wall 76 each of which provides a bearing support which allows free movement of the shaft 72 a paddle 79 attached to the adjustable front wall 78 prevents the dry material from dropping between the adjustable front wall 78 and the support wall 76 this paddle also allows the dry material to be completely shut off this is a useful feature which allows the same concrete mixing apparatus to be used to mix other materials for example stucco may be produced by shutting off gravel chambers 13 also shown in fig6 is the placement of four pneumatic sand vibrators 52 as described in connection with fig1 these pneumatic sand vibrators 52 are fixed to the outside surface of any of the walls of the sand compartment 4 which extend across the shortest dimension of the sand compartment 4 in the preferred embodiment which has three sand ports 5 which release sand from three sections of sand compartment 4 there are four pneumatic sand vibrators 52 each having a mounted height of approximately one third the distance from the bottom to the top of the hopper one pneumatic vibrator 52 is mounted in the horizontal center of each of the short walls of the center hopper section and one pneumatic vibrator is mounted in the center of each outermost end of the sand hopper 4 referring to fig7 a cement freeing assembly 80 is shown including break up tines 86 and cement feed augers 88 this cement freeing assembly is positioned near the bottom of the cement hopper 2 on its centerline cement hopper 2 has a planar bottom surface which differs from the bottom surfaces of the sand and gravel hoppers 4.