Variations on the hemp–lime mix - The Hempcrete Topic: Designing and Building with Hemp-lime - page 114

In this chapter we look in more detail at the hemp-lime mix itself, outlining the vari-
ables in both the hemp shiv and the binder that can affect the characteristics of the
finished hempcrete, and describing how the mix is adjusted for different applica-
tions.
Variations in the hemp shiv
In general, there should not be much variation found in the type of hemp shiv supplied for
building, which should meet the requirements as set out in Chapter 2 . That is, the pieces
should be about 10-25mm long, and may contain a certain amount of fines, but as far as
possible should not contain any dust (see page 26 ). The only variation commonly found is
that a finer-grade shiv (with shorter pieces of shiv) is commonly used to make hempcrete
for spray application, as the longer particles in the standard shiv can sometimes lead to
blocking of the spraying hose.
When placing by hand, coarse-grade shiv should always be used, since the matrix these
create when they are set in the wall provides an inherently stronger and more open (breath-
able) structure. The shorter pieces of hemp in spraying shiv do not create the same matrix
structure, but the additional adhesion given them by the forced projection of the spray-ap-
plied hemp compensates for any loss of structural strength.
Variations in the binder
There are two stages to the setting of a hempcrete wall. The first is the initial set, which has
to happen before the shuttering can be taken down. Because the shuttering needs to be re-
moved as early as possible so that the hempcrete can start drying out, the initial set needs to
be strong enough that the freshly cast hempcrete can support itself while the second stage
takes place. This happens slowly, over a number of weeks, as the hempcrete sets fully and
any excess water dries out of the wall.
At the risk of stating the obvious, the binder is the part of the hempcrete that is responsible
for both stages of the set. Although the binder for hempcrete is lime-based, choosing a
binder is not simply a case of choosing your preferred lime, since there are several func-
tions required of the binder, and most limes will not provide all of these by themselves.
A hempcrete binder needs to: