[Strawbale] Lime Plaster, The Magic Numbers

[CALXA at aol.com]

Hi Rob Tom,
 
Yes, ratios do change due to the lime and the sand (chemistry and particle  
size)  properties.
 
Historically the recommendation has been 1:3...but literature points out  the 
mixture may vary from 1:2  to 1:4, or even higher... All due to the  
particular properties of the lime, sand and the water used..
 
One ought to try different ratios to determine the best  
properties...plasticity - spreading properties, etc, drying shrinkage, etc,  before determining 
the best ratios for the materials used....  
 
BUT, that appears to be too difficult for most folks...and the result is  the 
1:3 ratio recommended by many folks....
 
It is not a simple mechanical mix - application...it really is an art to  
create the best plasters, mortars, etc...
 
An example might be: 
 
        Type S Dolomitic hydrate is  aprox 40% magnesium hydroxide/60% 
calcium hydroxide.The pH of magnesium  hydroxide is about 10; while the pH of 
calcium hydroxide is 12.4. The pH needed  to solubulize the surface of silica sand 
particles is about pH 11, plus. (the  higher, the better). So, you,see,  the 
magnesium content doesn't add to the  solubility of the silica. One needs to 
solubulize the surface area of the sand  particles so the chemical reaction can 
form calcium silicate....the cement that  holds everything together.
 
The value of the Type S Dolomitic hydrate is the particle size of both the  
magnesium and calcium hydroxides.As this material is pressure hydrated,  
virtually all of the magnesium oxide is hydrated, and the calcium hydroxide is  also 
fully hydrated into very small particle size particles - which results in a  
very high surface area. As calcium hydroxide is almost insoluble in water, the 
 high surface area exposed more reaction areas to react with the 
silica...forming  the calcium silicate. 
 
The high surface area of the magnesium hydroxide means that it too will  hold 
onto the water molecules, so the plaster doesn't dry out too quickly. A  
Water layer is absolutely necessary for the chemistry to work. In olden days,  
ONLY high calcium hydrated lime was used, and then only after aging the putty  
for over a year......all art learned from years of experience.....(Aging assured 
 any hard burned magnesium and calcium oxide particles finally hydrated into 
the  small particle size needed)
 
And then, still, the plasterer learned the art of testing the mix ratios  
before committing to the project....
 
And, after applying the plaster, it was covered in burlap like cloth, and  
the burlap kept damp, to insure the moisture level (humidity) was maintained, so 
 the chemistry reactions would go to completion....forming calcium  
silicate... 
 
Afterwards, excess lime would absorb carbon dioxide from the air (still  
needing water moisture to accomplish this), re-carbonating excess lime and  
forming calcium carbonate crystals, slowly filling the voids where the water was  
now evaporating...
 
This prevented shrinkage cracks, and weak plaster.....as these voids  were 
filled with calcium carbonate.  
 
Today, we are in too much of a hurry to age putties so long, or do much  
testing. The development of Type S Dolomitic hydrate does solve some of these  
problems, due to the small particle size and water holding capacity....
 
Yet, from a theoretical standpoint, it would require a higher lime/sand  
ratio, than the use of high calcium hydrate.
 
Hope this makes sense....
 :)
 
Best Regards,
 
 
Harry Francis
 
 
 



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