Nanocellulose has recently gained widespread attention within the concrete industry as a solution to environmental issues such as global warming and energy consumption.
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It is increasingly utilized in building materials because of its sustainability, renewability, and unique properties, such as strength, light weight, and tunable self-assembly. Derived from various natural sources, including plants, animals, and bacteria, nanocellulose is employed as an additive in cementitious materials in the form of nanofibrils, nanocrystals, and filaments.
Characteristics of Nanocellulose
Cellulose is the most abundant organic polymer found on earth. It is converted to nanocellulose through integrated physical, chemical (such as pulping and bleaching), and mechanical (like sonication) treatments. Due to its predominantly crystalline structure, cellulose is difficult to break down and exhibits strong resistance to oxidizing agents and alkali solutions. However, acids can easily hydrolyze cellulose into water-soluble sugars like glucose.
Nanocelluloses derived from various sources share similar properties, such as low density and large aspect ratio. Their nano-sized structure imparts them with a high strength, high surface area, and suitability to be employed as nanofillers. Their hydrophilicity facilitates easy chemical functionalization by modifying the hydroxyl groups present on their surface, enabling seamless integration with various building materials.
With properties like high strength, light weight, and relatively low cost due to abundant availability, nanocellulose stands out as a preferred reinforcing agent in composites.
Sustainability of Nanocellulose
Endless urbanization and deforestation have led to the depletion of natural resources like fresh water, fine sand, and gravel—the primary raw materials for the conventional cement and concrete industry. In terms of mass, cement is the largest artificial product on earth.
Carbon dioxide emissions from cement-based constructions also significantly contribute to global warming. In its 2019 report, the World Green Build Council aimed for a 40 % reduction in embodied carbon for all new building infrastructure by 2030, with aspirations for a carbon-free infrastructure by 2050. In this context, nanocellulose emerges as a sustainable alternative capable of alleviating the adverse environmental impacts of cement while reducing the use of natural resources like gravel, sand, and clay in the infrastructure-building industry.
However, the threat to the forests persists as wood remains the primary source of cellulose. This challenge can be overcome by using alternative sources of cellulose, such as algae, which offers the additional advantage of CO2 capture.
Cellulose can also be obtained from non-wood plants like flax, hemp, jute, and kenaf. Plant cellulose can also be economically derived from agricultural waste.
Applications of Nanocellulose
As an additive, nanocellulose can significantly improve the mechanical properties of a building material. However, to achieve this, an appropriate dosage is required, as an excessive amount of nanocellulose can induce self-aggregation and deterioration of the mechanical properties of the cement. To overcome this issue, dispersion methods like sonication are frequently employed.
Nanocellulose filaments can modify the viscosity of building materials, thereby enhancing stability through improved yield stress. They can also alter the microstructure of cement by reducing the pores and cracks, thus increasing the durability of the cement. Incorporating nanocellulose into cement composites enhances resistance to moisture and frost, thereby increasing the overall durability of the structure.
The hydration of cement is a critical chemical process that controls its overall performance. Cellulose nanofibrils can increase the stability of cement mixtures with a high water-to-cement ratio by allowing them to hydrate without precipitating down.
Alternatively, nanocellulose can be utilized to prevent shrinkage in cement mixtures with a low water-to-cement ratio, as it improves water transport to inner dry cement particles. However, such applications require precise nanocellulose dosage to prevent adverse results, such as increased shrinkage.
Nanocellulose, when in crystal form, can reduce the water absorption coefficient and alter the wetting and adhesion properties of mortar. Increasing the dosage of cellulose nanocrystals up to a certain limit can enhance the bulk density and specific density of the building material. However, further dosage increases may lead to decreased apparent density due to agglomeration of nanocellulose.
Bacterial nanocellulose has a higher crystallinity index, purity, tensile strength, water absorption capacity, and polymerization degree than plant cellulose. Thus, bacterial nanocellulose shows greater efficiency in increasing the structural strength and water absorption of cement composites.
The impact of nanocellulose on the mechanical properties of a building material also depends on the material type. For instance, nanocellulose crystals can improve the elasticity of calcium aluminate cement, but the same is not observed for ordinary Portland cement.
Factors such as the type, quantity, and dispersion of nanocellulose and the type of cementitious matrix collectively influence the characteristics of the final structure.
Recent Studies
A recent study published in ACS Sustainable Chemistry & Engineering investigated the role of cellulose nanocrystals in fiber cement. Nanocellulose crystals were used as an additive in fiber cement. The composite exhibited enhanced hydration kinetics, improved flexural strength, and shear thinning behavior. The authors also highlighted the significant role of reinforcing nanocellulose in fiber cement in the decarbonization of building materials without compromising the mechanical strength and curing time.
Another recent study, published in Construction and Building Materials, proposed using a mixture of nanocellulose and concrete mortar to reduce indoor Radon-222 gas emanation. Radon-222, a natural radioactive gas that emanates from bricks, can penetrate the human respiratory system and adversely affect lung tissues.
The researchers investigated various ratios of kenaf and oil palm cellulose nanofibrils mixed into bricks. The nanocellulose acted as liquid fillers, increasing the physical strength of the bricks while reducing Radon-222 emissions. This effect was achieved by reducing the amounts of stone, sand, and cement in the bricks and nano-ionizing their internal porosity.
Growing concerns about climate change have encouraged researchers to explore green materials, technologies, and products with reduced environmental and human health impacts compared to traditional counterparts. Nanocellulose satisfies the conditions of a “green” material and holds the potential to be a leading choice for sustainable materials in the 21st century.
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References and Further Reading
1. Guo, A., Sun, Z., Sathitsuksanoh, N., Feng, H. (2020). A Review on the Application of Nanocellulose in Cementitious Materials. Nanomaterials. doi.org/10.3390/nano10122476
2. Postek, M., Moon, R., Rudie, A., Bilodeau, M. (2013). Production and Applications of Cellulose Nanomaterials. [Online] TAPPI. Available at: https://umaine.edu/pdc/wp-content/uploads/sites/398/2015/02/Nanocellulose-Book_Preview.pdf (Accessed on February 9, 2024).
3. Alrubaie, M., Resan, S. F. (2023). Opportunities of using nanocellulose in construction materials. BioResources. doi.org/10.15376/biores.18.3.4392-4394
4. Raghunath, S., Hoque, M., E. Johan Foster. (2023). On the Roles of Cellulose Nanocrystals in Fiber Cement: Implications for Rheology, Hydration Kinetics, and Mechanical Properties. ACS Sustainable Chemistry & Engineering. doi.org/10.1021/acssuschemeng.3c01392
5. Shari, Khairul, M., An'amt Mohamed Noor, Nurfarah Aini Mocktar, Ros, Aziz, A., Nor Hakimin Abdullah. (2022). Internal bonding microstructures characterization between plant nanocellulose and concrete mortar mixtures for indoor Radon-222 gas emanation reduction. Construction and Building Materials. doi.org/10.1016/j.conbuildmat.2022.128841
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