Abstract: A closing mechanism for the outlet of a concrete container and comprising a vertical shaft carrying a feed worm having at least three grooves defined by threads. Each thread extending at the outside as well as at the inside over an angle not below 360/n.degree. and not exceeding 720/n.degree., where n is the number of threads, and having over its whole width angles of inclination not below 8.degree. and not exceeding 15.degree.. Preferably the upper edge of each thread forms an upwards open angle of between 65.degree. and 90.degree. with the shaft, and the transition between said upper edge and the peripheral edge of the thread is rounded off with a radius of curvature of between 35 % and 55 % of the width of the thread.