In the mixing device are radial openings (2) which issue form a collection chamber (3) which in turn is connected to a pump for feeding a cement/water slurry under high pressure. The high pressure presses the slurry through the openings. Centrally in each opening issues a compressed air tube fed with air under relatively high pressure, resulting in the cement slurry being finely distributed by the compressed air. In the tube, the cement slurry together with the filling material of burnt clay balls swirls around and comes into contact with the balls so that a complete surface coating of the clay balls is received by means of the cement slurry.
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