[Strawbale] Fwd: RE: Natural Hydraulic lime vs. Hydrated lime?

[Rob Tom]

------- Forwarded message -------
From: "Murray Hollis" <Murray at wordworks.com.au>
To: "'Rob Tom'" <ArchiLogic at yahoo.ca>
Cc:
Subject: RE: Natural Hydraulic lime vs. Hydrated lime?
Date: Wed, 02 May 2007 19:45:29 -0400

Rob
I prepared the following to send to the list, but it seems that I omitted  
to
actually send it. You are welcome to post it to the list if you feel so
inclined. I hope it is helpful.
Regards, Murray Hollis, Australia

> -----Original Message-----
> And bagged hydrated lime which will be slaked to make lime putty for
> plastering should be as fresh as possible since the dry powder does
> absorb CO2 from its surroundings, turning it back into limestone.
> Rob Tom

The traditional way to make 'lime putty' is to 'slake' quicklime (Calcium
Oxide --- CaO) --- I.e. react CaO with water. Mixing hydrated lime  
[Ca(OH)2]
with water is not correctly described as 'slaking'. There is no chemical
reaction in this case. The Ca(OH)2 has a solubility of about 2% in water  
and
otherwise it is Ca(OH)2 in suspension in the water. If CaO is 'slaked' by
reacting with water, it also produces a suspension of Ca(OH)2 in water. It
does this by undergoing a chemical reaction with the water and liberating a
large amount of heat (quite a hazardous process). The resultant Ca(OH)2 in
suspension (lime putty) tends to have a significantly smaller particle size
and consequently has slightly different physical properties.

As far a powdered Ca(OH)2 "absorbing" CO2 from the air, I believe that to  
be
largely a myth. Scientific tests have demonstrated that CO2 is absorbed by
Ca(OH)2 very slowly, especially if it is dry. Actually, "absorbed" is not
the correct description; the CO2 chemically reacts to produce CaCO3  
(Calcium
Carbonate --- limestone), but does so by a complicated and slow process.  
The
CO2 does not react directly with the Ca(OH)2, but rather the CO2 first  
needs
to dissolve in water to produce a weak acid called Carbolic Acid (H2CO3),
which then reacts with the Ca(OH)2 to produce CaCO3. If the Ca(OH)2 is
completely dry the CO2 has no water to dissolve in. However, there is
moisture in the air. It is this stage that I am not clear about; that is
just what form the water in contact with the Ca(OH)2 needs to take in order
for the reaction to form carbolic acid to occur. But I do know that the
process is very slow.

For example, when lime plaster (i.e. Ca(OH)2 suspension in water plus sand)
is applied to a wall, while it remains wet it has a much increased chance  
of
reacting with CO2 in the air to produce calcium carbonate. However, even
when it is wet the reaction proceeds very slowly. Measurements have shown
that the depth to which the calcium carbonate is formed over a period of
months following a normal plastering process is of the order of a tenth of  
a
millimetre. Lime plasters as old as 2000 years still contain a significant
amount of Ca(OH)2, so even after being exposed to the weather for that
length of time the lime component of the plaster still  is not entirely
'limestone'.

Finally, take care in interpreting what people mean by "lime", because this
term is used to refer to quicklime (CaO), hydrated lime (Ca(OH)2) and
limestone (CaCO3). And, as a final, final comment, Ca(OH)2 is called
"hydrated" lime because of the process by which it is formed by mixing  
water
with CaO, but  the terminology can be confused with the chemical "water of
hydration" which also is present. Ca(OH)2 chemically combines with water
molecules to produce Ca(OH)2.6H2O, which is still dry. This is a hydrated
molecule, but is not the reason that Ca(OH)2 is commonly referred to as
"hydrated". Also that water of hydration is not available to react with the
atmospheric CO2.

Sorry if all of this is a bit technical, but for those who want to know, it
might help to dispel some myths and misinterpretations.

Murray Hollis, Australia



-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at   C h a f f Y a h o o  dot  C a  >
(winnow the chaff from my edress in your reply)