Impact of chemical treatments on hemp hurds and hempcrete: evaluating strength and thermal properties

Hemp concrete, introduced in Europe in the early 1990s, is a lightweight, non-structural composite that uses hemp hurds as aggregates. It is valued for its low thermal conductivity and high heat capacity, providing excellent insulation. However, its mechanical strength is limited by poor interaction between the organic aggregates and cementitious binders. Existing knowledge is reviewed in this paper, and chemical surface treatments to improve hemp–binder compatibility are explored. In the experimental work, sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)2) and sodium silicate (Na2SiO3) were investigated as hemp treatments. Sodium hydroxide treatment removed amorphous components and induced fibrillation, roughening the surface and improving the composite’s compressive strength by 15–45%, although thermal conductivity increased by 20–40%. Calcium hydroxide treatment provided calcium ions that reacted with pectin to form adhesive bonds, raising the composite’s strength by 8–15% while reducing thermal conductivity by 7–27%. Sodium silicate enhanced roughness and created silica bonds at the interface, improving the composite’s strength by 10–25% and thermal conductivity by 7–17%. Changes in heat capacity remained within 12% across all three treatments. This research thus contributes to the optimisation of bio-based concretes by improving the compatibility between hemp hurds and cementitious binders, leading to improved mechanical and thermal performance.