United Abrasives- SAIT 20061 Type 27 4-1/2-Inch x 1/4-Inch x 7/8-Inch 13300 Max RPM C24N - Concrete Depressed Center Grinding Wheels, 25-Pack C24N - Concrete. Silicon carbide grain. Soft bond for fast stock removal. Masonry, concrete, stone. Cast iron and other ferrous metals. Special silicon carbide grain formula is excellent for grinding masonry, concrete stone and aggregate. All sizes reinforced with the highest quality fiberglass for maximum safety and performance Super-Lock Hub on all 5/8-11 threaded arbors in the safest, bibration-free design.