Making your mix
With the Australian Hemp Masonry Companies hemp lime building material the aim is to obtain an aerated mix resembling sticky crumble in which the hemp particles are well covered by the binder materials without the building material being too wet. The hemp should be as clean and dry as possible with minimal dust or foreign matter. If there is suspicion of damp or rot the material should not be used. Hemp for building should always be stored in a dry environment.
STEP 1: In a pan mixer, add the hemp, then add the water.
Allow the ingredients to mix for a few minutes till all of the hemp has been coated with water. It is important that all surfaces are wetted so that the binder can adhere to the hemp. It’s equally important that this wetting is done quickly because hemp is extremely absorbent and you don’t want the water absorbed, you want enough moisture on the surface of the hemp for the binder to adhere to.
Lime based masonry needs thorough mixing so that a light fluffy mix is achieved. This is quite a fast process.
STEP 2: Add the binder next and mix for 5 – 10 mins.
This is dusty initially. Wear a dust mask. Using a garbage bin lid to seal the mixer if using a drum mixer as this can reduce the dust initially. A water mister can be used with pan mixers. It’s important that all of the hemp is coated in binder. So stop the mixer and check that the mix to ensure the material is mixing evenly.
While it’s important not to make the masonry mixture too wet as you need the binder to coat the hemp, vary the water to suit the conditions by water spraying the mix slightly in very hot weather or windy weather. This is also necessary if there are any long delays before placing the mixed material.If the mixture is too wet and tamped firmly, a paste is forced to the outside of the mix and the lime solution leaches out of the shuttering resulting in a poor mix.
Do not use material that has been mixed for more than 6 hours. According to the hemp building industry in the UK any old mix material should be added to new mixes at a rate of 10% per mix.
STEP 3: The sand is added
This is the last ingredient to the mix to avoid the formation of pellets or balls. Once all of the hemp in the mixer is evenly coated with the binder, half of the sand is tossed into the mix, mixed briefly, and then as the material is being emptied into your wheelbarrow the second half of the sand is tossed through the remaining material. If this process is not done quickly the sand and the lime combine to form balls in the mix.
Your mixer should be selected to suit the task and site organisation. While it is possible for an owner builder to mix hemp masonry onsite in a normal 60 litre or 3 cubic foot rotary drum mixer, this is slower than using a pan type mixer . A challenge is ensuring that the minimal lime binder does not get left on the walls of the drum. Using a pan style mixer is highly recommended. For the material to be commercially competitive with other building materials, the builder is advised to mix in larger batches using at a minimum a 250 litre pan mixer.
For large commercial construction projects, larger mixers up to an 800 litre pan mixer operating on a slow speed of rotation is recommended so that bigger batches can be made up and there is no down time while workers are waiting for mix to place in the walls. For single home construction a 250 litre mixer is recommended.