By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Dec 5 2024A recent article published in Sustainability examined the effects of the thermal pre-treatment of hemp hurds on hempcrete’s physical, mechanical, and thermal properties. The resulting hempcrete was analyzed for density, compressive strength, water absorption, and thermal conductivity, evaluating its potential as a sustainable building material.
Study: Optimizing Hempcrete Properties Through Thermal Treatment of Hemp Hurds for Enhanced Sustainability in Green Building. Image Credit: Gatot Adri/Shutterstock.com
Background
Hemp, an annual botanical species, has emerged as a highly versatile crop. The woody core of the hemp stalk, called hemp hurd, finds applications in the construction industry. It is used to create thermal and sound insulation panels, known as hempcrete.
Hempcrete is a lightweight composite building material with appreciable benefits for sustainable construction. However, it suffers major drawbacks such as very long drying time and low mechanical strength. These properties of hempcrete directly depend on the physical and chemical properties of hemp hurds.
Different pre-treatments are employed to enhance hemp hurd’s performance in hempcrete mixtures and reduce its strong hygroscopic nature. While surface modification using chemical methods is efficient, it is costly and environmentally unsafe.
Alternatively, thermal treatments are cost-effective and eco-friendly. Therefore, this study explored the thermal pre-treatment of hemp hurds at temperatures ranging from 120 to 280 °C in an inert atmosphere.
Methods
Hemp hurd samples were obtained from a textile fiber research laboratory. Nine samples were thermally pre-treated in an inert atmosphere for six hours at 120, 160, 200, 240, and 280 °C, while one was air-dried (AD).
Subsequently, hempcrete specimens were prepared by mixing hemp hurds, hydrated lime, and water in three ratios (A: 80–12–8, B: 75–15–10, and C: 65–20–15 vol%). These were molded into dimensions of 4×4×8 cm3.
The moisture content of the hempcrete samples was determined after drying them at 105 °C to a constant weight. Meanwhile, their ash content was determined using a laboratory furnace at 550 °C.
The Dumas method revealed the nitrogen content and, consequently, the protein content in hempcrete samples. Alternatively, the Soxhlet method was employed to examine the residual fat fraction using petroleum ether as the extractant solvent.
Elemental analysis was conducted on the hempcrete samples to determine the C, H, N, S, and O content. Meanwhile, their bulk density was determined using a graduated glass cylinder. Furthermore, water absorption tests (24 hours) and thermogravimetric analysis (25–1000 °C) were conducted on the plant-based raw materials.
The morphology of the hemp hurd and hempcrete samples was examined using a field emission scanning electron microscope (SEM). The latter’s compressive strength, density, and thermal conductivity were determined at 28 days of curing.
Results and Discussion
Increasing pre-treatment temperatures of the hemp hurds considerably reduced the water absorption and density across all hempcrete formulations. This helped obtain lightweight insulating materials with high moisture resistance.
The density of all hempcrete samples decreased consistently with extended curing times and elevated hurd pre-treatment temperatures. Additionally, except formulation C treated at 120 °C (C120) and AD (CAD), all formulations had density within the optimal range for low-bearing, non-structural materials, promising for thermal insulation and sound absorption applications.
Lower densities at higher pre-treatment temperatures were attributed to hurd’s bulk density reduction and increased hydrophobicity. Formulation A, with the maximum proportion of hemp hurds, exhibited the minimum densities, while formulation C, with maximum lime content, had the maximum densities. Moreover, formulation A experienced a maximum decrease in density with increasing pre-treatment temperature after curing for 28 days.
Water absorption of hempcrete specimens decreased considerably with increasing pre-treatment temperature, attributed to the greater hydrophobicity and structural changes in the hemp hurds. Formulation A exhibited maximum water absorption capacity, reduced by 47% on increasing pre-treatment temperature from AD to 280 °C.
This variation indicated enhanced hydrophobicity of the hemp hurds due to thermal degradation of hemicellulose and cellulose with increasing temperature.
Compressive strength increased consistently with increasing pre-treatment temperatures due to improved bonding and rigidity within the hemp-lime matrix. However, thermal conductivity also increased with increasing pre-treatment temperatures.
Despite this increased conductivity, formulation A remained within the desirable threshold of 0.1 W/mK, maintaining efficient insulation properties. Alternatively, the conductivity of formulations B and C exceeded this limit. Therefore, a higher hemp hurd content with limited lime content was favorable to enhance insulation, but this might compromise the compressive strength.
Conclusion
Overall, the researchers comprehensively examined the influence of hemp hurds’ pre-treatment temperature on hempcrete’s physical and mechanical properties, with implications for hempcrete’s application as a sustainable building material.
Pre-treated hemp hurds enhanced the material’s structural and thermal characteristics and promoted long-term sustainability in construction by improving durability and lowering the need for other moisture-resistant additives. Moreover, thermal pre-treatment of hemp hurds can be strategically used to modify hempcrete’s properties for target construction applications.
The researchers suggest further optimizing the thermal pre-treatment parameters to enhance hempcrete’s performance at minimum energy consumption for overall sustainability. Additionally, its long-term durability should be evaluated under variable environmental conditions.
Journal Reference
D’Eusanio, V., Rivi, M., Malferrari, D., & Marchetti, A. (2024). Optimizing Hempcrete Properties Through Thermal Treatment of Hemp Hurds for Enhanced Sustainability in Green Building. Sustainability, 16(23), 10404. DOI: 10.3390/su162310404, https://www.mdpi.com/2071-1050/16/23/10404
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