An introduction to lime - The Hempcrete Topic: Designing and Building with Hemp-lime - page 51

chemical change to occur: carbon dioxide (CO 2 ) is given off, leaving behind calcium ox-
ide (CaO) - a substance known as 'quicklime'. Quicklime is a highly reactive material that
produces a large amount of heat when mixed with water. It needs storing and handling very
carefully, as it can easily cause serious injury during handling and can start fires if it gets
damp during storage.
When quicklime is mixed with water, in a process known as 'slaking', calcium hydroxide
(Ca(OH) 2 ) is produced. This is called 'hydrated lime'. This lime, when applied to a build-
ing in the form of mortar or plaster, will harden slowly by 'carbonation' - reacting with
carbon dioxide from the air.
Because the end result of carbonation is calcium carbonate - the same as the original raw
material - the process of transforming limestone to quicklime to hydrated lime to hardened
building lime is known as the 'lime cycle'. However, in reality the carbonated lime in the
building is a very different substance from the original limestone, in terms of its appear-
ance and properties and especially in its strength and hardness.
Air limes
When producing lime from relatively pure limestone or chalk deposits, such as those com-
monly found throughout the British Isles, there are two options for making hydrated lime
through slaking:
If slightly more than the sufficient amount of water needed to react with the quicklime
is used, the small excess of water is driven off by the heat to result in a powder known
as 'dry hydrate of lime'. This powdered hydrated lime (which is commonly sold in
builders' merchants as 'hydrated lime') is generally used as an additive to give some
extra workability to cement mortars. It is not of sufficient quality to produce reliable
and consistent lime mortars or plasters.
If significantly too much water is added during the slaking of the quicklime, then the
resulting lime runs into a free-flowing 'milk of lime'. This substance is sieved and
stored in a pit until it has settled into a dense white mass, called 'lime putty', covered
by a clear liquid - a saturated solution of lime known as 'lime water'.
Lime putty is always stored under water until needed, to prevent the process of carbonation
beginning. It is from this substance that high-quality lime mortars and plasters are made,
and it can be diluted with water, and coloured with natural pigments, to form limewash, the