Abstract: A machine for mixing fireproofing material, concrete or the like includes a cylindrical shaped rotatable drum with a rotatably driven axial shaft having mixer blades attached to the shaft. The drum has an opening for loading and discharging material to be mixed and a pivotally mounted cover for closing the opening. The cover is provided with an aperture and a funnel-shaped member within the aperture for supporting a sack of material to be emptied into the drum. The funnel-shaped member is closed by the sack supported therein thus sealing the aperture during the sack emptying and material mixing processes. Liquid is supplied to the mixer drum through another opening in the cover, which opening is blocked in the mixing position of the drum to prevent the emission of dust particles therethrough. Latching means are provided for releasably securing the cover means in closed or partially open position, to facilitate cleaning of the mixing drum.