Getting cement to whiten may require more than pressure alone. Cement is pourus and may require a special cleaning agent to assist the process. Using a combination of sodium hypochlorite and with tri-sodium phosphate seems to work quite well. Using a chemical resistant pump sprayer may work best to apply the agent accurately, for this solution is not friendly to grass or other landscape. Allow the whitener to dwell for approximately 15 minutes, than using a 0 degree turbo nozzle wil finish the job. These are pics of a job in South Plainfield, N.J.