Abstract: Of the type designed to be attached to the propeller-shaped bases or heads of a concrete grinder, to enhance the grinding effect of said propeller-shaped bases after the latter are used without disks, it is shaped like a cross, with four equal arms which are equiangularly distributed, in the shape of an isosceles trapezoid, each of which has a curved-convex end (1), forming part of the imaginary circumferential outline corresponding to the theoretical disk, and curved-concave side edges (2-2′), which are joined to each other without a break in continuity between adjoining arms and which define a deep concave indentation (3), whose path is circumferential and whose radius appreciably coincides with the theoretical radius of the disk, so that a pair of disks may be used in a concrete grinder with two heads, working on the same plane without touching each other, by means of offsetting them angularly from each other by 45°.