Impact of Alkaline Solution Interaction on Bentonite Shear Strength and Its Role in Waste Repository Barrier Design

The degradation of concrete in nuclear waste repositories generates high-alkaline pore water, significantly impacting the mechanical properties of bentonite used as buffer/backfill material.Direct shear experiments on GMZ01 bentonite flooded in NaOH and KOH solutions show that shear strength (SS) rises with solution concentration; K-linkages in KOH-treated samples show higher SS than in NaOH-treated ones. Simultaneously, Na-bentonite's shear strength in saline solutions (CaCl2, KCl, NaCl) fluctuates with ion type and concentration to exhibit strain softening action. While at lower concentrations SS follows NaCl < KCl < CaCl2, at higher concentrations the sequence changes to NaCl < CaCl2, hence emphasizing the effect of cation exchange mechanisms. By SEM and TEM, micro-structural study reveals enhanced surface roughness and cracking of smectite aggregates, hence raising internal friction angles. The modified effective stress concept (p') derived from the Mohr-Coulomb criterion clearly explains the observed SS variations under both alkaline and saline environments. These results offer crucial information for assessing the long-term durability and safety of built barriers in systems of nuclear waste disposal. Keywords: Alkaline solution, Salt solution, Bentonite, Shear strength, Effective stress, Cation exchange