CLAY SURFACE PROPERTIES BY WATER VAPOR SORPTION METHODS

Abstract

ABSTRACT Classification of expansive soils is a required component of geotechnical design to improve the long-term performance of structures. The current methods to classify swelling soils are based primarily on indices like the Atterberg limits, which are indirectly related to clay mineralogy and clay structure. The long-term goal of this research is to explore alternative ways to classify clays and to modernize geotechnical soil classification methods using measurement and analysis techniques based on clay surface properties. Clay surface properties such as specific surface area (SSA) and cation exchange capacity (CEC) are indicative of the clay mineralogy. For expansive clays, interactions of the clay surface with water molecules influence behavior. Consequently, water vapor sorption methods may potentially be used to determine clay surface properties. SSA is the measure of surface area per unit mass (m2/g). SSA measurement methods can be classified into three main categories: physical methods, positive adsorption methods, and negative adsorption methods. The most common techniques are positive sorption methods, where the sorption of molecules on the surface is measured. The sorbates can be either polar (e.g., N2) or non-polar (e.g., Ethylene Glycol Monoethyl Ether (EGME), water vapor). CEC is a measure of exchangeable mineral charge. Common measurement techniques include those based on replacing the cations in the natural exchange complex with a known cation species (e.g., ammonium displacement method) A water vapor sorption isotherm characterizes the relationship between relative humidity (RH) and the equilibrium moisture content of some material in that environment obtained along an adsorption (wetting) or desorption (drying) path at some temperature. A water vapor sorption isotherm is equivalent to a soil water characteristic curve (SWCC) since RH and soil suction can be directly related by Kelvin?s equation. A sorption isotherm, therefore, is a direct measurement of how the physical and chemical interactions between water and the soil surface result in water retention, and thus is an indirect measurement of the fundamental compositional and surface properties of the material (mineralogy, SSA, CEC). In the first chapter of this thesis SSA values obtained by Brunauer-Emmett-Teller (BET) Theory are determined from water vapor sorption isotherms obtained at low relative humidity values, and compared with values obtained using the more conventional EGME retention method. Thirteen samples of natural clayey soils and a suite of mixtures of end member Wyoming bentonite and Georgia kaolinite are tested. Additional comparisons are made for a suite of Mg+2, Ca+2, Na+, K+, and Li+-saturated forms of the bentonite to examine influences of exchangeable cation type and to extend the possible range of water vapor sorption isotherms over a wide range. The second chapter examines the performance and applicability of six existing isotherm models (Freundlich, Brunauer-Emmett-Teller, Frenkel-Halsey-Hill, Guggenheim-Anderson-de Boer, Double Log Polynomial, and Dubinin-Astakhov) for representing the water vapor sorption behavior of clayey soils. Dry mixtures of Wyoming bentonite and Georgia kaolinite are tested to cover a wide range of possible isotherms. Trends observed in the equation parameters with increasing bentonite content are discussed and analyzed separately for adsorption and desorption processes. In the third chapter, an original empirical method is proposed to indirectly estimate the CEC of clayey soils based on relatively simple water vapor sorption measurements. A master curve is obtained from measured water vapor sorption isotherms of 24 clayey soil samples. The validation of the proposed method is independently tested using twelve samples of natural clayey soils and nine water vapor sorption isotherms available in the literature.