Civil Engineering Reference
In-Depth Information
oxide, has been slaked. Type N hydrate
must be made up into a putty and stored
for a while to make sure that all the
magnesium oxide has slaked, as it takes
longer for magnesium to slake than it does
for calcium.
lime in the dry hydrate can cause 'popping'
of the surface as the calcium oxide reacts
suddenly with water (rain) and explodes,
leaving a pockmark in the wall.
How to make lime renders from dry
hydrate
Why dry-hydrate renders can fail
If you do want to use dry hydrate, follow
these guidelines.
There are several reasons that renders
made from bagged (dry) hydrate often fail.
The lime may have been either over-burnt
or under-burnt (or both); it may not have
been completely slaked, leaving lumps of
calcium oxide; or it may have been stored
for too long before use, so that an unknown
quantity of it has already carbonated and it
therefore becomes useless and unreactive.
For the purposes for which bagged hydrate
is used today - as an addition to cement
mixes - none of this matters too much.
Some of the French bagged limes have been
found to contain either pure chalk (a type
of limestone) or other impurities, such as
white clay, as well as the calcium hydrate.
Obviously, this means that for every part
of dry hydrate used, only some of it will be
able to carbonate. When made into render
or plaster it may therefore contain a lot
less lime than expected, and it is hard to be
sure, without chemical analysis of each bag,
exactly what the proportions are. Since it is
only the lime that forms the binder between
the sand particles, using impure dry hydrate
can have the effect of increasing the amount
of aggregate and decreasing the amount of
lime - so that if you used a 3:1 mix it may
actually be 4:1 or 5:1. In practice, a 'lean' mix
(insufficient lime binder) causes the render
to become powdery in places, which decays
over time as it is more prone to damage
from the weather. The presence of unslaked
1. Always use fresh hydrated lime, less
than one month old if possible. The date
of manufacture should be on the bag.
2. 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!
3. Make up the hydrate into a lime putty by
putting it into a bucket and adding water.
Stir well, and add only enough water
to make a very stiff mix. Leave it for 24
hours, and then make up a lime render
as for a lime putty mix (see page 95).
4. If you think your dry hydrate is not quite
pure calcium hydroxide, use this recipe
to compensate for the extra aggregate:
2½ parts sand:1 part lime putty. If you
have reliable information that your
hydrate is pure, then stick to the original
3:1 mix.
5. Once mixed up with sand, use in the
same way as any other lime render.
