Density and porosity of bentonites
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The calculation and measurement of the densities of smectites proved that calculation can lead to accurate values if the smectite d001-value, water content, molar mass of one formula unit, interlayer composition, and layer charge density are considered. The density measurement was supposed to be affected by accommodation of some He in micropores. The specific surface area determined by the common N2 adsorption is largely affected by micropores which in turn was used as proxy for microporosity. The investigated materials show a wide range of microporosities (5–65 % of the total porosity up to 50 μm). Micropores are supposed to result from the imperfect stacking of single TOT layers resulting in the quasi crystalline overlap region but they also result from partial access of the interlayer at the edges. Unfortunately, both types of microporosity could not be distinguished quantitatively. For measurement of the microporosity both N2 and CO2 adsorption were found to be suitable. The mesoporosity of bentonites also covers a significant range. One particularly mesoporous material was identified both by N2 adsorption and mercury intrusion. SEM investigation suggests the fibrous microstructure to be responsible for the extraordinary mesoporosity. For the characterization of the mesoporosity, N2 adsorption showed best resolution. The macroporosity may be classified according to the type of aggregates observable by SEM. The typical rose like arrangement of smectites results in 1 μm macropores and relict structures of the volcanic glass particles result in larger pores. The third type of macropore simply results from the relative arrangement of aggregates.