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Tensile Strength of Montmorillonite as a Function of Saturating Cation and Water Content¹

ABSTRACT

The tensile strength, tensile axial strain, and tensile strain energy were determined for Na-, K-, Ca-, Al-, and Fe-saturated montmorillonite over the water vapor pressure range (P/P_o) from 0.02 to 0.92. The procedure used was direct tensile stressing of oriented clay films. Measured tensile strengths ranged from 366 kg cm^-2 for Fe montmorillonite at 0.02 P/P_o to 19 kg cm^-2 for Ca montmorillonite at 0.92 P/P_o. Strength decreased rapidly with added increments of water during initial interlamellar expansion and continued to decrease at a slower rate with further additions of water. In general, strength decreased in the following order: Fe > K Na > Al > Ca. The tensile strength of Na montmorillonite was of the same order of magnitude as the force required to separate two Na clay platelets as determined from adsorption/desorption water isotherms.

Axial strain was independent of P/P_o above 0.4 for all systems except the Na-saturated clay. For Fe montmorillonite, strain was insensitive to water content above 0.15 P/P_o. Monovalent clay specimens required the most energy to rupture and divalent samples required the least energy. Aluminum and Fe clays required an intermediate rupture energy.

The greater tensile strength and low axial strain of the Fe montmorillonite suggested that a different mechanism controlled the failure characteristics of this clay. Hydroxy iron material may be the major cementing agent in the Fe system. The data show that saturating cation played a dominant role in the strength-energy properties of the other homoionic clays.

¹ Contribution from the Corn Belt Branch, Soil and Water Conservation Research Division, ARS, USDA, in cooperation with the Minnesota Agr. Exp. Sta., St. Paul, Minn. 55101. Paper no. 7476 Scientific Journal Series. Presented before Div. S-9, Soil Science Society of America, Detroit, Mich., Nov. 9–14, 1969.

² Research Soil Scientists, USDA; Assistant Professor and Professor, University of Minnesota; respectively.

Received for publication December 12, 1970. Accepted for publication August 9, 1971.