It's rare for a concrete structure not to be stained by something: iron rust, oil, chewing gum, or just plain dirt, to name some common offenders. The good news is that these materials can be removed.
THE CHEMICAL AGENTS NEEDED
Some of the chemicals used for stain removal are organic solvents that may be used with little or no modification. Others are solutions of inorganic chemicals in water. In many cases these are crystals or powdered compounds that are purchased as solids and then dissolved in water. In other cases the solutions are purchased ready made, such as various mineral acids, ammonium hydroxide, sodium hypochlorite, and hydrogen peroxide.
GENERAL APPROACH TO STAIN REMOVAL
The objective is to remove the stain in a way that leaves the previously stained area indistinguishable from its surroundings. For some kinds of stains this can be accomplished by dissolving and removing them. Unless special techniques are used the concrete absorbs the solvent, which carries the stain more deeply into the concrete and also spreads some of it over a wider area of the surface. The way to attack this problem is to use a poultice. The poultice is usually made by mixing some essentially inert fine powder with the solvent or solution to be used. The two are blended to make a smooth paste which is then applied as a poultice over the stain, using a trowel or spatula. The liquid portion of the poultice migrates into the concrete where it dissolves some of the staining material. Then the liquid gradually moves back beyond the concrete surface and into the poultice, from which it evaporates, leaving its burden of dissolved staining material in the poultice. After the poultice has dried it is scraped and brushed away. Finely divided materials that can be used to make such pastes or poultices are diatomaceous earth, fly ash, fuller's earth, ground limestone, hydrated lime, portland cement, silica flour, talc and whiting.