What rock turns into plaster when heated?

Limestone is quarried, broken up and heated to specific controlled temps. This resulting "quicklime" is then 'slaked' ( a special soaking process) in water to make a dry hydrate that is bagged and sold to the public. When the dry lime is again mixed with water into a plaster, it will cure- because of the AIR- on the walls and the reabsorption of air/CO2, turns the powder back into limestone again. Cement is the opposite- adding water stars the curing process immediately. Lime plaster needs time, and thin application, for the air to cure it back into limestone.

Limestone is NOT calcined at high heat-- the temp. is kept under 1800 deg to reduce the busted up stone to a 'quick' lime. Slaking does not take 6 months to 10 years. A controlled quick lime process takes only enough time for the added water to turn the lime back into a dry powder, which is now called Hydrated lime- sold in 50# bags- this is what most Americans buy to mix as plaster or mortar, or farm use. Quick lime is too dangerous to safely mix for the DIYer, so it is harder to find and buy. Quick lime IS sold as lump lime in bags, and the slaking process occurs then, under controlled conditions by experienced mixers. Hot lime gives off steam and heat in an exothermic event when the water and quick lime meet; this is where moisture in your eyes, nose, or skin causes burning during the slaking process.

SOAKING the powdered hydrated lime in water for months and years produces a better, creamier product to work with. ( Slaking happens only once) This lime is very drying and can be caustic to skin, but is not dangerous, will not give off heat/steam; thousands of natural builders are using it daily.