The cylindrical drum of a mortar/plaster mixer is locked in the mixing position by a rotatable pivot arm pivotally attached by one end to an end wall of the drum such that, when the drum is in the mixing position, the arm may be rotated between a locked position with the free end of the arm in locking engagement with the mixer frame to an unlocked position with the arm rotationally displaced from the locking position. As the mixer drum is tilted for discharge, the unlocked arm is further pivotable from the rotationally displaced unlocked position back to the locked position in response to drum rotation from the mixing position to the discharge position and reverse rotation back to the mixing position.