[Cob] Re: Sorry Jon...

[ocean]

Sorry Jon Kerr!  Based on your posting, you don't "know this one"...

First of all, I didn't say that hydrated lime was calcium oxide, but 
rather CALCIUM HYDROXIDE ( CaH2O2 )  Calcium oxide is quicklime.

And no, our bones are not made of quicklime/calcium oxide, but rather 
calcium carbonate ( CaCO3 ) the same material as limestone..

So let's take a moment to review the whole process!

"Quicklime" (calcium oxide or CaO) is formed when "limestone" (calcium 
carbonate or CaCO3) is quarried and fired/kilned at high temperatures.  
This resultant highly reactive & caustic material can be slaked by 
adding water (H2O) which forms "hydrated lime" (calcium hydroxide or 
CaH2O2).

When hydrated lime is added to water and applied to a surface as a 
limewash, the resultant finish gradually recombines with free carbon 
dioxide (CaO2) in the atmosphere and becomes calcium carbonate once 
more.  At this point your lime plaster has returned to its original 
state, limestone!

My understanding of the traditional preference for lime putty ("slaked" 
quicklime) vs. "builders" hydrated lime is that dry powdered hydrated 
lime in the bag has already begun exposure to the CO2 of the atmosphere 
so may already have recombined into CaCO3 even before application.  
This doesn't happen with traditional lime putty, which is kept in water 
or hydrated (for up to three generations!!) and hence doesn't begin the 
final process of recombining into CaCO3 until the lime plaster has been 
applied to a wall and beings to "dry".

Is this clear, or does anyone have any additional thoughts?

Ocean



On Jan 13, 2005, at 12:23 AM, Jon Kerr wrote:

> Ooh! I know this one!  This is one I actually know the answer to!  
> Whee!
>
> As Ocean mentioned before, hydrated lime is calcium oxide, which the 
> important part is the mineral calcium. Your bones are made of it! The 
> chemical processes it goes through change it from its original form, 
> which was limestone, into calcium hydroxide etc etc. As the lime 
> plaster is exposed to the air, the carbon dioxide (or was it oxygen?) 
> in it returns the lime putty to its original state. The end product is 
> limestone again!
>
> So whatever you coat with a lime plaster will eventually be covered 
> with a layer of rock.
>
> Jon