Basal spacings of smectite in compacted bentonite

Abstract The smectite basal spacings measured in samples with different water contents of bentonite compacted at various dry densities were reported and analysed. The materials used were two natural bentonites composed mainly of montmorillonite, one of them with a predominance of divalent cations in the exchange complex (FEBEX bentonite), and the other one with predominance of exchangeable sodium (MX-80 bentonite). The samples were prepared either by compaction of the bentonite with different water contents or by saturation at constant volume of samples previously compacted with its hygroscopic water content. The transition from the 1- to the 2-layer hydrate took place at lower water contents for the FEBEX than for the MX-80 bentonite. For a wide range of water contents (between 15 and 30 wt.% for the FEBEX and between 20 and 30 wt.% for the MX-80), the basal spacings measured were around 15 A, indicating the predominance of a 2-layer hydrate in the interlayer. The transition to the 3-layer hydrate, marked by a broadening of the 001 peak, or even a double peak, took place at water contents around 30 wt.% in both bentonites. The full development of the 3-layer hydrate was observed in all the samples that were fully saturated at constant volume. In fact, for a given water content higher density samples tended to have higher basal spacings. In the low density samples the basal spacing increased over time without substantial change of water content or macroscopic swelling pressure.