A concrete example of massive carbon reduction could result from rearranging the building material's molecules
Cement is the basic building block of the world, and its production one of the biggest contributors to climate change. But nanotechnology could hold the key to making it not only stronger, but also much greener.
Scientists at MIT and France's CNRS in have published research showing how to make concrete using cement that is more environmentally friendly by harnessing the power of nanotechnology.
The study, just published in the journal Nature Communications, is the culmination of five years of research by a U.S-French team led by MIT senior research scientist Roland Pellenq. It's a promising breakthrough to addressing climate change because, as Pellenq has noted, "Cement is the most-used material on the planet," and producing it emits huge amounts of carbon.
The researchers say 18 billion tonnes of concrete are produced every year—and this production accounts for 5-to-10 per cent of the carbon emissions caused by human activity. "One strategy to reduce this environmental footprint," they write, "is to enhance concrete's specific stiffness or strength by optimizing the molecular-level properties."
Cement and concrete are different. Cement is a fine powder, mostly limestone but also other materials, that mixes with water and binds the concrete. Concrete is a composite of water, aggregate and cement. It's possible to build things with cement alone, but concrete needs cement.
Here's where nanotechnology comes in. Cement is made by cooking a mix of calcium-rich and silica-rich material at 1,500 degrees Celsius, which releases carbon from the limestone into the air as greenhouse gas.
Pellenq's team compared the molecular structure of different mixes of cement, looking at the ratio of calcium to silica. They looked particularly closely at different mixtures, which they call C-S-H (for calcium-silica-hydrates) at the binding phase, when the molecules are interacting to become cement.
Comparisons of this type represent a new approach, the scientists say in their paper: "The C-S-H binding phases comprise small nanoparticles of 5 nanometres average diameter, products of small nanoparticles between [waterless] calcium silicates and water that form a gel-like network of variable stoichiometry (chemical relationships)." As far as they could tell, this type of research has not been considered before when it comes to cement.
After building a database of different ratios − molecular recipes, really − the team came up with one that may reduce carbon emissions from production by as much as 60 per cent. It's a ratio that's not used today, yet Pellenq calls it "a magical ratio" because it may have twice the resistance that normal cement has to fracturing, after some additional molecular-scale tinkering is done.
This may not sound remarkable at first glance, but consider the scale of scrutiny that nanotechnology demands. One nanometre is a billionth of a metre. The thickness of a newspaper sheet is about 100,000 nanometres. If a single nanometer were magnified to the size of a marble, on the same scale one metre would be the size of the Earth.
Another discovery is that different samples have different "nanosignatures". This means that the strength of different combinations is not so much a function of what minerals are used, but the way the nanoparticles are arranged − a discovery only possible through nanotechnology, and one that opens the door to using raw materials that release less carbon without compromising strength.
Stronger cement and better concrete may be good for the bottom line as well as the environment. It could be particularly helpful to the oil and gas industry, because fracture-resistant materials can prevent leakage and blowouts in well-casings.
Less carbon-intensive and stronger cements may be only the beginning for better, nano-engineered concrete. Other nanotechnological research is looking at cement that may be able to heal itself when it breaks − detecting the micro-cracks that develop from weather changes and automatically releasing self-healing chemical catalysts embedded in the material. Additional experiments are comparing how different molecular structures in cements conduct or resist electricity.
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