A screw conveyor to convey powdery material, in particular cement. The screw conveyor comprises: a transfer screw; and a trough containing the transfer screw; the trough being provided with an inlet section and an outlet section of the powdery material. The screw conveyor is characterized in that the screw comprises: at least a first screw portion having a first law of continuous variation of the pitches of the screw, which can be graphically represented by means of a first function having in at least one stretch a first derivative other than zero; and at least a second screw portion having a second law of continuous variation of the pitches of the screw, which can be graphically represented by a second function having in at least one stretch a first derivative other than zero. The first law of continuous variation of the pitches is different from the second law of continuous variation of the pitches.