Microstructure of Sand-Laponite-Water Systems using Cryo-SEM

摘要

This paper describes the use of cryo-scanning electron microscopy (cryo-SEM) to characterize the microstructure of concentrated laponite suspensions (3% by mass of the water), which are being considered for treating liquefiable deposits. In cryo-SEM a sample is cooled rapidly in nitrogen slush, sublimated to remove the unbound water and imaged under cryogenic conditions (～ -130 °C); hence it remains close to its natural state and dehydration is avoided. This is ideal for materials such as laponite suspensions, which have very high water content (＞3000%) and a delicate structure that would be damaged as a result of shrinkage. Cryo-SEM observations at magnifications varying from 250x to 40kx indicate that laponite suspensions prepared with deionized water have a cellular microstructure formed by elongated cells of a size several orders of magnitude greater than the clay particles, consistent with the structure of an attractive gel. Images of suspensions prepared with water of increasing ionic strength (10~(-4) M NaCl - 1 M NaCl) show an increased number of cross-links and more densely packed walls, and ultimately the formation of clay aggregates. The paper also presents cryo-SEM images of sand-laponite mixtures prepared using two different methods: a) dry-mixing sand and laponite and then permeating the resulting specimen with water; and b) permeating a clean sand specimen with a laponite suspension. The images highlight differences between the two specimens and provide direct insight into the micro-mechanism responsible for the observed macroscale geotechnical properties.