Specific Surface Area of Clay Using Water Vapor and EGME Sorption Methods

Specific surface area (SSA) is a valuable intrinsic property of clayey soils that can be directly related to more general properties and engineering behavior, including plasticity, cation exchange capacity, and swelling behavior, among many others. Laboratory methods available for estimating SSA, however, remain somewhat complicated and are not routinely used in geotechnical practice. This paper evaluates and compares methods for determining SSA using the conventional ethylene glycol monoethyl ether (EGME) method and various approaches involving sorption of water vapor under controlled humidity conditions. Surface areas estimated from water vapor adsorption and desorption isotherms are comparable for non-expansive clays with SSA values less than about 100 m2/g. Measurements are appreciably different for soils containing expansive clay minerals, where the SSA estimated via Brunauer-Emmett-Teller (BET) analysis of desorption isotherms is consistently about twice the SSA determined from adsorption isotherms. Surface areas estimated from water vapor and EGME methods are comparable at SSAs less than 100 m2/g, but measurements obtained using water vapor for larger SSAs are consistently about half of the EGME-based values. Simplified one-point methods for estimating SSA using water vapor sorption compare well with BET-based methods for SSAs less than 150 m2/g. Recommendations are provided for applying water vapor and EGME sorption methods in geotechnical practice.