No one knows who did it first or when. But by the 2nd or 3rd century BC. Roman engineers routinely ground up burnt limestone and volcanic ash caementum: a powder that hardens when mixed with water.
They made extensive use of the still-wet mush as mortar for their brick and stone works. But they had also learned the value of stirring pumice, pebbles or potsherds along with the water: Get the proportions right and the cement would eventually bind it all together into a strong, durable, rocky conglomerate called opus cementium or – in a later term derived from a Latin verb meaning ‘to bring together’ –specific.
The Romans used this wonderful stuff all over 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 unreinforced concrete dome in the world.
Two millennia later, we’re doing much the same thing, pouring gigatons of concrete for roads, bridges, high-rises and all the other great parts of modern civilization. Globally, humanity now uses an estimated 30 billion tons of concrete per year – more than any other material except water. And as fast-developing countries like China and India continue their decades-long construction boom, that number is only rising.
Unfortunately, our long-standing love for concrete has also contributed to our climate problem. The variety of caementum most commonly used to bind today’s concrete, 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 gigatonnes of global usage rates, and cement production appears to contribute about 8 percent of total CO2 emissions.
Admittedly, that’s nowhere near the fractions attributed to transportation or energy production, both of which are well above 20 percent. But as the urgency of tackling climate change increases public scrutiny of cement emissions, along with potential regulatory pressure from governments in both the United States and Europe, it has become too big to ignore. “It is now recognized that we need to reduce net global emissions to zero by 2050,” said Robbie Andrew, a 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 percent to zero by 2050. Many of their strategies are based on emerging technologies; even more are a matter of scaling up alternative materials and underutilized practices that have been around for decades. And everything can be understood in terms of the three chemical reactions that characterize the life cycle of concrete: calcination, hydration and carbonation.