Curing Cement’s CO₂ Weakness

It’s no exaggeration to say that our built environment is held together by Portland cement, although most people pay it little attention. Cement, a binding material made primarily from limestone, is quietly responsible for holding concrete together. But it is also responsible for about 5% of the world’s human-caused CO₂ emissions.

And production of cement and concrete is soaring, particularly in China. No one is suggesting that concrete can be replaced as a building material; after all, it’s often said that the world uses more concrete than any material other than water. Instead, the industry is looking for ways to reduce concrete’s carbon footprint without compromising its affordability and durability.

Two start-ups seek to do this by changing the chemistry of concrete. CarbonCure Technologies and Solidia Technologies are working to capture carbon inside the pores of concrete products. The two firms say their processes work cost-effectively using CO₂ that industrial gas firms such as Linde and Praxair recover from other processes.

Many builders would like the option of purchasing greener concrete, but changing how a traditional, often hidebound, industry works is an uphill climb. In recent years, the start-up firms Calera and Novacem also sought to capture CO₂ and store it in concrete, but both failed to bring a product to market.

“It became clear that the hype was outpacing the promise,” says Brent Ehrlich, products editor at ­BuildingGreen, a firm that tracks and rates green building materials. He cautions that a successful technology must be easy to adopt and bring obvious benefits in performance or green credentials.

CarbonCure and Solidia say that by focusing on retrofitting plants that manufacture precast concrete blocks, they can commercialize their technology quickly. And they claim that their extra-hard blocks are about 20% stronger than what builders are using today.

The traditional process to transform limestone into cement is inherently carbon-emitting. The main reaction involves heating limestone, silica, and other minerals in a kiln at 1,450 °C. The heat splits CaO from the limestone and releases CO₂ into the atmosphere. About half of the emissions come as a result of this simple chemistry; much of the rest is from burning fossil fuels—usually coal—to heat the kiln. For every ton of cement that is produced, close to a ton of CO₂ is emitted.

For concrete companies, this CO₂ legacy is their main environmental “hot spot,” says Sarah Buffaloe, LEED specialist at the U.S. Green Building Council, which sets the criteria for Leadership in Energy & Environmental Design certification. Targeting CO₂, therefore, is critical to distinguishing any concrete product as sustainable.

In recent years, cement makers have improved the efficiency of their kilns and tweaked recipes to require less limestone to reduce greenhouse gas emissions. Lafarge, a global supplier of cement and concrete, cut emissions by 20% between 2002 and 2010, according to Peter Quail, the company’s North American vice president of precast concrete.

“Now, we plan to reduce it by another 33% by 2020,” he says, “but because all the low-hanging fruit has been pretty well taken care of, we have started looking at a new generation of low-carbon cement.” Lafarge has its own research centers in France and Montreal. But Quail says the firm also looks for ideas from outside the company and recently partnered with Solidia.

Solidia and CarbonCure both take advantage of one of concrete’s chemical quirks. Its cementitious ingredients slowly react with CO₂ at the surface of concrete during its life span. By moving up that carbonation reaction to the point of concrete block manufacturing, much more CO₂ can penetrate the material.

Another way to look at it is that the technologies more fully close the concrete-chemistry loop. After removing CO₂ from limestone to make cement, they put it back into concrete where it can re-form limestone.

CarbonCure’s process adds CO₂ right into the concrete mixer, says Robert Niven, the firm’s chief executive officer. There it meets up with Portland cement, water, sand, gravel, and other inputs such as fly ash.

As these materials get mixed up, CaO from Portland cement reacts with water to form carbonate species such as Ca(OH)₂. Those species react with the added CO₂ and precipitate out as nanoscale particles of CaCO₃. The particles, in turn, help seed additional hydration products such as calcium silicate hydrate that further harden concrete, Niven explains.

The company does not actually make CarbonCure blocks but rather licenses its technology to masonry firms. Customers include Basalite Concrete Products and Anchor. “We can retrofit at very low capital cost, which allows us to expand and scale very quickly. It’s a volume game; we’re looking to make this the new standard of concrete production,” Niven says.

Compared with the CarbonCure method, Solidia’s approach has a greater potential to lower concrete’s CO₂ footprint, but it requires a special kind of cement. Solidia’s cement recipe uses more silica and less CaO than the Portland variety. This allows the kiln to operate at temperatures that are 250 °C lower, which cuts fossil-fuel CO₂ emissions by 30%, according to the company.

The resulting cement has a curious property: It does not react with water; it will cure only when exposed to CO₂. After blocks of wet concrete are formed, industrial CO₂ is pumped into the concrete curing chamber. The ability of the blocks to sequester CO₂ plus the lower kiln temperature add up to a CO₂ footprint that is 60% lower than common concrete’s, says Tom Schuler, Solidia’s CEO.

Schuler anticipates strong demand for greener concrete. But, he insists, “product durability, quality, and cost-effectiveness are our focus.” Solidia concrete is stronger, is less prone to cracking, uses less water, and cures in as little as one day as opposed to a week for standard concrete, he says.

CO₂-enhanced concrete is still in the early phases: Solidia is now testing its curing chamber mechanism on-site with Lafarge and other partner firms. And Solidia and CarbonCure are both conducting tests to ensure that steel support structures will not be more prone to corrosion inside their concrete.

Looking ahead, Solidia is working with Linde to adapt the process for uses where concrete is mixed, poured, and cured on the construction site. This so-called cast-in-place market uses about 80% of the 10 billion tons of concrete produced each year.

Even if the process moves into the cast-in-place market, “the carbon capture and storage potential of concrete will not be enough to solve our climate crisis,” ­BuildingGreen’s Ehrlich acknowledges.

Still, CO₂-absorbing concrete is a big innovation in an industry that can see a century pass with little change. “We’re trying to stay ahead of the curve on sustainability requirements,” says Lafarge’s Quail, who has worked in the concrete industry for 36 years. “I didn’t expect to see a game-changing technology come on-line in my lifetime. Now I’m dropping almost everything else to work on this.”