A concrete-displacement apparatus, especially a concrete-spray machine for wet concrete (i.e., a loose slurry of concrete adapted to be displaced by air under pressure) comprises a mixing vessel and a feed device having a tubular slider or valve member, the feed device and slider being traversed by compressed air. The tubular slider is formed with one or more openings and is rotatable in an outer sheath or conduit to control the flow of the fluent materials through the conduit.