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Flocculation Value and Gel Structure of Sodium/Calcium Montmorillonite and Illite Suspensions¹

ABSTRACT

The flocculation value, the minimum salt concentration which causes flocculation within 24 hours, was determined for mixtures of Na- and Ca-montmorillonite (API-25) and Fithian illite (API-35). The equivalent fraction of exchangeable Na⁺, E_Na, varied from 0 to 1. We prepared Na/Ca clay suspensions by adding the appropriate amounts of homoionic dry clays to a series of Na/Ca chloride solutions (0.2 to 80 mol_c m^–8; 0.2 to 80 meq liter^–1) with appropriate sodium adsorption ratio and shaking the mixture for 1 hour. The flocculation value of Ca-montmorillonite (0.25 mol_o m^–3) increased rapidly with small amounts of exchangeable Na⁺: the flocculation value for an E_Na of 0, 0.2, 0.4, and 1 was 0.25, 6, 9, and 12 mol_c m^–3, respectively. The flocculation values for Ca- and Na-illite were 0.25 and 55 mol_o m^–3, respectively, and increased linearly with E_Na. Van der Waals attraction forces are mainly responsible for flocculation of Ca-montmorillonite and illite systems. Edge-to-face attraction is dominant in flocculating Na-montmorillonite. The large effect of small amounts of exchangeable Na⁺ in montmorillonite suspensions results from demixing of exchangeable ions whereby Na⁺ is the predominant cation on the external surfaces when E_Na < 0.20.

¹ Contribution from U. S. Salinity Laboratory, AR-SEA, USDA, 4500 Glenwood Drive, Riverside, CA 92501.

² Soil Scientists. Dr. Shainberg's permanent address is Institute of Soils and Water, ARO, The Volcani Center, Bet-Dagan, Israel.

Received for publication November 26, 1979. Accepted for publication June 18, 1980.

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