Carbonation is usually a slow, natural process which occurs when compounds in concrete, such as portlandite and calcium silicate hydrate, react with CO 2 in the air to form calcium carbonate.

This microscopic image shows the lumpy calcium-aluminum-silicate-hydrate (C-A-S-H) binder material that forms when volcanic ash, lime, and seawater mix. Platy crystals of Al-tobermorite have grown ...