Cob: lime renders

Sat Mar 8 17:03:28 CST 2003

Dry Hydrate Lime Mixes: Type N and Type SThe bags of lime bought at a 
builder's supply store are almost always hydrated lime, which is quicklime 
that has been factory slaked only to the point that a powder is formed and 
not a putty. Hydrated lime is far less reactive and dangerous than quicklime 
and usually does not have the same properties as lime putty or hot lime 
mixes. In the U.K., our hydrated limes are made from quite pure limestone, 
otherwise known as high-calcium lime. In the U.S., due to differing geology, 
much of the limestone contains proportions of magnesium. It can still produce 
a good material.

Limes containing magnesium were often less favored when slaked outside of 
factory conditions, due to the fact that the magnesium component took longer 
to slake. However, this is no longer the case with Type S hydrates. Type S 
hydrates are autoclaved, ensuring that all the magnesium oxide has been 
slaked, as well as the calcium oxide. Although Type S dry hydrate can be used 
right out of the bag, it improves when made into a putty and gets better 
still if left to remain even longer in the putty stage. Type N hydrate is 
only partially hydrated; only the calcium portion of the lime is combined 
with water. Type N hydrate needs to be made up into a putty and aged before 
using to make sure that all the magnesium oxide has slaked.

When you buy powdered hydrated lime it is difficult to know anything about 
the lime. Usually you won' know what type of lime it is, how it was burnt, or 
how it was slaked. However, you may be able to find out how long it has been 
on the shelf and that, in the long run, may be the most important thing to 
know. If the dry hydrate has been in the bag too long, it may have already 
begun to absorb moisture from the air and begun to carbonate, thereby 
declining in quality. Some manufacturers date their bags. If not , you can 
always ask when a shipment arrived.

How to Make Lime Renders From Dry Hydrate
Always use fresh hydrated lime, less than one month old if possible. Look for 
the date of manufacture on the bag. 
As far as possible, check the production process and buy from a reputable 
company --although this still doesn't guarantee the quality of the product! 
Make up the hydrate into a lime putty by putting it into a bucket and adding 
water. Stir well, and only add enough water to make a very stiff mix. Leave 
it for 24 hours or longer, and then make up the render as described 
previously. 
Use 1 part lime putty to 2-1/2 parts sand, by volume, to compensate for the 
extra aggregate if you think your dry hydrate is not quite pure calcium 
hydroxide. If you have reliable information that your hydrate is pure, then 
stick to the original 3-to-1 mix. 
Once mixed up with sand, use in the same way as any other lime render.  

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<HTML><FONT FACE=arial,helvetica><FONT  SIZE=2 FAMILY="SANSSERIF" FACE="Arial" LANG="0">Dry Hydrate Lime Mixes: Type N and Type SThe bags of lime bought at a builder's supply store are almost always hydrated lime, which is quicklime that has been factory slaked only to the point that a powder is formed and not a putty. Hydrated lime is far less reactive and dangerous than quicklime and usually does not have the same properties as lime putty or hot lime mixes. In the U.K., our hydrated limes are made from quite pure limestone, otherwise known as high-calcium lime. In the U.S., due to differing geology, much of the limestone contains proportions of magnesium. It can still produce a good material.<BR>
<BR>
Limes containing magnesium were often less favored when slaked outside of factory conditions, due to the fact that the magnesium component took longer to slake. However, this is no longer the case with Type S hydrates. Type S hydrates are autoclaved, ensuring that all the magnesium oxide has been slaked, as well as the calcium oxide. Although Type S dry hydrate can be used right out of the bag, it improves when made into a putty and gets better still if left to remain even longer in the putty stage. Type N hydrate is only partially hydrated; only the calcium portion of the lime is combined with water. Type N hydrate needs to be made up into a putty and aged before using to make sure that all the magnesium oxide has slaked.<BR>
<BR>
When you buy powdered hydrated lime it is difficult to know anything about the lime. Usually you won' know what type of lime it is, how it was burnt, or how it was slaked. However, you may be able to find out how long it has been on the shelf and that, in the long run, may be the most important thing to know. If the dry hydrate has been in the bag too long, it may have already begun to absorb moisture from the air and begun to carbonate, thereby declining in quality. Some manufacturers date their bags. If not , you can always ask when a shipment arrived.<BR>
<BR>
How to Make Lime Renders From Dry Hydrate<BR>
Always use fresh hydrated lime, less than one month old if possible. Look for the date of manufacture on the bag. <BR>
As far as possible, check the production process and buy from a reputable company --although this still doesn't guarantee the quality of the product! <BR>
Make up the hydrate into a lime putty by putting it into a bucket and adding water. Stir well, and only add enough water to make a very stiff mix. Leave it for 24 hours or longer, and then make up the render as described previously. <BR>
Use 1 part lime putty to 2-1/2 parts sand, by volume, to compensate for the extra aggregate if you think your dry hydrate is not quite pure calcium hydroxide. If you have reliable information that your hydrate is pure, then stick to the original 3-to-1 mix. <BR>
Once mixed up with sand, use in the same way as any other lime render.  <BR>
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