A concrete solution from the cornfield
By Jennifer M. Latzke
Someday, not so very far off into the future, a little bit of the cornfield may be coming to a concrete project on a farm near you.
Two civil engineers at Kansas State University have conducted a proof-of-concept research project that uses ash left over from the cellulosic ethanol production process as an ingredient in cement.
Kyle Riding, assistant professor of civil engineering, and Feraidon Ataie, his post-doctoral associate, working with funding from the National Science Foundation, wanted to find a replacement for silica fume. Silica fume is an expensive ingredient that makes cement, and the concrete that’s made from it is more durable and less permeable to water and other elements. At an average cost of $800 per ton, silica fume is used in specific projects that require more durability, Riding explained, like bridge decks on roadways.
“Silica fume is a byproduct of the ferrosilicon industry,” Riding explained. It’s tiny particles that are emitted into the air during the process of manufacturing silicon metals used for things like semiconductors. They are trapped in filters in smokestacks and then go into the portland cement that’s used in concrete, Ataie explained. However, there’s only about 60,000 tons of silica fume available per year, which accounts for its high cost.
Riding and Ataie discovered, though, that the high-lignin residue left over from the cellulosic ethanol production process reacts in cement just like silica fume.
“Plants take that biomass and extract the sugars in the plant matter for use in ethanol,” Riding explained. “But there’s other materials left over that can’t be made into fuel. That high-lignin residue is now burned by most plants as a fuel source for energy to heat the ethanol process. What’s left is ash that is sent off to a landfill.
“But there’s still a lot of silica and minerals left in that ash,” Riding said. “We wondered if we could take that and use it for something else and save landfill space and provide another revenue stream for the cellulosic ethanol plants.”
Ataie and Riding wrote a paper that’s been accepted for scientific publication about their research into the thermochemical pretreatment of biomass in the ethanol process. They wanted to see which process of ethanol production—acidic or basic—worked better not only for a final ethanol product but also the ash for cement.
“In cellulosic ethanol production, typically the plant will soak the biomass in a diluted acid and heat it up for a bit,” Riding said.
“The main reason for this is to increase the yield of the ethanol and break down the cellulose structure, like corn stover, and get more sugars out of the microstructure,” Ataie said. The acid treatment seems to keep more silica in the ash, which is what gives the concrete its strength.
“We get amorphous silica, which has a disorganized atomic structure instead of a crystalline silica that has an organized structure,” Ataie said. “It’s also a happy coincidence that acid is the most effective method for producing ethanol from biomass.”
Ataie and Riding got samples of ash from the National Renewable Energy Laboratory in Golden, Colo. They started with a brown material that would be destined either for the boilers at the ethanol plant or for landfill.
“We burned it at 650 degrees Celsius, and we got a white ash that we used in concrete to see how it worked,” Ataie said. It exhibited the same behavior as silica fume, all at a drastically reduced cost.
And, by replacing 20 percent of the cement with this cellulosic material after burning, the research found that the strength of the concrete could be improved by 32 percent.
Future research could look into what biomass material makes the best ash for cement. This research began with corn stover, but the two said other materials might have more silica available for ash after the ethanol process.
“Rice straw is about 20 percent silica, sugarcane bagasse has a high content of silica,” Riding said. “Wheat straw is about 5 to 6 percent silica. So is corn. Woody materials are only about 1 to 2 percent silica, though.”
It also depends on what other materials are mixed into the burning process at the plant. The more pure the biomass, the more silica is available in the ash, Ataie said.
With more and more large-scale cellulosic ethanol plants coming online, there very well could be a market for 1 million tons or more of this ash in the cement and concrete business of the future.
“Consider wheat straw, with 5 percent of the dry biomass containing silica,” Riding said. “We could get 20 million tons of wheat biomass into an ethanol plant and produce a million tons of ash for the cement industry.”
“The concrete industry could absorb these millions of tons,” Riding said. “We estimate there is 1 cubic meter of concrete laid per person on Earth per year. That’s 6 to 7 billion cubic meters of concrete every year.”
This high strength concrete from cellulosic ash might be ideal for agricultural construction, too. “Wherever there needs to be more durability on the farm, this concrete from this ash could be used,” Riding said. “Animal enclosures, dairies, fertilizer storage, just look at the areas around the farm where durability is an issue.”
Riding and Ataie collaborated with the University of Texas, North Carolina State University, Antoine Borden, senior in civil engineering, Colorado Springs, Colo., with funding from the NSF and assistance from the NREL.
While this research was just to prove the concept could work, there is some interest from industry. And further research could improve the efficiency of the process altogether. Not only will tons of materials stay out of landfills, but this new revenue stream for cellulosic ethanol plants could also lower the cost of production.
And someday when a corn farmer pours the concrete slab for a new storage facility, it might have a little bit of his cornfield in it.
Jennifer M. Latzke can be reached by phone at 620-227-1807 or firstname.lastname@example.org.