Further insight into the microstructure of compacted bentonite–sand mixture

Abstract

Compacted bricks of bentonite/sand mixture are planned to be used as sealing plugs in deep radioactive waste disposal repositories because of their low permeability, high swelling capacity and favourable properties with respect to radionuclide retention. The isolating capacity of compacted bentonite/sand mixture is closely related to microstructure features that have been often investigated, in particular by using scanning electron microscope (SEM or ESEM) and mercury intrusion porosimetry (MIP). In this work, microfocus X-ray computed tomography (µCT) observations were used in parallel with MIP measurements to further investigate at larger scale the microstructure of a laboratory compacted bentonite/sand disk (65/35% in mass). Qualitative observation of µCT images showed that sand grains were interconnected with some large pores between them that were clearly identified in the bimodal pore distribution obtained from MIP measurements. Due to gravitational and to frictional effects along the specimen periphery, a higher density was observed in the centre of the specimen with bentonite grains more closely compacted together. This porosity heterogeneity was qualitatively estimated by means of image analysis that also allowed the definition of the representative elementary volume. Image analysis also provided an estimation of the large porosity in good agreement with MIP measurements.

Accounting for evolving pore size distribution in water retention models for compacted clays

“…The effects of mechanical paths are discussed firstly. Experimental data involving different pore size distributions of compacted bentonite–sand mixture samples at different dry densities and at the same water content are presented in Figure a . Bentonite–sand samples (70–30% proportion on dry mass basis) were compacted at a water content between 10% and 11% and at dry densities of 1.67 Mg/m 3 ( e 0 = 0.62), 1.80 Mg/m 3 ( e 0 = 0.5) and 1.97 Mg/m 3 ( e 0 = 0.37).…”

Estudo do comportamento microestrutural de solos tropicais compactados

“…Essa técnica também é muito utilizada na determinação da curva de retenção e da condutividade hidráulica do solo. Entre os estudos realizados nessa área podem ser citados Delage e Lefebvre (1984), Prapaharan et al (1985), Juang e Holtz (1986), Griffiths e Joshi (1989), Delage et al (1996), Romero (1999), Penumadu e Dean (1999), Simms e Yanful (2001), Buenfil (2007), Romero e Simms (2008), Mascarenha (2008), Koliji et al (2010), Sivakumar et al (2010), Alonso et al (2011), Romero (2013, Otalvaro (2013), Borges (2014), Ferrari et al (2014), Ghirian e Fall (2014), Mountassir et al (2014), Saba et al (2014), Wen e Yan (2014), Borges (2014 Sucção (kPa) Silva (2007), natural Silva (2009) Após a compactação os corpos de prova para a execução dos ensaios foram moldados (Figura 3.9).…”

Water retention behaviour of compacted bentonites: experimental observations and constitutive model

“…Note that such a relationship, relating the microstructural void ratio to the total water content (not the microstructural water content), is justified by the fact that, in compacted bentonites, the capillary water represents only a small fraction of the total water stored. The parameters of the model for the compacted mixture were calibrated based on experimental pore size distributions presented in [21,22]: e m0 = 0.29, β 0 = 0.1 and β 1 = 0.18. The model is validated against experimental data on wetting paths under unconfined conditions.…”