-GBA CEMENT WEEK-
If cement could be made without limestone, theoretically, that could eliminate many of the industry’s CO2 emissions. That’s Solidia’s first bet. Its second gamble: When that cement is used to make concrete, the process will actually absorb carbon dioxide.
Typically, when water is added to Portland cement and gravel to make concrete, it reverses the reaction that occurred in the cement kiln—almost—in a process called “curing.”
The calcium silicates (like alite and belite) combine with water to form calcium hydroxide and clay; the calcium hydroxide then reacts with CO2 in the air to form calcium carbonate again, releasing the water it had absorbed. The formation of calcium carbonate holds all the components of concrete together; if the concrete mix is put in a mold, over many weeks of curing, those familiar solid blocks are formed.
Here’s the problem: As long as enough of the cement binds the gravel together into concrete, the product is ready. In other words, it never goes through
a complete reversal, and thus doesn’t absorb the same amount of CO2 emitted during the cement-making process. One estimate suggests that concrete absorbs
about 17%of emissions produced over its lifecycle—which would be about 170kg of CO2 absorbed. What if it were possible to change the chemistry of cement such that it could absorb all the CO2?
Two startups have tried and failed in their attempts to change the chemistry of cement. UK-based Novacem invented a process that replaced calcium oxide with magnesium oxide. In 2012, it sold its intellectual property to a rival and folded. California-based Calera began with a pitch similar to Novacem’s, but after repeated disappointments, it shifted to focusing on specialized calcium carbonate for niche applications, such as wallboards. Both companies raised many millions of dollars before shutting or pivoting.
But these failures were yet to surface in Solidia’s early years. Back then, in lab experiments, one of the startup’s founding members Vahit Atakan, now its chief scientific officer, discovered that if he replaced limestone with the mineral wollastonite—a low-carbon alternative to limestone—he could make cement that eventually produced “
negative emissions” concrete. That’s because wollastonite’s chemistry is such that it would not produce any emissions when it is made to produce cement, but it would, like normal cement, absorb some CO2 when it gets cured as concrete.
But when Solidia began thinking about commercializing the product, the company hit significant hurdles. For example, changing the chemistry of cement would make the hundreds of cement plants currently in operation redundant, essentially turning them into stranded assets. In other words, it wouldn’t be in cement-makers’ financial best interests to invest in Solidia’s wollastonite-based product.
Another problem is that wollastonite is not as cheap or widely available as limestone. About 1.5 million kg of wollastonite is mined each year in the US, enough to make some 1.5 million kg of cement. That sounds like a lot—until you find out that US factories make nearly 100 billion kg of cement each year— that alone is about 50 Hoover dams worth of cement.
DeCristofaro says solving the wollastonite problem was the “seminal moment in Solidia’s history.”
On a pivot
Solidia knew it had no other choice: it would
have to make a synthetic version of wollastonite. The company spent a few years playing with various recipes, first in labs and later in a small factory, until it came up with a solution. It turned out to be deceptively simple.
Wollastonite-derived cement has a lot less calcium than Portland cement. So to replace wollastonoite, Solidia could reduce the amount of limestone and increase the amount of clay in the mix it sent to the kiln. With less limestone to convert to lime, the process could use less heat. Cutting out limestone reduced CO2 emissions from both the chemical reaction, and from the fossil fuels needed to heat the process.
Of course, the startup now needs to show that this lower-emission cement can be made into concrete that’s at least as good as others, and can be scaled up in a way that’s affordable. That’s what Solidia is working on right now. Recently, the company invited me to visit its small factory in Piscataway, New Jersey and peek at the technology. After putting on protective wear—hard hat, shoe gloves, and lab glasses—I got to see the process of making concrete using Solidia’s potentially game-changing cement.
) cement facility
