Wikipedia
Gypsum block
Gypsum block is a massive lightweight building material composed of solid gypsum, for building and erecting lightweight fire-resistant non-load bearing interior walls, partition walls, cavity walls, skin walls and pillar casing indoors. Gypsum blocks are composed of gypsum plaster, water and in some cases additives like vegetable or wood fiber for greater strength. Partition walls made from gypsum blocks require no sub-structure for erection and gypsum adhesive is used as bonding agent, not standard mortar. Because of this fundamental difference, gypsum blocks shouldn't be confused with the thinner plasterboard (also known as wallboard or gypsum board) used for paneling stud walls.