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Lime

Lime

There is hydrated lime, there is quicklime and there is lime putty. What are they? Consider limestone. It is rock that's composed of calcium carbonate (CaCO3), magnesium carbonate (MgCO3) and small amounts of other minerals that we don't need to care about. Now let's "calcine" the limestone rock. To do this, the rock is heated to 1,800° (Fahrenheit of course, we never heat anything in Centigrade). This removes the carbon dioxide from the rock's carbonate molecules giving you oxide molecules. The resulting material is "quicklime" or "calcined limestone". With the carbon dioxide gone, the stuff is now made of calcium oxide (CaO) and magnesium oxide (MgO). So far we've gone from limestone to quicklime. Quicklime is physically in an almost gravel like form and little pieces of quicklime are normally called "pebbles." When you see things that refer to "pebbled lime", that is quicklime that they are talking about.

In the past, contractors would take quicklime and soak it in water in metal boats or troughs at the job site. This soaking is called "slaking". Water molecules combine with the quicklime to form hydrated lime. These days you can purchase hydrated lime in bags. Or you can slake your own quicklime. Whatever you want to do. The manual slaking process can take anywhere from a few hours to several days to a couple of years to make aged putty! It depends on the purity of the quicklime.

Whether the quicklime was manually slaked on the site to produce hydrated lime or it was purchased from a commercial producer of lime products, hydrated lime is created when a single water molecule combines with a single calcium oxide molecule to make a single calcium hydroxide molecule: H2O + CaO = Ca(OH)2. And the same thing with the magnesium oxide molecules in the quicklime, a single water molecule combines with one magnesium oxide molecule to give you one magnesium hydroxide molecule: H2O + MgO = Mg(OH)2

Since all of the water molecules are chemically combined with the others, hydrated lime is still dry to the touch of human fingers. It is a powder. It isn't a wet substance.

Now we've gone from limestone to quicklime to hydrated lime. What next?

Add some water to the dry hydrated lime powder, stir it around and now you have lime putty. Lime putty is physically wet hydrated lime.

You can make limestone from lime putty or from hydrated lime by adding carbon dioxide molecules, thus completing a cycle of nature, if you want to do it. What happens?

Carbon dioxide plus calcium hydroxide equals calcium carbonate. Carbon dioxide plus magnesium hydroxide equals magnesium carbonate. When you add a single carbon dioxide molecule to a single calcium or magnesium hydroxide molecule you also get a water molecule released (H2O). You can save them to make lime putty later. Calcium carbonate and magnesium carbonate together make up limestone and now you're back to the rock.

CO2 + Ca(OH)2 = CaCO3 + H2O

CO2 + Mg(OH)2 = MgCO3 + H2O

If your heater is ready, you can start calcining the limestone again and make some quicklime.

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