Temperature effects on water retention behaviour and structural evolution of GMZ bentonite pellet mixtures

Abstract

Mixtures of high-density, multi-sized bentonite pellets are considered as a promising buffer/backfill material for deep geological disposal of high-level radioactive waste (HLW). During the operation of a HLW repository, the pellet mixtures will be subjected to ground water infiltrated from surrounding rock and elevated temperatures induced by nuclide decay heat, leading to a complex hydro-mechanical behaviour. In this paper, water retention tests with suction controlled by vapour equilibrium and osmotic techniques, as well as structural observations by mercury intrusion porosimetry (MIP) were performed on three kinds of specimens of Gaomiaozi (GMZ) bentonite at different temperatures (20, 40, 60 and 80 °C), including constant-volume pellet mixtures (PM) and compacted blocks (CB) with an identical dry density (1.45 Mg/m3) and free-swelling single pellets with the same initial dry density (1.95 Mg/m3) as those involved in the PM specimen. For the three kinds of specimens, the water retention capacity decreased with increasing temperature. From a perspective of thermal dynamics theory, it is concluded that in high suction range (> 10 MPa), increasing temperature inhibited the water adsorption and promoted the water desorption process consequently leading to a decreasing water retention capacity. According to the capillary law (Young – Laplace equation), in the low suction range (< 10 MPa), increasing temperature reduces the capillary water content in terms of decreasing the surface tension, contact angle and density of the capillary water, as well as expanding the entrapped pore air. Meanwhile, the MIP test results illustrate that during hydration, most of the large pores (2000–360,000 nm) were gradually converted into medium pores (100–2000 nm) while the change of small pores (6.4–100 nm) and inaccessible pores (< 6.4 nm and > 360,000 nm) were limited. Compared to the effect of suction, the temperature influence on this structural evolution process was relatively slight. Therefore, the temperature-dependent water retention behaviour was mainly caused by temperature-induced changes of water-clay interactions as well as physical properties of adsorbed and capillary water during hydration, rather than the limited changes of pore structure of the specimen.