[Image above] A scanning electron microscope image shows spherical particles in type C fly ash used by Rice University engineers to make cementless binder for concrete. (Credit: Multiscale Materials Laboratory/Rice University)

Ever since Joseph Aspdin heated powdered limestone and clay together in a furnace in the early 19th century, Portland cement has been the most commonly used type of cement in the world.

However, cement, in general, has an image problem—it is not the most environmentally friendly material. Cement manufacturing represents nearly 5% of global COemissions, according to the National Precast Concrete Association.

We’ve reported on a few ways researchers are studying to make concrete greener, including incorporating irradiated plastic water bottles into cement paste and this research that uses graphene at the nanoscale to replace nearly half of the materials that go into the production of concrete.

But scientists at Rice University have now developed an environmentally-friendly material that could eventually replace conventional Portland cement in concrete. Led by materials scientist Rouzbeh Shahsavari, the researchers’ new composite binder is made mostly of fly ash—which they found to have the same strength as Portland cement after a week of curing, according to a Rice University news release.

Fly ash has been used over the years as a concrete additive and partial replacement for Portland cement in concrete. But this time the researchers wanted to use fly ash to replace a larger percentage of Portland cement.

“The industry typically mixes 5 to 20 percent fly ash into cement to make it green, but a significant portion of the mix is still cement,” assistant professor of civil and environmental engineering and of materials science and nanoengineering Rouzbeh Shahsavari, states in the release.

“Previous attempts to entirely replace Portland cement with a fly ash compound required large amounts of expensive sodium-based activators that negate the environmental benefits,” he adds. “And in the end, it was more expensive than cement.”

Using Taguchi Analysis, Shahsavari’s team developed a method for determining the best strategies for mixing fly ash with other minerals. Their method not only improved structural and mechanical qualities of the resulting composite but provided an ideal balance of “calcium-rich fly ash, nanosilica, and calcium,” using < 5% of a sodium-based activator.

Shahsavari notes that there were a couple of challenges during the research.

“[One of] our two key challenges was how to not use any Portland cement and still get enough mechanical properties (because fly ash is usually mixed with Portland cement in certain proportion),” he explains in an email. The other challenge the team encountered was to “use as little as possible activators since they are expensive and may also not be abundantly available,” he adds.

The researchers are in the process of applying for patent protection and exploring economical ways to scale up their process.

“Our work provides a viable path for efficient and cost-effective activation of this type of high-calcium fly ash, paving the path for the environmentally responsible manufacture of concrete,” Shahsavari states in the release. “Future work will assess such properties as long-term behavior, shrinkage and durability.”

The researchers hope to incorporate other industrial waste into cementitious materials, eventually replacing cement.

“We hope the computational and experimental concepts and strategies of this work provides a platform for turning several other waste streams such as slag (from steel industry), rice husk, etc. into structural binders, bypassing cement entirely,” he adds in his email.

The paper, published in The Journal of the American Ceramic Society, is “High calcium cementless fly ash binder with low environmental footprint: Optimum Taguchi design” (DOI: 10.1111/jace.15873).

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