Effects of alkaline solution on shear strength of GMZ bentonite as a buffer/backfill material in China

The concrete in the auxiliary structure decomposes to produce high-alkaline pore water in the disposal repositories thus substantially affecting the mechanical properties of bentonite. Direct shear tests have been performed on GMZ01 bentonite inundated in NaOH and KOH solutions for evaluating the alkaline solution impact on its shear strength (SS). The experimental results reveal that the SS value is directly related to the concentration of the two kinds of alkaline solution. At the same concentration, the SS of the sample treated with KOH solution is higher than that of the NaOH solution. The concept of modified effective stress p' helps to understand the SS of bentonite eroded by alkaline solution on the basis of Mohr–Coulomb criterion. Upon further analyses, it was found that angle of internal friction φ' increased at higher alkaline concentration. The micrographs of scanning and transmission electron microscopy analyses revealed the cracking of smectite aggregates depicted as smaller stacks after corrosion of alkaline solution. As a result, the smectite layers surface was nonuniform which led to the increase of surface roughness as well as the φ′. The effective cohesion c′ remained unchanged in NaOH solutions, however, it was observed to increase with the KOH concentration which is mainly due to the strong K-linkage of K+.