons.

[4] Imai, G. (1980). Settling behavior of clay suspension. *Soils and Foundations*, 20(2), 61-77.

[5] Shakeel, A., Safar, Z., Ibanez, M., van Paassen, L. & Chassagne, C. (2020). Flocculation of clay suspensions by anionic and cationic polyelectrolytes: A systematic analysis. *Minerals*, 10(11), 999.

[6] Pedrotti, M. & Tarantino, A. (2018). An experimental investigation into the micromechanics of non-active clays. *Géotechnique*, 68(8), 666-683.

[7] Zhao, D. (2017). *Study on the creep behavior of clay under complex triaxial loading in relation to the microstructure*. Université de Lorraine, France.