Reinforcing Clay with Wheat Fibers for Sustainable Walls

By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Nov 25 2024

A recent study published in Construction and Building Materials examined the hygrothermal properties of red and beige clays reinforced with wheat fibers, utilizing the traditional heavy cob technique. The research explored these materials’ potential for low-carbon construction in modern wood frame buildings.

Background

Geo-sourced materials are essential in modern construction due to their low carbon footprint and recyclability. Raw earth is an abundant building material with low grey energy; it is locally available, easy to work, and meets several environmental sustainability criteria. Moreover, earth materials can regulate internal humidity due to their hygroscopic properties.

However, the main drawback of earth materials is their vulnerability when in contact with water. Adding plant fibers such as flax, hemp, or wheat can increase the resistance of earth materials to moisture and adverse weather conditions. Meanwhile, among various methods used to make earth materials, the traditional technique of constructing with cob stands out for its simplicity and ecological characteristics.

Accordingly, this study explored the influence of moisture content on the thermal and hygroscopic properties of wheat fiber-reinforced clay specimens manufactured using the heavy cob technique. Cob, a natural material, has several sustainable advantages over conventional building materials.

Methods

The researchers prepared clay specimens using red and beige clays (particle sizes up to 63 μm) and varying wheat fiber contents (0, 3, and 6 wt.%). The clay samples without fibers were mixed with water at a 25 wt.% ratio, while fiber-reinforced samples used moistened fibers prepared with a water/fiber ratio of 205 ± 15 wt.%.

The samples were formed into rectangular and cylindrical shapes using the cob technique, which incorporates untreated raw fibers. Various tests were conducted to analyze their properties:

• Fiber Behavior: Absorption and desorption characteristics were tested at 23, 40, and 50 °C.
• Porosity: Measured using gas pycnometry and an oil-immersion method.
• Thermal Properties: Evaluated using dry-state and NaCl solution measurements.
• Water Absorption and Vapor Permeability: Tested using partial immersion and wet/dry cup methods.

Temperature regulation was studied using a bi-climatic chamber and mini-wall models (80 × 80 × 15 cm³), simulating conditions in Djibouti (20–45 °C) and Johannesburg (-5–35 °C).

Results and Discussion

The study revealed that fiber-reinforced clay mixtures exhibited improved thermal properties with increasing moisture content. Thermal conductivity, specific heat, thermal diffusivity, and thermal effusivity all increased as water vapor replaced air in the pores of the dry samples when exposed to higher relative humidity. This demonstrated the ability of the material to adapt to changing environmental conditions.

The clay samples reached moisture equilibrium within four to eight days when exposed to varying levels of relative humidity, showcasing the humidity regulation capabilities of both fiber-reinforced and fiber-free samples. Sorption tests further highlighted the durability of fiber-reinforced samples, which maintained structural integrity and showed no signs of mold, even after being subjected to very high relative humidity levels (80–95 %) for over two weeks.

The fiber-reinforced clay mixtures also displayed an excellent moisture buffer value. This value decreased in environments with relative humidity between 20 % and 50 % but increased significantly in higher humidity conditions (50 % to 95 %). Similar trends were observed for the coefficients of moisture effusivity and specific moisture capacity, while moisture diffusivity decreased with increasing relative humidity. These findings highlight the material's ability to manage moisture effectively, particularly in humid conditions.

Fiber content played a significant role in enhancing the hydric properties of the clay specimens. However, it did not affect the water vapor resistance factor or the equivalent moisture penetration depth, indicating that fiber reinforcement primarily influences the material's moisture regulation and durability rather than its permeability.

A practical demonstration using a 15 cm thick cob wall further emphasized the material’s suitability for modern construction. The wall maintained a stable and comfortable indoor temperature range of 19.5 to 23 °C for a full week despite exposure to extreme outdoor temperatures ranging from -5 to 45 °C. This performance underscores the potential of fiber-reinforced clay mixtures for sustainable and efficient building applications, particularly for walls with a thickness of 15 cm or more.

Conclusion

The study demonstrated the potential of wheat fiber-reinforced red and beige clays as sustainable materials for modern construction. The fiber-enhanced mixtures showed excellent thermal and hygroscopic properties, effectively regulating high relative humidity without mold risks. Walls constructed with these materials could provide consistent indoor comfort while reducing the environmental footprint of building processes.

However, the study noted limitations, including the lack of data on actual water vapor permeability accounting for surface film resistance. Future research should address this gap to better understand the material’s behavior under varying humidity conditions. By refining these evaluations, the use of fiber-reinforced cob materials could further advance sustainable building practices.

Journal Reference

Kabore, A., Laghdir, A., & Ouellet-Plamondon, C. (2024). Natural thermal and hygrothermal regulation with heavy cob for low carbon construction. Construction and Building Materials, 451, 138832. DOI: 10.1016/j.conbuildmat.2024.138832, https://www.sciencedirect.com/science/article/pii/S0950061824039746

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