Practicalities on a hempcrete build - The Hempcrete Topic: Designing and Building with Hemp-lime - page 446

Clearly then, for hydraulic-lime-based binders to work, a window of time is required when
the ambient temperature remains well above freezing point while the binder reacts with the
water. For the fast-setting cements this is a matter of hours, but with natural hydraulic lime
(NHL) it takes days, depending on the strength of NHL used, with the more hydraulic ones
setting faster and harder. Always seek, and follow, the advice of the binder manufacturer
or supplier.
As a rule of thumb, work with any lime-based binder (including cement) requires a tem-
perature of 5°C or above. Depending on the type of lime or cement being used, the period
of time after application during which that temperature needs to remain constant varies. As
an example, when working with NHL 3.5, you really need the minimum temperature to
remain above 5°C for 7-10 days, with the risk of weakening due to a sudden temperature
drop decreasing with each day that passes, as more hydration occurs.
Although never used as a hempcrete binder in their pure state (i.e. without a pozzolan ad-
ded to make them more hydraulic), it is worth noting here that air limes (non-hydraulic
limes), which set through carbonation in contact with air, set very slowly indeed; some-
times the mortar inside a wall never sets completely. Because of this slow setting time,
these limes are even more susceptible to the effects of temperature, and this includes the
risk of excessive heat drying out the surface of the mortar or plaster and causing crack-
ing. If you are using these limes - for example, in fat lime (lime putty) plasters - ensure
that you understand exactly how the material will behave in a given situation. At least one
person on the team needs to be fully conversant with the preparation, application and after-
care of these products, or you are more than likely to run into trouble.
When it comes to hempcrete binders, individual binder product manufacturers specify the
minimum temperatures required for working with their products, and you should follow
this advice. However, it is advisable to familiarize yourself early on with the temperature
parameters required by the product you are using, as this stage of the work will be so much
easier and less stressful if you are not racing against inclement weather.
The obvious conclusion is that the best time for casting hempcrete is in the late spring
and summer, and this is indeed the preferred option, in terms of both avoiding the risk of
freezing and to encourage faster drying times (see below). However, it is possible to cast
hempcrete in colder weather, especially if the choice of binder is made carefully.
Prompt, as a natural cement, has a clear advantage here because it is naturally very quick
setting and is extremely hydraulic, using up a lot of water from the mix very rapidly. This