Ardex makes some wonderful tile installation materials. Here is a how to, or more importantly, how not to guide from them:
The best way to remove most contaminants is by scarifying, shotblasting or similar mechanical method. Removal must be deep enough to eliminate all penetrated contaminants.
Methods to Avoid
Acid washing (or acid etching) is not recommended because it is difficult to control, to fully remove the residue, and to properly neutralize. Further, the acid can penetrate into the porous concrete and chemically react with the cement, thus affecting the long-term integrity of the concrete. Acid washing will not satisfactorily remove grease and oil.
The use of sanding equipment is not an effective method to remove curing and sealing compounds.
All types of solvents should be avoided. Their use will drive oil, grease and other contaminants further into the concrete, only to permit their release back to the surface at a later time. Physically removing oil-contaminated concrete is the only sure way to ensure a clean substrate.
Sweeping compounds can leave an oily or waxy film on the surface of the concrete. Their use can create a bond-breaking layer that will result in a flooring system failure. Using a clean dry broom, sweep and vacuum the surface prior to placing any underlayment or topping.
Also, never use adhesive removers or solvents to remove contaminants from porous concrete. These materials can carry contaminants into the pores of the concrete, which will later migrate back to the surface resulting in a floor covering bond failure.