[Strawbale] Slaked lime

Derek Roff derek at unm...
Wed Apr 6 20:05:06 CEST 2011

Lime terminology is confusing and inconsistent at times.  The word  
"lime" is particularly ambiguous, since it is applied to every stage  
of the plaster making process, and also to various things outside that  

For me, slaked lime means calcium hydroxide.  To make lime, limestone,  
whose chief ingredient is calcium carbonate, is quarried, ground, and  
heated in a kiln or pit.  Quicklime, from the lime kiln, is mostly  
calcium oxide.  The slaking process reacts quicklime with water, to  
produce slaked lime, mostly calcium hydroxide.  As lime plaster cures  
on the wall, it changes to calcium carbonate.  So one could argue that  
slaked lime is the only kind of lime that is useful for plastering.

Of course there are always impurities and other minerals in the mix  
that affect the performance of the plaster, both in terms of both ease  
of application and in how well it performs on the wall.  Hydraulic  
lime is calcium hydroxide with enough iron, silicon, phosphorous,  
aluminum, magnesium, and other minerals, that it will set up under  
water.  Some people like plastering with hydraulic lime, some don't.

Perhaps I am missing the essence of your question, Jure.  In general,  
getting the best, purest, freshest, driest slaked lime will give the  
best results.  The minerals in the limestone used to make the slaked  
lime can be significant.  British plasterers in particular like lime  
that is very high in calcium.  This may be hard to find in other  
places.  A large proportion of magnesium is common in many sources for  
lime (sometimes called "dolomitic lime").  Limes in Italy and the USA  
are often high in magnesium, and some plasterers prefer to work with  
these dolomitic limes.

Magnesium and calcium react at different speeds in a simple lime kiln,  
which can result in overburnt or underburnt components in the slaked  
lime.  These can cause problems such as spalling- bits of plaster  
spontaneously coming off the wall.  Slaked lime sold as type S in  
North America (and Australia?) uses a more complex process in  
production, which can reduce the underburn/overburn problem.  Soaking  
slaked lime in water for months or years (perhaps mixing in sand to  
make lime putty) can help correct some lime deficiencies, and improve  
the working properties of the lime plaster.  John Glassford of  
Australia says that he can get good bagged type S lime in his area.   
He can mix it with water and sand, and use it the same day.  He  
reports little or no improvement with extended soaking.  People in  
other places report that extended soaking of the lime available in  
those areas can help a great deal.

Does any of this address your question?


Derek Roff

On Apr 6, 2011, at 11:20 AM, Sport Hotel, Jure Pozar wrote:

> Hi,
> Is slaked lime ok for the exterior plastering or does it need to be  
> a special kind of lime for this purpose?
> Cheers,
> Jure Požar,
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