[It says right on the bag that it is food grade which means it is the exact same stuff that we buy at Albertson's packaged as pickling lime.
The quote below is taken from Randy Holmes-Farley's article found here,
http://reefkeeping.com/issues/2005-01/rhf/index.htm#1
"Pickling lime is, in most cases, food grade calcium hydroxide. Mrs. Wages website (a place to buy pickling lime online) refers to it as calcium oxide in some places, but my understanding is that this product is calcium hydroxide. [As an aside, some of Mrs. Wages other information about pickling lime makes no sense at all, such as saying the reason that it contains lime is proprietary, but in the line above, showing how it works.]
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).
A couple of other definitions are worth noting. Limewater is the solution that forms when lime (or quicklime) dissolves in fresh water. The solution is exactly the same using lime or quicklime, as long as the same amount of calcium is added. Kalkwasser is simply the German word for limewater. Neither term is ever correctly applied to a solid, so any solid material sold as “kalkwasser” is either lime or quicklime."]
