This is from Randy's Article -- What your Grandmother never told you about limewater
What Is Lime?
According to the National Lime Association, “lime” is defined as either quicklime or hydrated lime. These materials are made by heating calcium carbonate until the carbon dioxide is driven off, forming quicklime (calcium oxide):
1. CaCO3 à CaO + CO2
Water can then be added to form hydrated lime (calcium hydroxide; referred to subsequently in this article as just lime):
2. CaO + H2O à Ca(OH)2
Both lime and quicklime are suitable for making limewater (kalkwasser) and otherwise supplementing calcium and alkalinity in reef aquaria. There are some important differences between the use of lime and quicklime that will be discussed in subsequent sections. These differences relate to the fact that quicklime is slightly more potent and gets hot when water is added to it (equation 2).
http://www.reefkeeping.com/issues/2005-01/rhf/index.php
I've never known that limewater only supplies calcium is a common misconception?