According to this article:
Hempcrete was discovered in a bridge abutment in France built in the 6th century. Since its rediscovery it has seen growing use in Europe. Industrial hemp is grown by certified commercial growers so the crop can be certified to be very low in THC. Hemp is not psychoactive. Given it has survived 14 centuries, people expect hempcrete buildings will have a long life.
It goes on to say:
Hempcrete is a bio-composite made of the inner woody core of the hemp plant mixed with a lime-based binder. The hemp core or “Shiv” has a high silica content which allows it to bind well with lime. This property is unique to hemp among all natural fibers. The result is a lightweight cementitious insulating material weighing about a seventh or an eighth of the weight of concrete. Fully cured hempcrete blocks float in a bucket of water. It is not used as a structural element, only as insulating infill between the frame members though it does tend to reduce racking. All loads are carried by internal framing. Wood stud framing is most common making it suitable for low-rise construction. Hempcrete buildings ten stories high have been built in Europe.