An apparatus and method for delivering an additive-enhanced slurry for use in making gypsum wallboard, and configured for connection to a centrifugal mixer with a tangential discharge outlet, includes a conduit having a main inlet in slurry receiving communication with the mixer outlet and extending to a discharge spout for discharging the slurry proximate a wallboard forming area. At least one volume restrictor is associated with the conduit for creating back pressure between the volume restrictor and the outlet for keeping the mixer fill, and at least one pressure reducer associated with the discharge spout and configured for reducing the pressure of the slurry dispensed from the discharge spout. The apparatus is configured for maintaining a generally laminar flow from the mixer outlet to the discharge spout.