Characterization, rheology and microstructure of laponite suspensions

Alain El Howayek, Purdue University

Abstract

The research presented in this thesis is part of a broader ongoing effort aimed at validating at the laboratory scale the use of laponite suspensions for treating liquefiable soils, as an alternative to sodium pyro-phosphate treated bentonite suspensions. The specific objectives of the work were: (a) the characterization of laponite—a material new to the geotechnical community, (b) a study of the rheology of concentrated laponite-water suspensions, and (c) an investigation into differences in cyclic triaxial behavior observed between dry-mixed and permeated specimens. Through the work performed new insights were also gained into the microstructure of laponite suspensions. Characterization of laponite relied on a number of techniques including TEM, TGA, XRD and sorption/desorption tests. Rheological measurements on the laponite-water suspensions were conducted using a Physica MCR 301 Rheometer. Falling head tests were performed to measure the hydraulic conductivity of Ottawa sand-laponite specimens. Microstructural observations on both the suspensions and the sand-laponite specimens were conducted employing cryo-SEM. Based on rheological measurements, laponite appears to be a viable material for treating liquefiable soils: a 3% suspension exhibits Newtonian behavior over a time window of a few hours, indicating that injection may be practical in the field; once delivered into the sand matrix, within a few hours, the suspension develops a gel structure with properties that in the case of bentonite have been associated with mitigation or elimination of liquefaction. Cryo-SEM observations indicate that the gel microstructure is formed by elongated cells with the longest dimension over three orders of magnitude greater than the 25–50 nm size of the laponite particles. Increased ionic strength (10−4 M NaCl–1 M NaCl), results in more densely packed walls. These observations are consistent with changes in the rheological behavior, and raise questions about existing phase diagrams for this material. Hydraulic conductivity measurements and cryo-SEM observations highlight differences between sand-laponite specimens prepared through dry-mixing and permeation. In the first, the gel in the sand pores appears denser and more concentrated in proximity to the particle contacts, and higher values of k are measured.

Degree

M.S.C.E.

Advisors

Bobet, Purdue University.

Subject Area

Civil engineering

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