Variations on the hemp–lime mix - The Hempcrete Topic: Designing and Building with Hemp-lime - page 115

provide the initial set to the hempcrete with enough strength to support the weight of
the drying hempcrete wall (it needs a strong hydraulic set)
allow the water to continue to dry out of the hemp shiv after the initial set (it needs va-
pour permeability)
provide a full set to the hempcrete over time (it needs long-term structural strength).

Most types of building lime, including the feeble and moderately hydraulic limes, have a
slow initial set (over several days) and take a long time to reach full strength. Even once
the initial set has taken, most limes do not, by themselves, set sufficiently strongly at an
early enough stage to support the weight of a hempcrete wall while it dries.

If a hempcrete wall is built with a lime that does not achieve a quick enough, or strong
enough, initial set, the consequences can be severe. When the shuttering is taken down, the
lower parts of the wall are vulnerable to compacting and slumping outwards at the face of
the wall, or - in the worst-case scenario - collapsing completely (see Chapter 5, page 77 ) .

To avoid this, it is essential that the binder contains something that ensures a fast, strong set
within 24 hours so that the shuttering can safely be removed. Not only must a successful
hempcrete binder achieve this strong initial hydraulic set, but also it must remain vapour
permeable during and after setting, so that moisture can leave the hemp shiv and the wall
as a whole. This double function is achieved in one of two ways by binder products cur-
rently on the market.

First, there are two proprietary binders specifically formulated for hempcrete, which are
available in the UK at the time of writing. Although their exact ingredients remain com-
mercially confidential, they are widely thought to be a mixture of air lime (hydrated lime)
- the main ingredient - together with a certain amount of Portland cement, believed to
be around 20-30 per cent, depending on the manufacturer, and possibly a small amount
of other pozzolanic ingredients (see Chapter 3 for more information about lime, cements
and other additives). The proportion of cement included is enough to give the required ini-
tial strong set, without being enough to inhibit the overall vapour permeability of the cast
hempcrete. These proprietary binders are Tradical ® HB, made by Lhoist, and Batichanvre ® ,
made by St Astier. They were both developed in France, although Tradical ® HB is now

made under licence in the UK.

The second solution to achieving the required initial set is to use an extremely hydraulic
naturally occurring lime, such as a natural cement. One such natural cement, Prompt, made
by Vicat, while originally sold for other purposes, has also been tested as, and is now mar-