Mixing hempcrete - The Hempcrete Topic: Designing and Building with Hemp-lime - page 319

Mixing hempcrete is easy to get wrong when you are not practised at it, and there is not
much margin for error. Getting the mix even slightly wrong can have serious negative ef-
fects on the finished wall. For this reason we recommend that you not only follow the
binder product manufacturer's guidelines to the letter, but also pay close attention to the
principles and method described here. It goes without saying that if you have not done it
before, you need to make up and cast some test mixes of hempcrete before starting on a
real project. The construction of a section of test wall enables you check and adjust both
your mixing and placing techniques.
Basic principles
The first thing to say is that personal protective equipment (PPE) is extremely important
when mixing hempcrete. If you attempt to do it without the correct equipment you will get
chemical burns from the lime in the binder. Lime burns are painful, and can cause serious
health problems in the case of getting it in your eyes (permanent loss of vision), or airway
(acute irritation and/or chronic respiratory problems).
Read Chapter 9 (Health and safety) very carefully, and make sure you put it into practice.
You are not the exception to the rule. Even if you are a bloke. What's more, if you are em-
ploying people to do this work, you (and also they) have a duty to ensure that safe working
practices are followed, that the correct PPE is worn or used, and that proper training and
instruction is given to anyone carrying out this work. You also need to keep a first aid kit
handy with the correct items in it, the most crucial of which is eye wash.
The key principle to remember when mixing hempcrete is the importance of the mix in-
cluding all the constituent materials - hemp shiv, binder and water - in the right propor-
tions. For those used to working with cement mortars and concrete, this may seem strange:
with these, there is much more leeway for the mix having slightly too much, or too little,
cement in it. The gauging of the cement binder to the required sand and/or gravel is usually
done quite roughly with a spade or shovel. And the worst consequence of a cement mix
that is too wet or too dry is likely to be your boss grumbling about its workability, rather
than it having a serious effect on the building's structural integrity. Not so with hempcrete.
Because the aggregate in hempcrete is a plant material, and because the desired consisten-
cy is a light and fluffy (low-density) mix, with every piece of hemp shiv coated in binder,
exactly the correct proportions must be added.