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Concrete sealers are applied to concrete to protect it from corrosion and staining. They either block the pores in the concrete to reduce absorption of water and salts or form an impermeable layer which prevents such materials from passing.
The American Concrete Institute, Portland Cement Association, and National Ready Mix Concrete Association, state that most damage to concrete is attributable to moisture intrusion: alkali-silica reaction (ASR), chemical intrusion, freeze/thaw, and corrosion of reinforcements.