for delivering successive blocks to the synchronized belt conveyor 3 the sides of the block conveyor 1 are protected by a conventional outer fairing 1 c which is only shown in fig9 for the sake of clarity of the drawings the clamping mechanism comprises a plurality of actuators 10 which can be pneumatic hydraulic mechanical or electromechanical actuators for example transversely moving one clamp 8 out of clamps 8 9 between a clamping position in which the clamps 8 9 hold the blocks against one another and a release position in which the clamps 8 9 do not hold the blocks each actuator 10 is mounted at the upper end of one of the vertical branches of a u shaped frame 11 comprising two vertical branches attached in the lower portion by a lower horizontal branch 11 a the free upper end 11 b of the other vertical branch is attached to clamp 9 opposite clamp 8 to which the actuator 10 is coupled the actuators 10 are protected in the upper portion by transverse protective sheets 20 the block conveyor 1 comprises a reciprocating mechanism for moving the clamping mechanism forward when the clamps 8 9 are in their clamping position and to move the clamping mechanisms backward when the clamps 8 9 are.