There are two ways of removing stains from concrete; the dry or mechanical methods and the wet methods. The dry methods include grinding, sandblasting, planing, and scouring scabbing as well as flame cleaning and short-blasting.
For the wet methods, you can use water or specific chemicals depending on the nature of the stain. The chemicals work by bleaching the staining substance or dissolving it so that it is blotted from the surface.