The road to low-carbon concrete

Enlarge / Cement works, Ipswich, Suffolk, UK. (Photo by BuildPix/Construction Photography/Avalon/Getty Images) Construction Photography/Avalon via Getty Images

No one knows who did it first, or when. But in the 2nd or 3rd century BCE, Roman engineers regularly ground burnt limestone and volcanic ash into camentum: a powder that began to harden as soon as it was mixed with water.

They widely used the still wet slurry as mortar for their bricks and stones. But they had also learned the value of incorporating pumice, pebbles or shards of pot with the water: if you get the right proportions, the cement would eventually bind everything together into a strong, durable conglomerate that looks like to rock called opus. caementicium or - in a later term derived from a Latin verb meaning "to reunite" - concretum.

The Romans used this wonderful material throughout their empire: in viaducts, breakwaters, coliseums and even temples like the Pantheon, which still stands in the center of Rome and still has the largest concrete dome unarmed of the world.

Two millennia later, we're doing much the same thing, pouring concrete by the gigatons for roads, bridges, skyscrapers, and all the other big chunks of modern civilization. In fact, globally, humanity currently uses approximately 30 billion metric tons of concrete per year, more than any other material except water. And while rapidly developing countries such as China and India have continued their construction boom for decades, that number is only going up.

Unfortunately, our long love affair with concrete has also made our climate problem worse. The variety of camentum most commonly used to bond concrete today, a 19th-century innovation known as Portland cement, is made in energy-intensive kilns that generate more than half a ton of carbon dioxide for every ton. of product. Multiply that by the gigatonnes of global usage rates, and cement manufacturing turns out to contribute about 8% of total CO2 emissions.

Admittedly, this is far from the fractions attributed to transport or energy production, which are both well above 20%. But as the urgency to tackle climate change heightens public scrutiny of cement emissions, as well as potential regulatory pressures from governments in the United States and Europe, it has become too important to ignore. “It is now recognized that we need to reduce net global emissions to zero by 2050,” says Robbie Andrew, senior researcher at the CICERO Center for International Climate Research in Oslo, Norway. "And the concrete industry doesn't want to be the bad guy, so they're looking for solutions."

Major industry groups such as the London-based Global Cement and Concrete Association and the Illinois-based Portland Cement Association have now released detailed roadmaps to reduce that 8% to zero by 2050. Number of their strategies rely on emerging technologies; it's even more about developing alternative materials and underutilized practices that have been around for decades. And everything is explained by the three chemical reactions that characterize the life cycle of concrete: calcination, hydration and carbonation.

The direct approach: eliminate emissions from the start

Portland cement is made in giant rotary kilns that perform the calcining reaction:

calcium carbonate (limestone, chalk) + heat → calcium oxide (quicklime) + carbon dioxide.