Surface fractal dimension of bentonite affected by long-term corrosion in alkaline solution

Abstract

To study the change mechanisms of the fractal dimension of bentonite affected by a corrosion process in alkaline solution, sodium bentonite was mixed with 1 mol/L NaOH solution and sealed for reaction for up to 720 d (days). The fractal dimensions of bentonite specimens at different stages of the corrosion process were obtained from N2 adsorption and swelling deformation tests, and the results of both methods suggested that the fractal dimension tends to increase with duration of the corrosion process. From X-ray diffraction (XRD) tests, it was found that montmorillonite, the main component of bentonite, was dissolved gradually in the alkaline solution. However, the decline in montmorillonite content had no relation to the change in fractal dimension, which was validated by swelling deformation tests of bentonite–quartz mixtures, as the swelling characteristics of the bentonite–quartz mixtures could be expressed by a unique curve of the em–ps relation (em is the void ratio of montmorillonite, ps is the vertical stress) with an identical fractal dimension. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were used to observe the surface microscopic morphology of the original and alkaline-corroded bentonite. It was found that montmorillonite layers in the original bentonite were often aggregated into plate-like particles. However, after corrosion in the alkaline solution, these montmorillonite aggregates were cracked into small stacks and the surface of montmorillonite layers became more uneven, resulting in a more complicated surface and a higher fractal dimension.