The water-swellable clay composition 12 comprising an intimate mixture of bentonite, with polypropene and/or polybutene in one embodiment, or with a plasticizer in another embodiment, is thoroughly blended in a homogeneous blend with an elastomer, such as butyl rubber, in a sigma blender 14 to fully masticate the elastomer to provide a homogeneous bentonite composition into an extruder 16.