A twin shaft "compulsory mixer" (C) having has a static mixing chamber bottom (80, 82, 84). Two centrally disposed discharge gates (91, 92) are each defined from an end to about the middle of the mixing chamber (80). At the middle bottom of the mixing chamber, a tensioning rod (150) is placed normally across the abutted ends of the two rectilinear slots (118). The two gates (91, 92) are mounted to a pillow block (132) and spherical bearing arrangement (130) and actuated at the respective mixer chamber ends by conventional piston drives (112, 114). Reducing the gates in length, reduces deflection making chamber sealing possible. Further, the tensioning rod (150) adjusts for chamber deflection. Finally, the eccentrically mounted elongate rectilinear gates (91, 92) can be rotated in opposite directions to deposit concrete on opposite sides of a concrete off-loading and elevating conveyor (B). Discharge of mixed concrete at maximally controlled rates can occur in a balanced fashion to an underlying conveyor for elevation and discharge from the large capacity compulsory mixer forming the foundation of the portable modular plant.