An introduction to lime - The Hempcrete Topic: Designing and Building with Hemp-lime - page 68

Lime in hempcrete building
Throughout this topic we refer to hempcrete being a mixture of hemp shiv, water and a
'lime-based' binder. This is in order to convey the idea that various different hempcrete
binders exist (and new ones are being formulated all the time), all of which are based
on different combinations of naturally occurring lime products, sometimes together with
pozzolans or other additives.
Due to the construction process used in cast-in-situ hempcrete, a pure air lime or hydraulic
lime binder is not sufficiently strong, although they may be suitable in pre-cast applications
such as panels or blocks, or if building up the wall very slowly. This is because in the cast-
in-situ method the wall must support its own weight while it dries, but blocks and panels
can be cast, and dried, flat on the ground.
Those binders currently used extensively and successfully in the UK for cast-in-situ hemp-
crete are either a natural cement or a proprietary formulated hempcrete binder. The pro-
prietary binders' exact constituents are a closely guarded secret, but they are thought to
contain a majority of air lime, possibly with some hydraulic lime or a pozzolan, and a pro-
portion of Portland cement to give the fast hydraulic set necessary at the start of the setting
process.
For more information on binders, see Chapter 6 .
The use of lime in the plaster and render finishes for hempcrete walls, and in limewash if
used, is discussed in more detail in Chapter 18 .