Spinning Straw Into Carbon Savings

Anyone taking a broad look at shrinking the carbon footprint of the construction industry has to look at concrete, whose production alone is responsible for somewhere between 3% and 7% of carbon dioxide emissions worldwide. Wood is sometimes proposed as a more climate-friendly alternative.

There are many formulations for producing concrete, and several can reduce carbon emissions compared to industry norms. One new approach would replace some of the Portland cement in concrete with high-lignin ash, a waste product from grain farming and bioethanol production. It sounds a little like the age-old idea of putting straw in the mortar, for added strength, but on a chemical level this is miles beyond. It looks like a good solution to several interconnected problems.

Baled Straw

Baled wheat straw, a high-volume farming by-product in need of a use. Image by Wikimedia Commons user Huwmanbeing.

Problem: Corn ethanol consumes a valuable crop which would do better feeding people.

Solution: Use just the stalks from the corn (and also from wheat, rice, or dedicated drops like switchgrass) to make cellulosic alcohol.

Problem: Leftovers from that process (unlike corn ethanol leftovers, which make good cattle feed) are mostly lignin and currently have no destination but the landfill.

Solution: Put the lignin ash into concrete.

So now you’re substituting a total waste product from an alternative fuel process for a product with a terrible carbon footprint. That looks like a beneficial substitution already, but the happy news today is that lignin ash bonds chemically with the cement to make the concrete 32% stronger. Presumably that would allow you to use less concrete in many applications, on top of the concrete being greener, ton for ton.

The new study is the work of Feraidon Ataie of Kabul, Afghanistan, now a Ph.D. candidate at Kansas State University. A large new market for corn and wheat stalks would tend to improve prices for Kansas farmers as well as to cut the cost of a biofuel. It looks like there might even be a little additional bioenergy produced when the straw byproduct is burned to turn it into high-lignin ash.

Pouring concrete

Pouring a concrete basement slab. Image by Wikimedia Commons user Vesta.

Cement is the binder in concrete—the part that heats up and sets up when you add water, producing a substance that is not water soluble. Aggregate (sand and gravel) is the bulk ingredient. Cement is produced from rocks rich in calcium carbonate by cooking them at high heat, simultaneously giving off clouds of carbon dioxide both from the burning fuel and from the calcium carbonate, which splits into CO2 and quicklime.

Daniel Mathews writes about plants, animals, geology, and culture—most often writing in book form. (Bible form, if you listen to his fans. And now also in iPhone app form.) But he got his start in green homebuilding. Fresh out of Reed College, he went into the Oregon woods and built himself a tiny house out of timbers he cut and a cedar shakes he bucked and split all within 100 feet of the site. Fortunately he keeps up with the times, and focuses today on high-tech paths to a small carbon footprint. He lives in Portland with his wife, son, daughter, cat, dog, vegetable garden, and lots of music.