Google:
http://www.marinedepot.com/Ecosystem...FIAD3P-vi.html
May also be the calcium part of any number of two part additives (B-IONIC, Kent A & B, etc). Solubility of calcium Carbonate is very low (< 500PPM), so that Ca++ in the HCO3- or CO3-- usually will react to form an insoluble precipitate. Even when mixed dry, the mix is so hygroscopic that the water the mixture attracts is enough to cause a reaction between the three ions, resulting in clumping of the mix and loss of solubility of the Ca++ as CaCO3 if left open to the air. Much as does hydroxy-apatite [Ca3(PO4)2]3.Ca(OH)2, the ~64% inorganic part of bone in mammals (but bone is mostly the phosphate salt rather than the carbonate salt (as aragonite or calcite in seawater or coral skeleton)).
Usually CaCl2 for solubility reasons (a chloride salt of an earth metal). Some products also add trace elements to one of the two parts, but for the sake of solubility, it is usually the bicarb part unless it is incompatable with the pH of the alkalinity supplement.
HTH
