Video of slaking of lime
When limestone is heated above 900˚C (cherry red), it decomposes into calcium oxide, CaO. In this movie, a piece of travertine (a type of limestone made in hot springs) is used. When mixed with water - as shown in the video - it reacts violently to make slaked lime, Ca(OH)2. When exposed to the atmosphere, slaked lime reacts with carbon dioxide to recreate limestone, CaCO3. This procedure was used to seal walls and floors in homes more than 5000 years ago: the slaked lime was smoothed onto the wall, where it reacted to form a firm dry coating.