A common material known as Portland cement already incorporates a small percentage of fly ash – a byproduct of coal-burning power plants. Researchers from Rice University are finding ways to use greater amounts of the ash in an effort to build a “greener” kind of concrete.

Fly ash is created when power plants burn anthracite or bituminous coals. With a composition of silicon dioxide, aluminum oxide, and calcium oxide, the fly ash material contains a mineral, cement-like composition – one that offers binding capabilities.

Manufacturers of Portland cement often supplement the binder with small amounts of the silicon- and aluminum-rich fly ash – just not enough of it, according to Rouzbeh Shahsavari, an assistant professor of civil and environmental engineering.

“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,” said Shahsavari in a recent press release.

The Rice University professor, along with graduate student Sung Hoon Hwang, wanted to create a binder made mostly with fly ash.

Previous attempts to entirely replace Portland cement with a fly ash compound required large amounts of expensive sodium-based activators – an ingredient that negated the cost and environmental benefits, said Shahsavari.

“The chemicals used to activate it as a cementitious binder were expensive,” Shahsavari told Tech Briefs. “Our formula has minimal amount of those chemical activators and no cement, hence reduced cost.”

Using a quality-control computational process known as Taguchi analysis, the Rice researchers determined the optimum mix: approximately 80 percent fly ash, five percent sodium-based activator, along with small amounts of nanosilica and calcium oxide.

And most significantly: no cement.

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

Fly ash binder does not require the high-temperature processing of Portland cement, yet tests demonstrate a consistent compressive quality after seven days of curing.

“This is our first try,” said Shahsavari, adding that the strength is similar to Portland cement and could potentially be used on pavement, walls, and other structural components. More work still needs to be done to study long-term behavior of the material.

The results are reported in the Journal of the American Ceramic Society.

“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,” said Shahsavari.

What do you think? Will fly ash lead to better, greener concrete? Share your comments and questions below.