Add a plasticizer, which acts to lubricate the molecules, and the foam gets spongier and softer.
Addition of the super-plasticizer was made to increase strength and decrease permeability, not to increase workability.
British Dictionary definitions for plasticizer
any of a number of substances added to materials in order to modify their physical properties. Their uses include softening and improving the flexibility of plastics and preventing dried paint coatings from becoming too brittle