Making carbon-negative portland cement
The dome of Rome’s Pantheon, at 43 meters wide, is both a marvel and an omen. It remains the largest, unreinforced concrete dome ever built, anticipating — by nearly two millennia — modern concrete wonders like the kilometer-high Burj Khalifa in Dubai or the 27-million-cubic-meter Three Gorges Dam in China.
But when Emperor Hadrian’s engineers mixed the lime-based cement used in the Pantheon’s concrete, they had no inkling that the process released vast amounts of CO2, nor, that one day CO2 would warm our whole planet and destabilize our climate.
Cement-making generates about 8 percent of annual anthropogenic CO2 emissions; less than emissions from power generation or transportation, but more than petroleum refining or any other industry. If the cement industry were a nation, its emissions would exceed those of India — or Germany, Japan, and South Korea, combined.
How can our society use cement while avoiding a climate catastrophe? Entrepreneurs have struggled for decades to develop formulas that sequester more CO2, provide equal strength with less mass, or mix the ingredients of cement in less polluting proportions. None of the alternatives worked very well: sequestration captured just a fraction of the CO2, and messing with cement’s recipe worried regulators concerned with structural integrity. What’s needed is a true drop-in replacement for cement that doesn’t generate CO2.
DCVC has found the company that knows how to do it: Brimstone, an Oakland, CA-based startup developing zero-carbon Portland cement, which recently announced a $5.1 million financing led by DCVC and Breakthrough Energy Ventures.
DCVC believes that Deep Tech companies tackling trillion-dollar problems can create new industries and revitalize old ones — including cement production. Brimstone’s co-founders, former Caltech scientists Cody Finke and Hugo Leandri, have invented a chemical process that produces cement, indistinguishable from ordinary Portland cement, without contributing to atmospheric carbon.
More than 97 percent of all cement now produced in the United States is Portland cement. It’s made by crushing limestone, chalk, and marl, and heating it to more than 1,450 °C (2,640 °F) in huge rotary kilns. This thermal decomposition process breaks down calcium carbonate into calcium oxide, or “quicklime,” and carbon. The quicklime combines with raw materials such as slag and fly ash — known as “supplementary cementitious materials,” or SCMs — to form nodules of clinker, which are ground up and mixed with gypsum to make cement powder. Cement powder can be transported anywhere and mixed with water and aggregate, such as gravel or sand, to make concrete.
Unfortunately, the carbon released inside the kiln combines with oxygen from the air to make CO2, which escapes into our atmosphere. For every 1,000 kilograms of cement produced, the process releases 650 to 920 kilograms of CO2. A large kiln can emit as much carbon as a power station — and that’s just from the chemistry of thermal decomposition — It doesn’t include the energy needed for the kilns, which are typically heated by coal, fuel oil, or natural gas.
The carbon released during cement production wasn’t an issue in Roman times. But today, the world’s economies use a staggering amount of cement — some 4.1 billion metric tons per year. China is the biggest consumer, by far. Since 2017, the country has been making more cement every year than the rest of the world, combined, according to the U.S. Geological Survey. In a three-year span from 2011 to 2013, when the Three Gorges Dam was under construction, China used more cement than the United States had in the entire 20th century.
Brimstone’s replacement for Portland cement doesn’t start with calcium carbonate, and therefore doesn’t generate CO2 during thermal decomposition. The company’s process uses a different type of rock — basalt — and it produces both calcium oxide and the SCMs needed to create Portland cement. Basalts are igneous rocks formed when volcanic lava cools, and are very plentiful. They’re rich in calcium oxide, which can be extracted without generating CO2. Basalts also contain silica, which means Brimstone’s process generates synthetic fly ash — a critical supplement that’s grown more costly as the coal and steel plants, that have historically supplied fly ash, shut down.
“We benefit from the simplification of making both materials at one location,” says Finke, who is Brimstone’s CEO. “Market conditions have allowed us to think of new solutions.”
In 1824, a Leeds bricklayer named Joseph Aspdin obtained the first patent for Portland cement. Aspdin named the material for its resemblance to the Jurassic limestone quarried on the Isle of Portland, in Dorset, England.
The main ingredient was, in fact, limestone: calcium carbonate (CaCO3) that precipitated out of ancient oceans, rich with calcium-armored invertebrates. Aspdin heated limestone and clay in his kitchen stove to create a material that could be used for mortar or stucco. Joseph’s son William Aspdin, and others, refined the formula. The material was first used at scale in the London sewer system, built to divert cholera-carrying waste to the Thames estuary. Portland proved, almost miraculously, strong and durable. Many of those cement sewer tunnels built in the 1850s are still in use.
Today, Portland cement is used all over the world as the basic ingredient in concrete, mortar, and other building materials. Cement and concrete are remarkably versatile and affordable materials that have driven architectural and engineering innovation for millennia; they aren’t going anywhere. More than coal, oil, or natural gas, cement is the indispensable material of modern life. It forms the foundations of our cities, roads, bridges, tunnels, and factories, and even our nuclear plants and wind farms.
The cities of the 19th and 20th centuries were built with concrete. But the 21st century will witness the largest human migration to cities in history. Those future urbanites will need homes, roads, places of work, and more. The OECD projects that large cities — with populations over 5 million — will nearly triple in size, on average, by 2050. Most of that building will occur in developing and poorer nations, and will be achieved by pouring cement.
To save our planet, cement must now be decarbonized. There is no scenario for limiting global warming in which the production of traditional Portland cement can continue unabated. To meet the 2015 Paris Agreement goals, the cement sector would need to cut its annual emissions by 16 percent by 2030, and much more beyond that date, according to the International Energy Agency.
To build a world fit for the 10 billion people who will be alive in 2050, while avoiding a climate catastrophe, cement will need to be reinvented, once again. DCVC is proud to help Brimstone pioneer a radically simple solution.
Kelly Chen is a Partner at DCVC. Zachary Bogue is the firm’s cofounder and Managing Partner.