Effects of Temperature on Structural Properties of Hydrated Montmorillonite: Experimental Study and Molecular Dynamics Simulation

Montmorillonite (MMT) is highly sensitive to environmental changes and therefore plays a key role in the structural evolution of rocks and soils and even damage and disasters. The effects of important environmental factors (the temperature and water content) on MMT structural properties require in-depth study. The structure and morphology of sodium montmorillonite (Na-MMT) and its thermal products (micro-nanoparticles) were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). A molecular dynamics (MD) simulation was performed to investigate how temperature (below the failure temperature of the Na-MMT crystal layer) affects the structural properties of hydrated MMT. (1) The laboratory results showed that increasing the temperature significantly affected water molecules, and the particle aggregates exhibited inhomogeneous thermal expansion. The interlayer structure collapsed at 500–700°C. (2) In the simulation, the pull-off force inhibited interactions among oxides, crystal layers on both sides of the sample, and the bonding structure of water molecules, thus exposing the stress on the bonding body for analysis. The MMT ultimate stresses in the X, Y, and Z directions all trended downward with increasing water content and temperature. (3) Environmentally induced damage was most likely to occur in the Z direction. Increasing the number of interlayer water molecules increased the layer spacing and considerably weakened van der Waals forces, such that the roles of the electrostatic force and the interlayer hydrogen bond network gradually became significant. The most significant impact of increasing the temperature was reflected in the hydrogen bonding network, resulting in the destruction of the interlayer water bridge, the gradual failure of the layered bonding structure, and the formation or development of cracks. This improved understanding of the structural properties of MMT aggregates under environmental change advances research on the evolutionary behaviour of nano-, micro-, and macrostructures of rocks and soils.


Introduction
Structural changes in rocks and soils under environmental change are often accompanied by damage and even disasters, posing potential hazards to human and property. In recent years, increasing numbers of scholars have been focusing on components of geotechnical systems that are sensitive to environmental changes, because these materials play a determining role in changing the system [1][2][3][4][5][6][7][8][9][10][11]. Macrobehaviour is the cumulative manifestation of microstructural changes. e breakdown of aggregates leads to the expansion of existing cracks, the formation of new microcracks, and the structural collapse of minerals, thereby changing geotechnical properties. Environmental changes frequently trigger and accelerate this process [1][2][3][4][5][6][7]. Humidity and temperature are environmental triggers that significantly impact the structure and physicochemical properties of many rocks and soils.
Pores with different scales in rocks and soils provide storage space for water molecules. Changes in humidity may trigger mineral dissolution-crystallization and structural expansion or contraction, leading to pore collapse, the development of existing cracks (pores), or the creation of new cracks [5,[12][13][14][15][16][17], which in turn leads to the structural deterioration or even destabilization of rocks and soils. Fire, friction, and burial depth all change the ambient temperature. Rapid temperature increases affect the mechanical properties of rocks and soils [18,19]. Frictional sliding in faults during accelerated slip leads to a large increase in temperature which thermally activates physicochemical processes. Friction during landslides can produce temperature increases above 800°C, which accelerates sliding [20,21]. e stability of the rock and soil structure largely depends on that of the main constituent minerals (or weak minerals) to environmental changes. Over the last few years, many scholars have conducted detailed studies on special rocks and soils, such as loess [4,22,23], sliding zone soils [24,25], bentonites [26], and soft rocks [27,28], demonstrating that carbonates, soluble salts, and clay minerals are extremely sensitive to environmental change. Layered clay minerals are often considered to play a key role in the multiscale evolution of many rock and soil structures by controlling the mechanical properties and solute transport in many natural and engineered environments.
Damage to rocks and soils or even the initial structural alteration of a large landslide may begin with changes in the deformation or deposit pattern of clay minerals. Montmorillonite (MMT) is a predominantly hydrophilic clay mineral that is widely distributed in nature. MMTcan absorb water molecules in saturated and unsaturated environments.
is phenomenon is known as hydration and involves the electrostatic forces exerted by interlayer ions and hydrogen bonds formed between water molecules and the OH group on the MMT 010 surface [29]. e expansion and contraction of MMT result in the development of existing cracks (pores) or the creation of new cracks, which may initiate the structural destruction of rocks and soils. e pore water chemical composition affects the hydromechanical behaviour of natural stiff clays, whereas hydration and swelling are considered the most important processes in MMT [19,30].
Numerous scholars have investigated the microstructural changes in sodium montmorillonite (Na-MMT) induced by temperature or humidity by using techniques such as mercury intrusion porosimetry [31], scanning electron microscopy (SEM) [32], and high-precision X-ray computed tomography [33][34][35]. us, some macroscopic destruction mechanisms have been elucidated and possible structural changes have been predicted, providing an in-depth understanding of the destruction process of Na-MMT. However, research on microstructural evolution has been limited to qualitative studies at the micrometre level. In addition, environmentally induced structural changes in MMTremain difficult to characterize because it is difficult to observe hydrated samples in the natural state. A complete molecular understanding of the effect of temperature on the structural properties of hydrated MMT aggregates is essential to determine structural evolutionary processes in geotechnical systems, such as crack initiation and propagation. e objectives of this study were to use laboratory tests to determine the effect of temperature on MMT properties, to perform large-scale molecular dynamics (MD) pull-off tests on hydrated MMT at different temperatures (the pull-off measurement determines the effect of humidity on the interaction force between two surfaces in air [36,37]), and to explore the mechanical properties and failure mechanism of hydrated MMT in detail.

Na-MMT Clay (Na-MMC).
e Na-MMT used in this study had a sample purity > 98% and a grain size of 16-20 μm. e Na-MMT was obtained from the Nanocor company. First, the samples were air-dried at room temperature (approximately 20°C). e approximately 500-g airdried samples were then thermally treated in a muffle furnace at temperatures of 100, 200, 300, 400, 500, 600, 700, 800, 900, and 1000°C. A heating rate of 10°C/min was used to heat the samples to the prescribed temperature level, which was then maintained for 1 h to ensure even heating of the specimens ( Figure 1 shows the test samples diagrams of sodium montmorillonite and its heat-treated products). e heated samples were naturally cooled to room temperature and used for various laboratory tests, such as SEM and Fourier transform infrared (FTIR) spectroscopy.

Characterization.
All the samples were characterized by FTIR spectroscopy using a Nicolet 6700 spectrometer (with a maximum resolution of 0.019 cm −1 ) over the 4000-400 cm −1 range. A FEI Quanta 450 SEM was used to acquire microstructural images of Na-MMT and its thermal products at different magnifications. e phase changes in Na-MMT at various temperatures were analysed using an X-ray diffractometer (X'Pert Pro MPD diffractometer from PANalytical B.V.) with the following parameters: CuKa, a tube current of 40 mA, a tube voltage of 40 KV, and a continuous scanning step of 0.02°. An Anton Paar TTK450 was used over a test temperature range of room temperature-700°C.

MD Simulation.
Parallel computing was performed in the supercloud using the LAMMPS code. MD simulations were performed at six different temperatures (300 K, 375 K, 475 K, 575 K, 675 K, and 775 K below the failure temperature of the Na-MMT crystal layer) to determine the mechanical properties of hydrated MMT under a pull-off test. e water molecules in the clay-water system were described using the SPC/E water model developed by Berendsen et al. [38]. e CLAYFF [39] force field was used to describe the interactions in the clay system. e simulated Na-MMT had a molecular formula of Na 6 (Si 62 Al 2 ) (Al 28 Mg 4 ) O 160 (OH) 32 .nH 2 O [40] and lattice parameters of 4a � 4 × 0.523 nm, 2b � 2 × 0.906 nm, α � c � 90°, and β � 99°. e interlayer spacing c was in the 0.96-1.85 nm range; that is, c was 1.25, 1.53, and 1.85 for increasing water contents n of 32, 64, and 96 nm, corresponding to approximately 9%, 19%, and 28%, respectively. e unit cell was expanded to generate a periodic supercell of size 20 × 10 × 6 in the a, b, and c dimensions. Figure 2 shows the dry, 1-, 2-, and 3-layer hydrated MMT supercells (0 W, 1 W, 2 W, and 3 W, respectively). e stress-strain relationship was obtained by performing an MD simulation in the NPT ensemble (the Nose-Hoover hot bath method was used for temperature control) for uniaxial pull-off loading of the Na-MMT samples, where the pull-off was applied gradually at a constant strain rate (10 −4 /fs). OVITO software was used to visualize the simulation, and the results were deeply analysed in combination with Python program code.

FTIR Spectroscopy.
e different thermal products were characterized using FTIR spectroscopy. In Figure 3, the OH contraction vibration peak of crystalline water in Na-MMT appears at 3637 cm −1 , the contraction vibration peak of Al-Al-OH and Al-Mg-OH appears at 3452 cm −1 , the OH, Al-Al-OH, and Al-Mg-OH bending vibration peak of crystalline water appears at 1646 cm −1 , the stretch vibration peak of Si-O appears at 1042 cm −1 , the bending vibration peak of Al-Al-OH and Al-Mg-OH appears at 917 cm −1 , and the Si-O bending absorption peak appears at 798 cm −1 .
ere is no clear redshift in the spectral peaks of the thermal products over 100-600°C in Figure 3. However, the various groups exhibited different degrees of redshift when the temperature reached 700°C. Figure 4 shows the XRD spectra of Na-MMT measured under different temperatures. From 25°C to 200°C, the MMT characteristic diffraction peak (001) is sharply enhanced, the diffraction angle 2θ increases from 6.999°to 8.82°(a 26% increase) [41], and interplanar spacing d (001) shrinks from 1.262 nm to 1.002 nm (a 20.6% decrease).

X-Ray Diffraction (XRD).
ese results show that the number of MMT interlayer water molecules decreases, the interlayer spacing decreases and the interlayers become highly ordered, and the crystal structure becomes more ordered. From 500 to 700°C, the intensity of the MMT characteristic diffraction peak ese results show the decomposition of the MMT unstable components (silica-alumina oxide crystals), leading to the collapse of the crystal layer and an increase in the interlayer spacing. e diffraction angle 2θ � 26.4°is associated with the small quantity of free silica in the MMT, which does not change significantly with increasing temperature. Figure 5 shows the SEM micrographs of the raw and heated samples. Figures 5(a)-5(c) correspond to magnification factors of 2000, 6000, and 20,000, respectively. Significant pore cracking is not observed below 400°C. e SEM micrographs show intensive crack formation on the Na-MMT surface above 400°C and significantly larger particles than at low temperatures. e Na-MMT underwent significant inhomogeneous thermal expansion. e structure was clearly damaged above 700°C, and clear traces of melting were observed. It can be observed from the test samples that, with the rise of temperature, the colour of montmorillonite also shows a very significant change, which becomes more obvious after the temperature exceeds 700°C, and approximate orange yellow at 900°C, and approximate primrose yellow when the temperature reaches 1000°C. Combined with XRD and FTIR spectroscopy test results, it is found that when the temperature is lower than 700°C, the temperature has very little influence on the single molecular structure of sodium montmorillonite, while the image of montmorillonite aggregates is very obvious, with obvious thermal expansion phenomenon. Figure 6 shows the uniaxial stressstrain diagrams in the X, Y, and Z directions for dry Na-MMT under a pull-off test at 300 K. When the strain in the X and Y directions ranged from 0 to 0.058 and from 0 to 0.065, respectively, the stress increased linearly with the strain. e slope of the stress-strain curve in the elastic region was used to obtain Young's modulus in the X and Y directions as approximately 193.04 GPa and 199.28 GPa, respectively. Table 1 is a comparison between the aforementioned predictions and the corresponding experimental results [37]. All Young's moduli predicted in this study were slightly smaller than simulation results from the literature.   0.105 in the X and Y directions, respectively. Note that the stress-strain curve in the Z direction exhibits wavy failure with inhomogeneities and multiple peaks, indicating strain failure and separation at low stress in some portions of the Na-MMT sample. e maximum stress of 1.17 GPa was obtained at a strain of 0.126 in the Z direction. Figure 7 shows how the ultimate stress of hydrated MMT changes during the pull-off test in the X, Y, and Z directions. e ultimate stress of Na-MMT samples at the same initial temperature exhibited a clear decreasing trend as the water content increased. e ultimate stress of Na-MMT samples with the same initial water content gradually decreased as the temperature increased, whereas hydrated MMT was more strongly affected by the temperature. e most significant decrease in the ultimate stress of Na-MMT induced by the water content and temperature occurred in the Z direction. e ultimate stress decreased to zero at 775 K for the 2-layer hydrated Na-MMT and at 675 K for the 3-layer hydrated Na-MMT. us, the MMT structure in the Z direction was damaged in this environment without the influence of external forces.

MD Simulation.
An in-depth analysis of the failure mode during the pulloff process was performed. e strain correlation function (SCF) is used to describe how chemical bonds in a structure  Advances in Civil Engineering 5 change during pull-off. e SCF can be interpreted as a time correlation function (TCF) under strain loading and is shown in [24]: is a binary operator that is unity if the respective atom pair (e.g., Al-O) is bonded and zero otherwise; and b is the value of b averaged over all simulation time and pairs. Both aluminium oxide (octahedral and tetrahedral) and tetrahedral silicon oxide were damaged under pull-off in the X and Y directions. Figure 8 shows the uniaxial pull-off failure diagram of 3-layer hydrated Na-MMT along the X direction at 300 K under a strain of 0.5. As shown in Figure 8(a), C(ε) of all bonds [35] other than the HO-OH bond decreased significantly with some fluctuations, implying frequent fracture and formation of new fracture surfaces. As these surfaces formed, C(ε) stopped decreasing at a well-defined strain. Tetrahedral silicon and octahedral aluminium constitute the main chains in the crystal layer (the tetrahedral aluminium and octahedral magnesium content is relatively low) and largely determine the mechanical properties in the X and Y directions of the layered structure.
Hydrated Na-MMT exhibited a different type of failure under pull-off in the Z direction. e layered structure on both sides of the sample was almost undamaged, and the interlayer ionized water formed a water bridge to prevent failure as the strain increased (Figure 9(a) shows the pull-off failure of the 3-layer hydrated Na-MMT at 300 K). Increasing the temperature attenuated the effect of the water bridge (e.g., Figure 9(c) shows the state of the interlayer water molecules in the 3-layer hydrated Na-MMT at 675 K).

Discussion
e aforementioned results confirmed that heating significantly changed the structural properties of hydrated Na-MMT. However, hydrated Na-MMT exhibited different change patterns with increasing temperature. e interlayer water molecules were mainly affected below 400°C [42], whereas there was a significant thermal expansion of particles above 400°C. e MD pull-off test inhibited the interactions between the oxides and the crystal layers on both sides of the samples, thus exposing the stress on the bonding body for analysis. e results showed that increasing the temperature and the water content produced a clear decreasing trend in the Na-MMT mechanical properties in the X, Y, and Z directions with low-stress damage. Clearly, a very strong layered main chain structure was formed by silicon, aluminium, and magnesium oxides and their respective hydroxides in the X and Y directions for hydrated MMT. A pull-off force of several GPa was required for this structure to fail. Failure was more likely to occur along the stacking (Z) direction than in the X and Y directions. e electrostatic forces in the interlayer bonding system involve counterions, van der Waals interactions, and the hydrogen bonding networks of water molecules [5,28,37,43]. In the simulation, the pull-off force suppressed the bonding of crystal layers on both sides of the sample, thus exposing the evolution patterns of the interlayer forces. e increase in the number of water molecules resulted in an increased interlayer spacing, weakened van der Waals forces [44], and the formation of a water bridge by electrostatic interactions between Na + -water molecules and the hydrogen bonding network. An analysis of the radial distribution function (RDF) shows that increasing the temperature scattered more Na + ions on the substrate surface and had a limited effect on the Na + hydration energy [45,46]. e threshold for the hydrogen bond distance of interlayer water molecules is usually less than 3Å [29,47]. Increasing the temperature increased the activity of the water molecules, which severely damaged the hydrogen bonding network (Figures 9(b)-9(c)).
MMT aggregates formed by sedimentation, decomposition, and other processes act as a "bridge" between skeletal particles and other minerals in the rocks and soils. ese aggregates are not stacked into an ideal structure and exhibit various defects [30,48], such as pores of different scales, which weaken the bonding between MMT aggregates, while providing a wide storage space for water molecules (a "gap" defect [30]). It can be inferred that increasing the humidity and temperature causes the interlayer spacing and activity of water molecules to increase (Figure 9(d)). e presence of defects results in damage to MMT aggregates under low external forces. erefore, layered clay mineral aggregates with developed cracks are found in different rock and soil microstructures, including the sandstone in ree Gorges Reservoir Region (Figure 9(e)), the Zigui sliding zone soils (Figure 9(f )), and the mudstone of the Xiaoba Formation in southwestern China (Figure 9(g)).
ese cracks further evolve under environmental changes until the primary structure is completely damaged. Rocks and soils then adapt to the environmental conditions to attain structural stability.
MMT is extremely hydrophilic and sensitive to environmental changes (in unsaturated environments, MMT preferentially adsorbs water molecules over being controlled by the system pore size [49]). Even a very low weak mineral content in rocks and soils may produce changes in the overall structural system [3]. erefore, MMT-dominated layered clay minerals are often considered to play a key role Advances in Civil Engineering in the evolution of multiscale structures (i.e., by weakening or destruction of the structure) of many special rocks and soils, such as sliding zone soils, loess, and mudstone. [3,15,24,25].
Many scholars have used MD simulations to extensively study the adsorption-desorption, elasticity, interlayer ions, and other characteristics of hydrated MMT, which have furthered the understanding of the hydration, swelling, and mechanical properties of MMT [50][51][52][53][54][55]. is study was a continuation of previous research to further explore MMT bonding under temperature and water content changes and the MMT evolution characteristics during destruction processes. e study results provide a deep understanding of the molecular-scale evolution characteristics of rocks and soils that contain MMT and elucidate the evolution mechanism of the structural system. In particular, the results show how structural destruction, or even landslide and fault weakness, is triggered. us, a theoretical basis for the protection and risk prevention of rocks and soils is provided.

Conclusion
Heating significantly changes the structural properties of hydrated Na-MMT. Various change patterns in these properties were observed with increasing temperature. e interlayer water molecules were mainly affected at low temperatures.
e thermal expansion of particles became significant above 400°C. e crystal layers of the Na-MMT began to collapse above 500°C. An MD simulation was used to determine effect of temperature on the stress-strain curves of different hydrated Na-MMT samples. Pull-off was used to test the bonding of Na-MMT aggregates in different directions. e Na-MMT mechanical properties deteriorated to different extents in different directions as the water content and temperature increased. e failure mode of Na-MMT aggregates in different directions was analysed. Na-MMT was more prone to damage along the stacking direction under environmental changes. During the failure of the layered bonding structure, electrostatic interactions between water molecules and Na + and the hydrogen bonding network of water molecules resulted in the formation of a water bridge. e most significant impact of the temperature was reflected in the hydrogen bonding network of water molecules. e disappearance of the water bridge weakened bonding in the stacking direction. MMT in nature exhibits various defects and is therefore highly sensitive to environmental changes. us, the environmentally induced change mechanism of the MMT structure and the associated complex interactions are accessible and facilitate a complete understanding of the structural evolution process of crack generation and expansion in massive molecular aggregates of hydrated MMT.

Data Availability
In this paper, the tested montmorillonite was purchased from the high-purity Na-montmorillonite produced by Nanocor, an American company, and all legal organizations can contact Nanocor or contact an agent for purchase (e.g., Beijing Anwu Technology Co., LTD., wishoo@163.com).
e software codes used to support the findings of this study were supplied by Jie Meng under license and so cannot be made freely available. Requests for access to these data should be made to Jie Meng (2201910220@cug.edu.cn).

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.