Thermal Diffusivity of Hemp Concrete: Experimental and Numerical Studies

• Author / Creator

  Al-Tamimi, Ahmed

• The heat transfer from a hot to a cold medium occurs first in transient state before reaching a steady state. Hence, heat flux and surface temperatures fluctuate with changing times during the transient phase till they stabilizes after a period, and reach a steady phase. Heat conduction depends on thermal conductivity, specific heat capacity, and density (i.e., thermal diffusivity) during the transient state, while thermal conductivity dominates the steady state. In reality, the heat transfer occurs in the transient phase for building envelopes, while the steady state is used mainly for materials lab testing, such as the hot box technique. Therefore, materials for building envelopes must be assessed for thermal diffusivity and not just for thermal conductivity.
  The first part of this study collates the thermal diffusivity of various types of concrete mixtures from literature, to explore the effect of mixture components and external factors on thermal properties. The outcome of that literature review indicates the significant effect of aggregate and binder properties on the overall thermal properties of the composite. Hemp concrete's reported thermal diffusivity values were at the bottom, showing low thermal conductivity and high specific heat capacity.
  Hempcrete in the literature shows low thermal conductivity and high specific heat capacity, which are rarely reconciled, giving an advantage for building envelope application, to maximize energy storage and minimize thermal energy losses. There is a need to optimize the hempcrete mixture's components and balance thermal and mechanical properties with locally supplied materials to reduce the cost and make the hempcrete industry more available and affordable. Four groups of binders were experimentally investigated for hempcrete's strength and thermal properties. The results showed that the presence of lime in the binder matrix of hempcrete is mandatory to enhance strength and overcome the negative impact of hemp to some extent due to pectin and hydrophilicity. Alkali-activated binders exhibit less impact with hemp hurds due to the alkaline solution that improves hemp surface and enhances the bond, which might not significantly be absorbed by hemp hurds. The thermal conductivity, diffusivity, and heat capacity are considerably higher due to the higher density and amorphous NASH. The maximum strength obtained in this study is alkali-activated metakaolin; thus, other mixtures might need optimizations to boost the strength. Increasing hemp content reduced density, strength, thermal conductivity, and volumetric heat capacity. Large hemp hurds disordered the arrangement of particles due to the flaky shape, while very fine hurds requires higher binder content and increase the possibility of interfacial failure under loading. The sizes of hemp hurds in the range of <3mm for width and <10mm for length achieved lower porosity and higher strength due to well gradation.
  The second part of this work aims to enhance the compatibility and bonding between hemp hurds and binder matrix by conducting chemical treatments for hemp hurds. Such treatment aims to consume pectin and reduce the water absorption of hemp. Hemp hurds were treated with three chemical solutions: NaOH, Ca(OH)2, and Na2SiO3. Sodium hydroxide treatment partially removed the amorphous components, roughening the surface and reducing particle size. Calcium hydroxide provides calcium ions that react with pectin and adhere to the hemp surface. Sodium silicate solution slightly roughened the surface, and silica bonded to enhance the interface zone. Hempcrete samples with treated hemp showed a significant improvement in strength due to changes in the morphology of hemp and consuming pectin with slight changes in thermal properties.
  The last part of this study is the evaluation of hempcrete mixtures produced in this study in a full-scale model for thermal performance in terms of thermal storing energy and losses. The evaluation has been done in two stages: First, literature thermal data for various concretes, including hempcrete mixtures, were assessed with the help of the ANSYS transient model. Second, the evaluation was conducted on the experimental data of hempcrete in this study. Hempcrete mixtures produced in this study showed a high potential due to their thermal performance with lower lost energy, between 12 and 24 %. In comparison, the literature on hempcrete describes losses between 19 and 60 %.

• Subjects / Keywords

  • Thermal diffusivity
  • Hempcrete
  • Thermal modeling
  • Storing energy

• Graduation date

  Spring 2024

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-v6rn-ex10

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.