Insights into the influence of hemp size and proportion on the strength and thermal performance of hempcrete

Hempcrete is a sustainable bio-composite known for its excellent thermal insulation. This study explores the effects of hemp hurd size and content on the mechanical and thermal performance of hempcrete to determine the optimal mix parameters. Four hemp hurd types were examined: HS1 (coarse, long flaky hurds from Canadian harvest), HS2 (medium flaky hurds from Canadian harvest), HS3 (fine hurds from Canadian harvest) and HS4 (well-graded short hurds from a French harvest). Five hemp contents (20, 40, 50, 67 and 100 wt.% of binder) were tested. HS4 hurds (3–10 mm) provided the best grading, reducing porosity and enhancing strength, achieving compressive strengths of 1.33 MPa (dry) and 0.77 MPa (wet). Coarser hurds (HS1, HS2) increased porosity, while finer hurds (HS3) raised binder demand and interfacial failure risk. Hempcrete with HS4 exhibited the highest thermal conductivity (0.131 W/(m∙K)), as fine particles hindered heat diffusion while coarse ones increased void content. Increasing hemp content decreased density, strength, conductivity and heat capacity, with thermal diffusivity governed by the balance between conductivity and capacity. Thermal modelling indicated energy losses of 27% at 24 h for 20 wt.% hemp and 14% for 100 wt.%. Overall, well-graded hurds (HS4) with optimised hemp content achieved the optimal strength–insulation balance.