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What are the best methods for cleaning concrete, and how do I know which method to use?

Concrete surfaces are not always uniform in color when forms are removed; they may have a somewhat blotchy appearance and there may be a slight film of form-release agent in certain areas. There may be mortar stains from leaky forms or there may be rust stains. Flatwork can also become discolored during construction.
Where appearance is important, all surfaces should be cleaned after construction has progressed to the stage where there will be no discoloration from subsequent construction activities.  There are three techniques for cleaning concrete surfaces; water, chemical, and mechanical (abrasion).
Water dissolves dirt and rinses it from the surface.  Chemical cleaners, usually mixed with water, react with dirt to separate it from the surface, and then the dirt and chemicals are rinsed off with clean water.  Mechanical methods--sandblasting is the most common--remove dirt by abrasion.

Before selecting a cleaning method, it should be tried on a test area to be certain that it will be helpful and not harmful. If possible, identify the characteristics of the discoloration because some treatments are more effective than others in removing certain materials.  
Water cleaning methods include low to high pressure water blasting, and steam. Low pressure is the most common and will soften the dirt then flush it off with a slightly higher pressure rinse.  High pressure water blasting and steam cleaning should be performed by skilled operators.  Serious damage may occur if the surface is subject to freezing temperatures while wet, and water can bring soluble salts to the surface, forming a chalky, white deposit called efflorescence.
Chemical cleaning is usually done with a water based mixture formulated for specific materials such as brick, stone, and concrete. An organic compound called a surfactant (surface-active agent), which acts as a detergent to wet the surface more readily, is included in most cleaners.  A small amount of acid or alkali is included to separate the dirt from the surface.  For example,
hydrochloric (muriatic) acid is commonly used to clean masonry walls and remove efflorescence.  There can be problems related to the use of chemical cleaners.  Their acid or alkaline properties can lead to reaction between cleaner and concrete as well as mortar, painted surfaces, glass, metals, and a host of other building materials. Since chemical cleaners are used in the form of water-diluted solutions, they too can liberate soluble salts from within the concrete to form efflorescence.  Chemicals commonly used to clean surfaces and remove discoloration include weak solutions (1% to 10% concentration) of hydrochloric, acetic, or phosphoric acid. Diammonium citrate (20%-30% water solution) is especially useful in removing discoloration stains and efflorescence on formed and flatwork surfaces.

Mechanical cleaning includes sandblasting, shot blasting, scarification, power chipping, and grinding.  These methods wear the dirt off the surface rather than separate it from the surface.  They, in fact, wear away both the dirt and some of the concrete surface; it is inevitable that there will be some loss of decorative detail, increase surface roughness, and rounding of sharp corners.  Abrasive methods may also reveal defects (voids) hidden just beneath the formed surface.

Chemical and mechanical cleaning can each have an abrading effect on the concrete surface that may change the appearance of the surface compared to that of an adjacent uncleaned surface.  
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