Written by Megan Schires
Hemp is one of the oldest crops domesticated by humans. With its wide variety of uses and applications, it’s easy to understand why it’s been a desirable product throughout history. Hemp seeds and flowers are used in health foods, medicines, and organic beauty products; the fibers and stalks of the hemp plant are used in clothing, paper, and biofuel. Today even a waste product of hemp fiber processing, so-called hemp shives, is being utilized to create sustainable building materials like hempcrete.
Hempcrete is a bio-aggregate concrete, where the hemp shives - small pieces of wood from the stalk of the plant - are mixed with either a lime or mud cement to create a durable, eco-friendly building material. Hempcrete is lightweight and non-structural, but can instead be integrated with traditional building construction systems. Similar to traditional concrete, it can be either cast-in-place or prefabricated into building components like blocks or sheets.
The high silica content found naturally in the woody parts of the hemp plant means it bonds well with lime. The lime binding agent used in hempcrete is in the form of calcium hydroxide, which then begins absorbing carbon dioxide from the atmosphere to create calcium carbonate, or limestone. This means hempcrete is not only durable, but actually a carbon negative material. Once cast, hempcrete requires significantly less water than traditional cement to cure, contributing to the preservation of this precious natural resource.
Because hempcrete is made from a natural waste material, its entire life cycle as a building product is environmentally-friendly, all the way to its eventual reuse or recycling in the case of demolition. Even the cultivation of the hemp plant requires less water, pesticides, and fertilizer than other crops. Hemp is easy and quick to grow in almost any part of the world and provides two harvests per year. As it grows, it sequesters CO2, prevents erosion, naturally stops weed growth in its surroundings, and also detoxifies the soil. What is left after harvest breaks down into the soil, providing valuable nutrients and making hemp a desirable rotation crop for farmers.
Once the hemp becomes hempcrete, its benefits continue. In the case of a fire, the lime coat provides adequate fire-resistance for inhabitants to evacuate. It also reduces fire spread and risk of smoke inhalation because it burns locally and without smoking. Hempcrete won’t cause any skin or respiratory problems and is also vapor-permeable, creating a healthy indoor environment. Its lightweight structure and the air pockets created among the particles mean hempcrete is both earthquake-resistant and an efficient thermal insulator.
Hemp is naturally resistant to both mold and pests, as well, and even 1500 years ago humans were making use of these particular properties. In India’s sacred Ellora Caves, artwork from the 6th century CE has been preserved due to the ancient people’s use of hemp plaster. A research team led by Indian scientist M.R. Singh found that these artists had crushed the hemp plant and mixed it with lime to form a plaster. The hemp’s ability to naturally repel pests and regulate humidity means that the artwork in these caves survived the test of time, while that of the Ajanta Caves, built before those of Ellora and not utilizing hemp in their plaster, was eventually destroyed primarily by silverfish insects.
Today in the US, companies like Hempitecture work to research and develop the potential of hempcrete as a sustainable building material, working with local governments to farm the hemp and create prototype structures. Hempitecture is working to make hempcrete a more widely accepted and used building material could have economic and social benefits as well as environmental.