Introduction Clay mineral crystals are mostly formed by stacking layers of silicon-oxygen tetrahedral sheets and aluminum-oxygen octahedral sheets. They basically have water absorption and expansion because of their unique layered crystal structure. Clay minerals after hydration and expansion usually cause serious engineering problems, which have a negative impact on engineering construction. They have received extensive research attention. It is generally believed that the hydration expansion of clay minerals is mainly divided into two stages, i.e., crystal expansion and osmotic expansion. Crystal expansion is caused by the expansion of water molecules into the interlayer of clay minerals. Permeation expansion is due to the lattice substitution of clay minerals, which leads to the imbalance of valence and electricity of the crystal. Massive exchangeable ions are gathered on the surface of the crystal. After these ions are dissociated in water, they will repel each other with negative charge under the action of the diffusion double layer, resulting in expansion. Compared with osmotic expansion, the degree of crystal expansion is small, kaolinite, illite, pennine and other interlayer cation-free clay minerals do not have an expansibility. However, the problem of expansion disaster occurs under the working condition rich in pennine, indicating the research value of pennine hydration expansion. However, the water absorption characteristics, expansion characteristics and hydration mechanism of pennine in nano-scale are not yet clear, and related research needs to be carried out. In this paper, a molecular dynamics study on water absorption of mesophyll pennine was carried out to explain the hydration characteristics of mesophyll pennine nanocrystal structure. The changes of energy, structure and chemical bond of mesophyll pennine in the process of hydration expansion were discussed.Materials and method The ring cutter samples with different pennine contents (i.e., 0%, 20% and 100%) were prepared with pennine mixed with quartz. The no-load expansion rate was measured. The particle size of the soil sample was controlled to be 200 mesh (0.075 μm), the density of the soil sample was 1.8 g/cm3, and the initial water content was 10%. The hydration degree of pennine with different water contents was analyzed by thermogravimetric analyzer, and the water content controlled was 0%, 5% and 10%. The microstructure and crystal parameters of pennine samples were analyzed by X-ray diffractometer and scanning electron microscope. The crystal model was established based on the results of the microscopic test. The sorption module was used to carry out the adsorption test. A hydration model containing different amounts of water molecules was established through the adsorption test. The geometry optimization task under the Forcite module was used to optimize the structure of the model. The molecular dynamics simulation of the model was carried out using the Dynamics task under the Forcite module, and the expansion deformation of pennine was simulated via dynamic calculation under the NPT ensemble. The Clayff force field suitable for clay minerals was used in the simulation process. The simulated temperature is 298 K and the simulated pressure is 0.001 GPa. The time step is 1 fs, the number of iteration steps is 100 000, and the truncation radius is 12.5 ?. The charge calculation method is the Forcefield Assigned. The non-bond energy Coulomb interaction was calculated by the Ewald sum method. The van der Waals interaction energy was calculated by an atom-based method.Results and discussion The experimental results of the no-load expansion rate show that the no-load expansion rate of the ring knife sample increases with the increase of the content of pennine, indicating that the pennine has an expansibility. The thermogravimetric analysis shows that the higher the initial water content is, the higher the degree of hydration of the pennine will be. In addition to the free water adsorbed between the particles, some water molecules are bonded between the crystal layers via hydrogen bonding. The results of molecular dynamics simulation show that pennine is electrically neutral due to the simultaneous substitution of high-valence cations and low-valence cations, so the crystal expansion occurs. After the water molecules enter the interlayer, they are embedded in the hexagonal holes of the silicon-oxygen backbone layer. The oxygen atoms in the water molecules form a hydrogen bond connection with the hydroxide ions in the talc layer, which controls the hydration limit of the pennine crystal. In the process of hydration and expansion of pennine, it is dominated by electrostatic interaction energy, and followed by van der Waals force, and the bond stretching energy is the smallest. Compared with montmorillonite, pennine hydration is more difficult. The peak of the radial distribution function of water molecules shifts to the right as the degree of hydration increases, indicating that the spacing of water molecules and the diffusivity of water molecules increase. After massive water molecules enter the crystal, the total number of hydrogen bonds increases, the average coordination number of oxygen atoms and hydrogen atoms increases. However, the hydrogen bond length increases, the bond angle decreases, and the crystal expands.Conclusions The experimental and simulation results showed that when water molecules entered the interlayer of the crystal, pennine expanded slightly, which was consistent with kaolinite and illite without cations in the interlayer. The hydration expansion of pennine conformed to the linear growth law as a whole. Pennine crystals continued to expand until the limit as the hydration degree increased. The lattice constants in the limit state were a of 21.52 ?, b of 18.61 ?, and c of 14.54 ?. The limit adsorption amount of the crystal was controlled by the crystal structure. After water molecules entered the interlayer of the crystal, the adsorption site was in the hexagonal holes of the silica backbone layer, and each hexagonal hole could only adsorb one water molecule. When the water-absorbed pennine expanded under dynamic conditions, the electrostatic interaction energy was the largest, the van der Waals force was the second, and the bond stretching energy was the smallest. When water molecules entered the interlayer of pennine, hydrogen bonds could be formed or strengthened. The coordination number of hydrogen and oxygen and the length of hydrogen bond formed increased, but the bond angle decreased with the increase of the number of water molecules.