The most commonly used man-made material on Earth is concrete. Its traditional manufacture mixes sand and gravel with cement, creating an environmental impact in terms of airborne pollution and damage to land use for quarrying. Producing one metric ton of the most commonly used cement type, Portland cement, releases up to one ton of carbon dioxide -- it is estimated that the global cement industry is responsible for seven percent of yearly carbon dioxide emissions into the atmosphere.

But new, environmentally friendly methods for producing concrete without cement are being developed. One such method being investigated at Kaunas University of Technology (KTU), Lithuania, makes use of fly ash, the coal combustion product that gets driven out of coal-fired boilers along with flue gases. The final product is as strong as traditional concrete, and it also offers greater resiliency to acid damage and temperature extremes.

"At first, the idea that concrete can be produced without using cement seemed radical,” said Vytautas Bocullo, a researcher in the civil engineering and architecture faculty at KTU. “Now, after several years of intensive work we succeeded to develop alkali-activated concrete… instead of Portland cement we are using alkali-activated industrial waste products: fly ash, biofuel bottom ash, aluminum fluoride production waste.”

Theoretically, any material containing silicon and aluminum compounds could be used because those materials melt and bind in ways that are similar to cement when treated with a special alkaline solution. The higher pH imparted by the alkaline also serves to protect armature against corrosion.

To maximize cost-effectiveness of producing the alternative concrete, Bocullo relies on the use of local materials. Substance preparation varies depending on the material.

"For example, fly ash of coal can be used instantly,” he said, “but the biofuel ash needs to be grinded up to the fineness of the cement. In order to improve the qualities of the final product, several substances can be mixed -- but before that their chemical composition and additives need to be investigated for their impact on the environment and on the compressive strength of the concrete.”

KTU research groups are also experimenting and developing other types of concrete mixtures, including ultra-high-performance concrete and self-renewing concrete.

Another recent study which focused on the role of fly ash in alternative concrete appears here.