As scientists work to improve the strength and durability of concrete, it is perhaps not surprising that the "wonder material" graphene is proving to be a promising additive. As the world's strongest man-made material, it may have a lot to offer the construction community, and this latest example comes from researchers at Northwestern University, who have developed a new graphene-filled cement that is highly water-resistant and crack resistant.
As a key ingredient in the production of concrete, the world's most widely used material, cement has a huge environmental footprint, accounting for about 8% of global greenhouse gas emissions. The researchers hope to reduce this burden by developing concrete forms that last longer, thereby reducing the need for further concrete to replace damaged structures.
One of the reasons concrete structures fail is the formation of cracks, which begin as small cracks. Once water seeps into these breaks, it causes them to grow in size until the whole thing needs to be repaired or replaced. We've seen some interesting ways in which scientists might be able to intervene in this process, such as by "self-healing" concrete, repairing cracks with the help of blood enzymes, fungi and special glues.
Graphene may also play a role here. We've already seen promising concrete prototypes that use flake graphene to reduce water permeability, and even how old tires can be turned into graphene to produce stronger concrete. And the technology is starting to make its way into the real world, with engineers recently pouring the world's first graphene-enhanced concrete slab in the UK.
Researchers at Northwestern University have been working on another pressurized cement formula that incorporates tiny particles of this wonder material to make concrete more resistant to water. The team experimented with a range of particle types, including carbon nanotubes, carbon nanofibers and graphene nanosheets, and then tested their properties through an advanced technique called scratch testing. This subjected the material's microscopic sites to a tapered probe to test its fracture reaction.
"I was able to look at many different materials at the same time," said ange-Therese Akono, lead author of the study. "My method is applied directly at the micron and nanoscale, which saves a lot of time. And then based on that, we can learn how materials behave, how they crack, and ultimately predict their resistance to fracture."
The experiments also allowed scientists to adjust the composition of the cement to improve its performance. Through this process, the team identified graphene nanosheets as the winning ingredient. They found that graphene could be added in small amounts to improve the fracture resistance of the finished product. This is achieved by reducing the porosity of the material, so that water permeability is reduced by 78%.
Akono said: "The role of nanoparticles in this kind of application has not been understood before, so this is a major breakthrough. As a trained fracture mechanics specialist, I wanted to understand how cement production could be altered to enhance fracture response."
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