an air pocket above and adjacent to the supply impeller, due to the erratic movements of the continuously-agitated mortar. This erratic effect is capable of being intensified by the ability to reverse instantaneously and at any given moment the direction of rotation of the set of mixers, owing to the flexibility of the associated hydraulic transmission. PA0 c) In similar fashion, the supply impeller, having stub blades or a screw conveyor, operates within a semi-tubular sheet-metal duct arranged close to the sweep zone of the impeller, and placed directly below the full length of the supply tank and the set of mixers. This semi-tubular duct encases the "lower two-thirds" of the sweep zone of the supply impeller, while leaving the "upper one-third" free to open directly into thesupply tank and the set of mixers. PA0 a) This chamber projects from the main body of the machine and is arranged in direct alignment with the semi-tubular casing of the supply impeller, without having any dead space or interruption in the continuity of the flow of fluids in the course of transit. This chamber advantageously takes the form of a tubular casing of sheet-metal or cast-iron, arranged around the sweep zone of the extension of the impeller, thus performing the function of a passage for forcing the mortar to the pump. PA0 b) Simultaneously, this chamber performs the function of a decompression chamber and of clearing out of the sand resulting from a possible segregation and tending to accumulate at the supply orifice of the pump. For