Variations on the hemp–lime mix - The Hempcrete Topic: Designing and Building with Hemp-lime - page 116

keted as, a binder for hempcrete. Natural cements set very quickly and very hard, but retain
the vapour permeability that Portland cement lacks. Prompt Natural Cement is made from
a specific type of limestone, using the same process as that used to produce hydraulic lime,
and is made in France, where this type of stone is found.
There is a further characteristic of hempcrete binders containing a high proportion of air
lime that is worth noting here. Since air limes set very slowly, through a process of carbon-
ation with the air, and since the water introduced during mixing has to gradually evaporate
out of the wall, it is usual for hempcrete made with such a binder to shrink slightly - often
by 2-3mm at the edges - as it dries. This is not a problem for the cast hempcrete material
per se, but it does have implications for airtightness in certain situations (see Chapter 21,
page 296 ).
Hempcrete binders.
The main characteristics of the three binders are set out in the table overleaf. 1 At the time
of writing, these three products are the only tested and reliable binders available for hemp-