Apparatus for forming a constructional block in which a lower die is filled with a given and at least substantially excess amount of material after substantially closing the lower die with an upper die for forming a mold by drawing the dies together, the material is highly compressed while, simultaneously with drawing them together under the action of a means for controlling and regulating its passage, in a first stage, the excess material is allowed to escape partly outwardly of the mold and, in a second stage, substantially before obtaining the desired dimension between the two faces of the block through the relative drawing together of the dies, the passage of the material escaping outwardly of the mold is stopped.