Calcium carbonate is the name of a chemical substance , CaCO3.
Ground oyster shells are a natural product mainly containing CaCO3,
along with other minor ingredients.
Limestone is a sediment mineral composed mainly of calcium carbonate,
very similar to oyster shells.
Chalk (Blackboard chalk is NOT made of chalk!) is a form of calcium carbonate, having the same chemical composition
as ground calcium carbonate, limestone, marble, and precipitated calcium carbonate.
Horticultural lime, also called hydrated lime or Ca(OH)2
is produced by adding water to CaO.
Quick lime is CaO, a very aggressive substance. It is produced from
CaCO3 containing limestone or shellsand by heating it to 1200°C,
hereby CO2 is driven off the CaCO3 molecule, leaving CaO.
The material that is used in the mushroom industry as a casing ingredient is
always some form of calcium carbonate, CaCO3: either ground
chalk, ground limestone or ground shellsand.
Ground limestone sources:http://www.google.com/products?q=" ground Limestone" OR "limestone flour"