Unconfined Compression Behavior of Unsaturated Compacted Ca-Bentonite: Influence of Ca(OH)2 Solution and Elevated Temperature

Jun Ha Baek¹, Yeowon Yoon², Yunsik Gong³, Jihoon Lee³, Ho Young Jo^3,*, Ji-Hoon Ryu⁴

Received 2025-03-14 ; Revised 2025-04-04 ; Approved 2025-04-22

Abstract

This study investigated the unconfined compression behavior of unsaturated compacted Ca-bentonite (dry density = 1.6 Mg/m3) after exposure to 0.02 M Ca(OH)2 solution and elevated temperatures (25 °C and 150 °C) for up to 28 days. Unsaturated Ca-bentonite demonstrated brittle failure behavior regardless of the experimental conditions. At 25 °C, the unconfined compression strength (qu) of Ca-bentonite specimens mixed with either deionized (DI) water (pH 6) or Ca(OH)2 solution (pH 12) was statistically unchanged. However, at 150 °C, qu values increased significantly compared to 25 °C. A similar trend was observed for Young’s modulus, indicating increased stiffness at higher temperatures. The enhancement is attributed primarily to reduced water content and corresponding increases in suction rather than direct mineralogical alteration, as confirmed by X-ray diffraction analysis. This study provides the early-stage mechanical response of unsaturated Ca-bentonite under repository-relevant thermal and alkaline conditions, offering understanding into its performance before saturation is achieved.

Key Words : Ca–bentonite, High-level radioactive waste disposal, Buffer material, Alkaline solution, Unconfined compressive test, Stress–strain behavior