Abstract
Modified calcium bentonite (Ca-bentonite) is extensively used in engineered barrier systems (EBSs) for municipal and industrial disposal sites due to its high swelling potential and low hydraulic conductivity. However, few studies have focused on the micromechanism of hydration and swelling under the effect of inorganic chemical solution. In this study, free swell index (FSI) and the type and content of modified Ca-bentonite bound water under the inorganic chemical solution were quantitatively studied by using the free swell test and nuclear magnetic resonance (NMR). According to the results, modification of sodium and polymer significantly increases the FSI of Ca-bentonite, bringing it close to that of natural sodium bentonite. In addition, the chemical stability of polymer-modified bentonite is significantly higher than that of sodium-modified bentonite but less than that of natural Na-bentonite. The FSI of modified Ca-bentonite decreases with the increase of cation valence and ionic strength. T2 distribution curves of the two types of modified bentonite are three-peak curves. With the increase of ionic strength, the content of total water and permeated hydrated water (accounting for 69%–95%) in bentonite decreases gradually, whereas the surface hydration water (accounting for 2%–31%) and free water content (accounting for 0–15%) increase. A uniform linear relationship exists between the FSI and corresponding total peak area of NMR (independent of ion valence, concentration, and bentonite type). Furthermore, a linear relationship exists between the FSI of the same type of bentonite and the T2 relaxation time. Research results can provide data and theoretical basis for quantitative analysis and mechanism of the hydration swelling of bentonite.
Highlights
Bentonite, which mainly consists of montmorillonite clay minerals, is widely used in engineered barrier systems (EBSs) due to its low hydraulic conductivity that restricts pollutant migration and minimizes the risk of groundwater contamination
When bentonite particles interact with chemical solutions, the hydrated cation in the solution is adsorbed to the surface of the bentonite particles to maintain the electrical neutrality
The mechanism of bentonite hydration swelling can be explained by the theory of diffusion double layer; the essence of which is the change of water content among different water types on the surface of bentonite particles [14]
Summary
Bentonite, which mainly consists of montmorillonite clay minerals, is widely used in engineered barrier systems (EBSs) due to its low hydraulic conductivity that restricts pollutant migration and minimizes the risk of groundwater contamination. The low permeability of the bentonite mixture is mainly determined by the swelling ability of the bentonite. The reason is that its main mineral component, montmorillonite, has strong hydration and swelling properties, which cause filling of the pores between the soil particles, as well as narrowing and curving of the permeation channels, thereby reducing permeability to achieve the function of blocking and preventing the migration of pollutants. The hydration swelling mechanism of bentonite can be explained through the theory of diffusion double layer.
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