A thin, circular mixing apparatus for receiving from separate sources (1) dry calcined gypsum powder and (2) the liquid ingredients of the core of a gypsum wallboard, with the liquids being fed to a small diameter radially inward centrifugal pump portion, the dry ingredients being fed to a medium diameter circular portion into which the centrifugal pump elements continually force the liquids in an outwardly spiralling path and having a radially outermost circular portion into which the mixture is further forced in a continued outwardly spiralling path and out from which the mixture is discharged, between rotating radially outwardly projecting teeth.