Industrial hemp is a cultivar of the Cannabis sativa plant grown for its wide range of commercial uses, including textiles, food products, and construction materials. When the tall, fibrous stalk of the plant is harvested and processed, it yields two main co-products: the long outer bast fibers and a woody inner core. This inner material, known as the hemp hurd, is a low-density, highly porous aggregate that has recently gained attention as a sustainable raw material. The unique physical and structural characteristics of the hurd enable its transformation into a variety of innovative products, forming the basis for a new generation of bio-based industrial applications.
Defining the Hemp Hurd
The hemp hurd, also referred to as shiv or shive, is the pithy, woody center of the industrial hemp stalk. The stalk is structured like a cylinder, with the outer layer composed of long, strong bast fibers surrounding this less dense, spongy core material. The hurd is a co-product of the process aimed at extracting the bast fibers, which are used for textiles and ropes.
Obtaining the hurd involves decortication, which separates the stalk’s two main components. Before mechanical separation, the stalks often undergo retting, where they are left in the field to naturally break down the pectin and gums that bind the bast fibers to the hurd. This exposure to moisture, such as dew or rain, weakens the connection between the layers.
The field-retted or dried stalks are then fed into a decortication machine, which uses rollers and crushers to mechanically break the stalk material. This crushing action shatters the inner, brittle hurd core into small, broken pieces, while the tougher outer bast fibers remain largely intact. The resulting mixture of coarse, woody hurd particles and long fibers is then separated through mechanical processes like air classification or screening.
The resulting hurd material consists of small, irregularly shaped fragments, typically ranging from 5 to 35 millimeters in length, depending on the desired end use. Once considered a low-value byproduct of fiber production, the hurd now represents a significant portion of the harvested plant biomass. This material is primarily composed of cellulose, hemicellulose, and lignin, much like wood, but with structural characteristics that lend themselves to specialized industrial uses.
Physical Properties and Structure
The value of hemp hurd is rooted in its physical structure, which is characterized by significant internal porosity. This cellular architecture gives the material an exceptionally low bulk density, making it very lightweight for its volume. The low density is an advantage for applications where minimizing weight, such as in construction or shipping, is beneficial.
This porous, sponge-like structure contributes to the hurd’s high surface area and allows it to exhibit an ultra-absorbent nature. Hemp hurd can absorb a significant amount of liquid, making it a preferred material for managing moisture. This absorbency, combined with its composition, also contributes to its natural resistance to mold and mildew, a desirable trait in bio-based materials.
The cellular makeup also gives the hurd insulating capabilities against both temperature and sound. When used in construction, the material’s ability to trap air within its porous matrix provides an effective barrier to heat transfer, contributing to a high thermal resistance, or R-value. This thermal performance is often compared favorably to traditional wood products, and in some cases, it can surpass mineral-based insulation materials.
The irregular, open structure of the hurd efficiently dampens sound waves, making it an excellent material for sound absorption applications. Furthermore, the hurd contains a relatively high silica content, which is a naturally occurring mineral. This chemical characteristic enables a strong chemical bond when the hurd is mixed with an alkaline binder, such as lime, for construction applications.
Primary Industrial Applications
The unique combination of low density, high absorbency, and insulating properties has positioned hemp hurd as a versatile raw material across several industries. The most recognized application is in the production of Hempcrete, a bio-composite material used as a non-structural infill in building construction. Hempcrete is created by mixing the hurd aggregate with a lime-based binder and water, which cures into a lightweight, insulating material.
The hurd pieces typically used in Hempcrete are chopped to lengths between 6 and 25 millimeters, and the lime binder coats these particles to hold the matrix together. While Hempcrete does not bear structural loads like traditional concrete, it functions as a highly effective thermal envelope for walls, floors, and roofs. During the curing process, the lime binder re-absorbs atmospheric carbon dioxide, effectively sequestering carbon within the building material over its lifetime.
Another major use for hemp hurd is as a superior animal bedding material, particularly for horses and small livestock. The ultra-absorbent nature of the hurd allows it to manage moisture and odor more effectively than traditional wood shavings or straw. Additionally, the material is naturally low in dust, which promotes better respiratory health for the animals and farm workers.
Hemp hurd also serves as a raw material for composite and insulation products. It is processed into specialized insulation batts or panels that offer high thermal performance and moisture regulation. The material is also incorporated into engineered wood products, such as particleboard, where it can replace wood fiber, resulting in lightweight boards with improved moisture resistance and strength.