Civil Engineering Reference
In-Depth Information
to calcium carbonate again. Just as making
quicklime needed heat, the reverse process
PRODUCES heat. For this reason, NEVER
add water to quicklime - always do it the
other way round or it may explode!
other raking and mixing. Care must be
taken when placing the bath, as the heat
generated can burn any grass or wood
underneath it. Add some of the quicklime
to the water, and immediately rake and mix
it: a garden hoe is the best tool for the job.
The process of adding quicklime to water
is called
slaking
, and great care needs to
be taken as the temperature soars and the
whole mix starts to bubble and boil.
CaO + H
2
O ᇕ Ca(OH)
2
+ heat
(calcium oxide + water ᇕ calcium hydroxide
+ heat)
So quicklime added to water gives us . . .
lime putty!
Slowly add the rest of the quicklime, raking
continuously. The purer the quicklime is, the
faster the hydration process occurs. Keep on
raking and mixing until the lumps have all
broken down, and then leave to cool comp-
letely. Sieve the resulting putty, which feels
like double cream at this point, through a
1⁄16
Lime putty can be ordered from a supplier,
ready-made in airtight sacks or tubs, or you
can make your own.
Protective clothing, goggles and a mask
should be worn when working with quick-
lime.
” grid to take out any pieces of limestone
that were not burnt in the kiln, as these
will not react. These can go back in the kiln
for burning next time. What you now have
is slaked lime, or lime putty, which is best
stored for at least three months before use.
This is to ensure that most of the calcium
oxide has hydrated, which takes time. It's
important not to expose lime putty to the
air, or it will begin to carbonate before you
use it. If this happens, the lime will not be as
reactive and will make a much weaker mix
than you require.
How to make lime putty
Recipe:
2
parts water to 1 part quicklime (by
volume).
To make lime putty (calcium hydroxide,
Ca(OH)
2
), great care must be taken and
protective clothes, mask and gloves MUST
be worn, as a tremendous amount of heat
is generated, producing steam and spitting
lime.
Traditionally, lime putty was stored in a pit
in the ground, where it would remain for
several months or years before use. The
advantage of a pit is that any excess water
rises to the top, leaving good-quality putty
below. The Romans forbade the use of any
lime putty that was less than three years
old, and all the old practitioners say that the
older the lime is the better it is for the job.
Lime putty in pits was a valuable commodity
and would often be bequeathed from one
NEVER ADD WATER TO QUICKLIME,
BECAUSE IT CAN EXPLODE. ALWAYS ADD
QUICKLIME TO WATER.
First, pour water into a metal bath (do
not use plastic), twice as much by volume
as the lump lime to be used. Two people
are needed: one adding the lime and the
