Plaster Gypsum Perlite is composed of a retarded gypsum, Perlite, and selected aggregates with special additives. It is supplied as a dry powder in pre-weighed bags ready to use on-site, which requires only the addition of clean water to produce a cohesive mix. Plaster Gypsum Perlite is a high-quality product and provides underlayment on various facets of the projects.