We've written a lot about alternative fuels, such as using plastics and garbage to power a cement kiln in Texas. Now, in what's at least a three-way win, civil engineers at Kansas State University (KSU) have figured out how to use the waste products of biofuels to make a stronger concrete that's also got a smaller carbon footprint.
Since concrete is one of the world's most-used industrial materials, there's been ongoing work to make it greener. Most efforts are aimed at finding a replacement for Portland cement, one of concrete's main ingredients. For example, Wagners Composite Fibre Technologies, which makes engineering composites, also makes Earth Friendly Concrete (EFC). By eliminating Portland cement, this concrete's carbon emissions are 80 to 90 percent lower than the levels usually associated with that material.
EFC uses a geopolymer binder system made from the chemical activation of blast furnace slag, which is waste from iron production, and fly ash, which is waste from coal-fired power generation.
The KSU researchers have also found a replacement for Portland cement, but they're using ash that results from the waste products of biofuels, in turn made from the waste products of agriculture -- in this case, corn stover, wheat straw, and rice straw. All of these are used as feedstocks for the class of biofuels known as cellulosic, meaning they are derived from feedstock based on inedible waste plant material, instead of feedstock derived from seeds or grains of plants grown as (usually non-food) crops.
Corn stover -- corn stalks and leaves -- is the same material DuPont is using in its Nevada, Iowa biofuel facility, which it expects to complete in 2014.
Since the production of cellulosic ethanol is expected to increase, so will the wastes resulting from those processes, say Feraidon Ataie, doctoral student in civil engineering, and Kyle Riding, an assistant professor of civil engineering, in an article (subscription or payment required) detailing their work in the Journal of Materials in Civil Engineering. That means that the availability of high-lignin residue, the byproduct of cellulosic ethanol, will continue to increase as a feedstock.
At present, this material doesn't have many other uses. It's usually burned to produce electricity, or the ash that results from burning is tossed out and landfilled.
The team found that, after pre-treating the high-lignin ash byproduct, adding it to cement made the cement stronger because the ash reacted chemically with the cement. After some experimentation, they determined that using the agricultural residue ash (ARA) to replace 20 percent of the cement by mass increased the concrete's strength by 32 percent. Ataie and Riding also spent considerable time studying the effects of thermochemical pretreatments to make the ash reactive in specific ways.
According to the article, the team found that:
Pretreatments are effective in partial removal of alkali metals and other impurities out of both wheat straw and rice straw leading to ARA with lower loss on ignition (LOI), higher internal surface area, and higher amorphous silica content than that of unpretreated ARA. It was shown that the ash alkali content correlated with the ash LOI and amorphous silica content.
It makes sense that research that reduces the carbon footprint of cement, makes that cement a lot stronger, and keeps lignin ash out of landfills is taking place in an agricultural state like Kansas. It will also be interesting to see how, and when, this innovation can reduce the cost of making ethanol biofuels by giving commercial value to what was once a waste product.
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