Abstract: The invention relates to a clay-containing mixture or blend capable of forming a moisture resistant gel, which comprises a powdered or ground smectite and/or a smectite-containing natural rock and at least 0.8-10% by weight, calculated for the smectite content, of an at least partially water-soluble and/or water swellable polymer and optionally at least 0.5% by weight of a solid activating agent. Such a mixture may also additionally comprise as a diluting agent more than 0.5% by weight of one or more solid inert filler(s); this diluted version is termed in the specification as a blend.