[Strawbale] Natural Hydraulic lime vs. Hydrated lime?

[Murray Hollis]

Allow me to offer some technical details about 'lime'.

The term 'agricultural lime' is usually reserved for calcium carbonate
[CaCO3], at least in Australia. Hydrated lime is calcium hydroxide
[Ca(OH)2], which is made by reacting calcium oxide [CaO], which is also
called quicklime, with water. That is a highly exothermic reaction that can
be quite hazardous. Both calcium carbonate and calcium hydroxide are used
for agricultural purposes, but the calcium carbonate is slower acting,
because it is much less soluble than calcium hydroxide, and therefore less
hazardous to plants. For agriculture, they use calcium hydroxide when they
want to rapidly increase the ph of the soil, but more commonly they use
calcium carbonate, which will quite gradually increase the ph.

Calcium hydroxide is the form used for building purposes, such as in
concrete and mortar mixes. However, Calcium hydroxide is not generally sold
as pure calcium hydroxide (except as a chemical reagent), but rather with
varying levels of impurities (typically about 10% impurities) and different
impurities depending on the source of the calcium carbonate (limestone) that
is used to make the calcium oxide (at temperatures of roughly 600 degrees
centigrade, which might not be quite the right temperature).

Actually, Calcium hydroxide has 'water of hydration' on each molecule
[Ca(OH)2.6H20], but although, chemically, such molecules are called
'hydrated', this are not the reason it is called 'hydrated'. It is called
'hydrated', because of the water that is used to react with the CaO. Each of
CaO, CaCO3 and Ca(OH)2 are called 'lime', so when people talk about lime,
you need to be aware that they could be talking about one of three quite
different materials.

I hope this is useful for those who desire to know.

Murray Hollis, Gundaroo, Australia

> -----Original Message-----
> From: strawbale-bounces at listserv.repp.org [mailto:strawbale-
> bounces at listserv.repp.org] On Behalf Of Rob Tom
> Sent: Wednesday, January 17, 2007 5:37 PM
> To: Raftercat5 at aol.com
> Cc: strawbale at listserv.repp.org
> Subject: Re: [Strawbale] Natural Hydraulic lime vs. Hydrated lime?
> 
> On Tue, 16 Jan 2007 20:29:14 -0500, David Elfstrom
> <listbox at elfstrom.com>
> wrote:
> 
> > You want Natural Hydraulic Lime. Almost certainly the one you saw in
> > the feed store's catalog was hydrated lime.
> 
> > Raftercat5 at aol.com wrote:
> 
> >> We thought
> >> we found a source locally, an animal feed store.   I think it was
> >> hydrated
> >> lime that was listed in the store's book that we can  order.
> 
> 
> Raftercat;
> 
> The hydrated lime that you saw in the feed store catalogue will almost
> certainly be agricultural lime (ie low grade, for amending soil) and
> unsuitable for plastering (ie too many impurities left in it from the
> calcining process.
> 
> For plastering quality hydrated lime, you'd need to visit a plastering
> contractors' supply house. (ie check you local Yellow Pages directory).
> 
> 
> === * ===
> 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
> 
> 
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