Abstract: The interaction between colloidal oxides and clay minerals is the major factor affecting the microstructure, swelling, and shrinkage of red clay. In this paper, the colloidal oxides are removed by the selective chemical dissolution method. The effects of colloidal oxides on the microstructure and swelling shrinkage of red clay are explored via swelling shrinkage test, particle size analysis, mercury injection, and SEM. Results illustrate that the colloidal oxides adhere to the clay mineral surface in the form of an envelope. The removal of colloidal oxides significantly changes the microstructure of soil. Moreover, the disappearance of the coating leads to the deterioration of soil structure bonding, weakening of agglomeration, increase of particle dispersion, decrease of micropores, and increase of pores. In addition, physical parameters of soil (specific gravity, optimal water content, maximum dry density, liquid plasticity, free expansion rate, and linear shrinkage rate) decrease since the removal of colloidal oxide. The research findings offer scientific basis for correctly understanding the special physical characteristics and enriching related basic theories of red clay.

Key words: red clay, colloidal oxides, microstructure, swelling-shrinkage property