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Hemp-lime concrete: what is it?
Hemp-lime concrete or hempcrete is a mix of the chopped up woody stalk (shiv) of industrial hemp shiv (Cannabis sativa) with a lime-based binder and water.
Hemp stalk showing the fibre and core
Industrial hemp is a fibrous plant similar to flax and jute. It is a fast-growing (three to four months) carbon sequestrating broad-leafed plant which requires minimal fertilisers, pesticides and herbicides and only moderate amounts of moisture. Hemp was first harvested by the Chinese over 8,500 years ago, then introduced to west Asia, Egypt and Europe between 2000 and 1000 BCE (Small and Marcus, 2002). Industrial hemp is grown for fibre and grain: fibres are used to produce textiles (the name ‘canvas’ is derived from cannabis), string and rope, and paper; grain is used for food, animal feed and oil. While of course it comes from the same family as the cannabis drug, breeding hemp over centuries for tall slender plants with long fibres has reduced the tetrahydrocannabinol (THC) content (the psychoactive ingredient) to less than 0.3 per cent, making the plant a poor source of drug material. China leads the world in the production of industrial hemp; it is grown in over thirty countries including the United Kingdom, where it must be grown under the Home Office Low THC Cannabis (Hemp) licence.
Industrial Hemp Crop in Peasenhall Road, Walpole, Suffolk
Once a crop is cut, the hemp stalks are left in the field until sufficiently rotted (retted) by rain and dew; then they are baled and stored indoors to allow drying to halt the retting process. The outer fibres are separated from the inner woody shiv using a decorticator, which also shreds the shiv into chips. The chips, originally considered to be a low-value waste product of the fibre industry, predominately marketed as animal bedding, have begun to be used as a bio-based construction material. By 2004, about four per cent of the hemp shiv produced in the EU was used in construction (Karus and Vogt, 2004).
Hemp shiv before casting
Component DOI: http://dx.doi.org/10.15180/150405/006