Concrete and graphene
Written by Alinor“I found Rome a city of bricks and left it a city of marble.” is a quote many of us can attribute to emperor Augustus. Unfortunately, he leaves out the true hero of Roman architecture - concrete. So humble a material was the backbone of imperial Roman majesty, and so humble a material is the backbone of modern industrial society. We can thank concrete for the skyscraper, for the city, for the dam, and for the shelter. It’s cheap, strong, durable, and metaphorically flexible. On the other hand, its lack of literal flexibility are among its many shortcomings. Concrete has a relatively low tensile strength - which is why the Romans built in arches and why we today reinforce it with steel bars. Steel bars present but another problem - steel rusts, and rust expands, leading to cracks and ultimately structural failure. In a similar vein, concrete can absorb water, rendering freezing climates and polluted air yet another weakness. Perhaps most pressing of concrete’s negatives are in its significant carbon footprint. The concrete industry alone accounts for at least 8% of global emissions (Nature 597, 593-594 (2021)). In our mission to decarbonize concrete, we might find the solution in carbonizing concrete; here’s how.
Beyond reducing the carbon dioxide emissions of the cement process, an approach to decarbonize concrete involves making it last longer; concrete that lasts longer means needing to make less concrete for maintenance. The cracks in the concrete that form as a consequence of rebar rust and freeze-melt cycles are due to the tensile stresses that arise when expansion happens inside the concrete. Bolstering tensile strength could be achieved by adding silica or titania nanoparticle to fill in the pores, resulting in denser arrangements. Alas, once a crack forms, nanoparticles offer little resistance to further crack growth. Integrating carbon nanotubes or nanofibres may stifle crack growth and increase both the compressive and tensile strength, and while many trials report slight improvements to tensile strength (Hassan, 2018), many papers also report slight detriments (Cherkashin, 2017). Furthermore, carbon nanotubes were observed to have increased the corrosion rate of the steel rebar - ultimately negating the slight benefit to tensile strength, if there was any benefit at all (Hassan, 2018). This lack of enthusiasm in carbon nanotubes and nanofibres isn’t uncommon, but thankfully, the end of the line for carbon nanotech can be avoided by going beyond lines, tubes, and fibres.
Graphene is another carbon nanomaterial commonly under the nanotechnology limelight, and on the concrete stage, graphene is the star of the show. Research concerning graphene as a potential additive to concrete includes forms different to pure graphene. Graphene oxide (GO), the most popular variant, consists of the normal hexagonal matrix but with oxygen and hydrogen atoms bonded to some of the carbon atoms. Similarly is reduced graphene oxide (rGO), which has a reduced number of oxygen and hydrogen atoms bonded to the surface. The oxygen and hydrogen atoms attached to the graphene lattice allow for stronger interaction with the cement, and making graphene oxide more easily dispersible in the concrete. Graphene nanoplatelets (GNPs) are less than 100 stacks of graphene sheets with their size in the micrometers. Graphene alone is the strongest material ever experimented with, and introducing its incredible mechanical properties into composite materials such as concrete have so far yielded incredibly promising results. Numerous studies focusing on the dispersion of graphene into concrete gives us a range of improvements over a rainbow of mechanical properties - tensile, compressive, flexural strength, stiffness, and energy dispersion. Just as all rainbows are technically double rainbows, graphene-reinforced concrete composites have been shown to have a rainbow of other benefits- reduced water ingress, reduced gas penetration, increased thermal diffusivity, and even some electrochemical properties (Shamsae in review, 2018).
A large amount of capital has gone into developing the actual incorporation of graphene into the concrete. Graphene, like the rest of the carbon nanoallotropes, has chemical properties which make it difficult to thoroughly and homogeneously disperse in composite materials, especially those with a degree of wetness like concrete. Uneven distribution of graphene can lead to the formation of aggregates, actually hampering the mechanical properties of concrete. Recent advances have been made in the quest to improve the dispersion of carbon nanostructures amongst the concrete mix, but those involving complex make concrete lose its attractive price. A balance between dispersion quality and mass producibility has to be struck for this promising technology to go beyond mere promise. Concrete is the king of construction material because of its low cost, ease of use, and strength. Any adulterant which threatens any of these key reasons is simply infeasible to employ on a large scale, let alone aid in the fight to decarbonize concrete. Integrating graphene into concrete on large scale is dependent on graphene production on a large scale. While we have made significant progress, there is still a lot of work to be done regarding the mass production of graphene. Luckily, research directed towards carbon capture and sequestration might help with the availability of graphene for composite concrete while also providing a means to store the carbon long term. For the immediate future, the dream of commercially viable carbonized concrete is, optimistically, manifest in using nanocarbon black with cement, as collaboratively explored by MIT (Massachusetts Institute of Technology) and CNRS (Le Centre national de la recherche scientifique). Only with further research into nanotechnology can we see this dream come through the gates of horn, and not that of ivory.
For extra reading regarding concrete carbonization:
Cherkashin, A.V. & Pykhtin, K.A. & Begich, Yasmin & Sherstobitova, Polina & Koltsova, Tatiana. (2017). Mechanical properties of nanocarbon modified cement. Magazine of Civil Engineering. 72. 54-61. 10.18720/MCE.72.7.
Hassan, A., Elkady, H., & Shaaban, I. G. (2019). Effect of Adding Carbon Nanotubes on Corrosion Rates and Steel-Concrete Bond. Scientific reports, 9(1), 6285. https://doi.org/10.1038/s41598-019-42761-2
Tzileroglou, C., Stefanidou, M., Kassavetis, S., & Logothetidis, S. (2017). Nanocarbon materials for nanocomposite cement mortars. Materials Today: Proceedings, 4(7), 6938–6947. https://doi.org/10.1016/j.matpr.2017.07.023
Shamsaei, E., de Souza, F. B., Yao, X., Benhelal, E., Akbari, A., & Duan, W. (2018). Graphene-based nanosheets for stronger and more durable concrete: A Review. Construction and Building Materials, 183, 642–660. https://doi.org/10.1016/j.conbuildmat.2018.06.201
Liu, C., Hunag, X., Wu, Y.-Y., Deng, X., Zheng, Z., & Yang, B. (2022). Studies on mechanical properties and durability of steel fiber reinforced concrete incorporating graphene oxide. Cement and Concrete Composites, 130, 104508. https://doi.org/10.1016/j.cemconcomp.2022.104508
For introductory reading regarding scientific concepts:
https://en.wikipedia.org/wiki/Ultimate_tensile_strength
https://en.wikipedia.org/wiki/Allotropy
https://en.wikipedia.org/wiki/Concrete_degradation