Chas Hurst wrote:
...


Limestone is a sedimentary rock composed chiefly of calcium carbonate, which
in its' pure form is white. Limestone varies widely in color due to other
material that accreted in the sediment.
The lime associated with building is not calcium carbonate but calcium
hydroxide which is quite caustic.

Calcium Oxide is caustic in its anhydrous form. When it absorbs water
it goes to CaO * 6H20 or maybe CaOH as you suggest. Pardon me if I got
this wrong -- it's been nearly 50 years since I was in high school
chemistry class!