Classification and physical characteristics of bound water in loess and its main clay minerals

The presence of bound water on the surfaces of loess particles has significant impacts on the physical, chemical and engineering properties of loess, which are crucial for geotechnical engineering and geohazards prevention in loess regions. However, to date we know little about the distribution and properties of bound water in loess. In this study, thermogravimetric analysis (TGA), isothermal adsorption experiment (IAE) and pycnometer method were adopted to determine the contents, type and physical characteristics of bound water in Malan loess (ML) and its main minerals (montmorillonite, MT; and illite, IL). The results indicate MT and IL absorbed the highest and lowest volume of bound water, respectively, and the results for ML were situated in the middle. During TGA, the tightly bound water (TBW) on MT, IL and ML is completely removed at 293 °C, 295 °C and 235 °C, respectively, while the corresponding temperature for loosely bound water (LBW) removing are 164 °C, 138 °C and 140 °C, respectively. IAE results showed the condition for the formation of TBW are: relative humidity (RH) <69% for MT, and RH <58% for IL and ML, and that of LBW are: RH >69% for MT, and RH >58% for IL and ML. The total amount of bound water obtained by TGA and IAE were almost same, but the amounts of LBW and TBW obtained by the two methods were a little different, caused by the different mechanism and the experimental processes. The density of bound water on MT, IL and ML ranged from 1.41 to 1.19 g/cm3, 1.34 to 1.17 g/cm3 and 1.39 to 1.14 g/cm3, respectively, which were much higher than that of free water. Combined with the measured specific surface areas, the maximum thickness of bound water on MT, IL and ML was calculated to be 10.65 Å, 2.14 Å and 5.25 Å, respectively, indicating that the bound water were multilayer, monolayer and double-layer adsorption, respectively. The properties of bound water are obviously different from those of free water, and the content and the thickness of bound water absorbed on the soil particle surfaces are quite different for different minerals and the loess as a whole. The methods used in this study are helpful for revealing and explaining the soil hydration mechanism. It is also valuable for the understanding the physical and engineering properties of loess and its related minerals.