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There are generally two main types of form releaser: barrier type or chemically active type.
For barrier type (e.g. form oil), it creates a barrier between the form and the fresh concrete. However, the quick evaporation of diesel oils affects clean air.
For chemically active type (e.g. release agent), an active ingredient (e.g. fatty acid) chemically combines with calcium (lime) in the fresh cement paste. This calcium/fatty acid product (grease or metallic soap) is stable and causes the formwork to release from the hardened concrete. It is this slippery, greasy, non-water soluble soap which allows the easy releasing of formwork from hardened concrete.