Abstract: To investigate the hydration evolution process of sodium montmorillonite, the water vapor isothermal adsorption-desorption experiment was conducted for samples with relative humidity ( ) ranging from 0 to 0.98. The water adsorption rate curves and BET curves were proposed to define the hydration evolution process of sodium montmorillonite and the corresponding control factors. By measuring the variation of value, the influence of adsorption water on the thickness of clay mineral layer was studied for sodium montmorillonite hydration characteristics. Based on the theory of Fourier transform infrared spectroscopy, the rationality of the proposed method is also quantitatively validated with water molecular structure vibration information. The relationship between adsorption water characteristics of sodium montmorillonite and its hydration mechanism is explained in terms of the energy required by the phase transition of adsorption water and the mass loss of the adsorption water by thermal analysis. The test results indicate that sodium montmorillonite adsorbsion is mainly on the external surface to form surface adsorption water at 0 0.15, the stage of 0.15 0.40 is the cationic hydration stage between sodium montmorillonite layers, and the stage of 0.40 0.98 is the hydration stage on the inner and outer surface of the crystal layer. The water molecules gradually cover the montmorillonite completely, forming a multi-layer adsorption layer. The hydration process of sodium montmorillonite is controlled by interlayer sodium ions and crystal energy, and the hydration of interlayer sodium ions affects the starting sequence of hydration evolution of sodium montmorillonite.

Key words: hydration mechanism, sodium montmorillonite, cation, basal surface of crystal layer, define the boundaries