Water retention behaviour and microstructural evolution of MX-80 bentonite during wetting and drying cycles

Abstract

MX-80 bentonite used in engineered barrier systems would be subjected to wetting and drying cycles. To assess the response of the material under such circumstances, a comprehensive experimental characterisation of the water retention behaviour of compacted MX-80 granular bentonite was performed in this study. A new methodology is proposed to investigate this behaviour under a constant volume condition for specimens prepared at different dry densities. The material was subjected to different hydraulic paths, including cyclic variations of the water content. As a result, an irreversible modification of the retention behaviour was observed when the material approached a fully saturated state during the first main wetting, and a new hydraulic domain was consequently created. The water retention capacity of the material increased as a result of such modification. Microstructural observations were performed at different stages of the hydraulic paths to relate the permanent change in the retention behaviour to the evolution of the fabric during the wetting and drying cycles. A clear transition from a double-structured to a single-structured fabric, followed by a permanent change of the microfabric, was found following the first wetting. Available data on the hydration of smectite particles were used to relate the microstructural evolution to the change in the water retention properties. This correlation shows the evolution of the active porosity at the particle level within the microstructure, which consequently affects the macroscopic response of the bentonite in terms of its water retention behaviour

Estimation of water retention behaviour of MX-80 bentonite partially saturated with saline solution

“…Figure 8 shows that those suctions are within the range of the typical published values, e.g. [1,3,[5][6] and are further confirmed by psychrometer readouts ( Table 1). The water retention curve for bentonite partially saturated with 1 molar NaCl (Fig.…”

“…The paper adds to significant existing body of knowledge on MX-80 bentonite water retention behaviour e.g. [1][2][3][4][5][6][7][8].…”

Water retention model for compacted bentonites

“…The existence of two water retention domains in compacted bentonite-based materials has been highlighted by several Authors (see for instance Lloret et al 2003;Villar 2007;Loiseau et al 2002;Agus et al 2013;Seiphoori et al 2014;Wan et al 2015;Gatabin et al 2016). Experimental data show that:…”

“…In particular, the influence of clay fabric and its evolution have been discussed in the literature highlighting the role of compaction water content (Thom et al 2007), the hydro-mechanical path (Lloret et al 2003;Romero et al 2011;Dieudonne et al 2014a;Seiphoori et al 2014;Della Vecchia et al 2015) and the chemical 35 composition of the pore fluid (Mata et al 2002;He et al 2016;Thyagaraj and Salini 2016). Accordingly, in recent years, water retention models have been developed to account for the role of the compacted clay fabric.…”

“…The influence of the dry density on the water retention curve of compacted MX-80 bentonite has been recently studied by Seiphoori et al (2014). Figure 2 presents the water 135 retention curves, obtained for three different values of dry density, in terms of both water content and degree of saturation evolution with suction.…”