The present invention provides a plaster composition that includes calcined gypsum and a naphthalene sulfonate-aldehyde condensate alkali salt polymer having a weight average molecular weight from about 17,000 to about 47,000. The present invention also provides a method of forming a plaster composition that includes belending calcined gypsum and a naphthalene sulfonate-aldehyde condensate alkali salt polymer having a weight average molecular weight from about 17,000 to about 47,000 together to form a dry blend and contacting the dry blend with water to form a hardenable aqueous slurry.