54 Technology

54.1 Cement

Lobet

“Our dream is to decarbonize cement, and we want to do it as fast as possible”

The chemical reaction at the heart of today’s cement-manufacturing process is a major reason why the production of this essential building material accounts for an estimated 7 percent of global carbon dioxide emissions from energy and industrial sources.

But a small group of tech entrepreneurs says they’ve found a better way. Cody Finke, co-founder and CEO of Brimstone Energy, is one of them.

Today, producers use limestone, a common rock, as the basis of cement. They mine it, grind it and then heat it up to temperatures roughly a quarter as hot as the surface of the sun. This drives carbon dioxide out of the rock and into the atmosphere.

Limestone is a form of calcium carbonate, a chemical compound with the formula CaCO3, made up of three main elements: calcium, carbon and oxygen. Extracting the calcium and oxygen needed for cement leaves behind the single carbon atom and two oxygen atoms — CO2. Fifty to 65 percent of the CO2 emissions from cement production result from the fact that the source rock is limestone, not from heating cement kilns to high temperatures.

Brimstone’s alternative recipe swaps out this limestone for calcium silicate rocks, which are also very common. The silicates contain the same calcium oxide, commonly known as lime. But ​“there is…no CO2 in the rock,”

Using an alternative source rock also allows Brimstone to lower the kiln temperatures — another way the company can reduce CO2 emissions. Typical cement production requires temperatures of approximately 900 degrees Celsius for one key part of the process and 1,450 degrees Celsius for the other. Brimstone’s process, in contrast, requires temperatures above 500ºC for only 20 percent of its heat energy.

Because of these lower temperature requirements, Brimstone can use electric kilns for most of its process instead of kilns fired by coal, petroleum coke or natural gas. For now, the company still expects to burn a fossil fuel for the hottest 20 percent of its process or use hydrogen if it’s available. It’s possible to electrify the entire process, Finke said; it’s just uneconomical to do so today.

Lobet (2021) Changing up the recipe to make low-carbon cement

St. John

“There are a lot of novel technologies out there that work, and work fine. But they’re not scalable, they’re not commercially viable — and usually it’s because the feedstock is not available in full volume, or not available where it’s needed.” “There are a lot of novel technologies out there that work, and work fine. But they’re not scalable, they’re not commercially viable — and usually it’s because the feedstock is not available in full volume, or not available where it’s needed.”

Cement and concrete production are responsible for 8 percent of human-caused carbon dioxide emissions worldwide, and novel SCMs like Terra’s offer one path to reducing that massive carbon footprint. SCMs lower emissions from concrete production because they reduce reliance on Portland cement — by far the most common type of cement made today and also the driver of concrete’s carbon impact. The production of Portland cement requires super-high temperatures that are achieved by burning fossil fuels, and the carbon-rich limestone used in its production also leaks CO2 into the air.

Major cement and concrete companies such as Cemex and Holcim already use millions of tons of SCMs today, mostly fly ash from coal plants and slag from steel mills, both to reduce their concrete’s carbon footprint and to strengthen the material. But the same climate imperatives that are pushing the cement industry to cut its carbon emissions are also driving the closure of coal plants and steel blast furnaces, making these components less ubiquitous and more expensive to get.

Terra CO2’s SCM, by contrast, is made from a variety of silicate rocks, including granite, basalt, alluvial sand and gravel, glacial flood gravel and clay-sand mixtures. Silica rock for the most part doesn’t have any embodied CO2.” That’s in contrast to limestone, the primary ingredient of Portland cement, which ​“by weight is about 50 percent embodied CO2” — carbon that’s released into the atmosphere when it’s processed into clinker, the precursor to Portland cement.

Terra CO2 puts these rocks into a reactor that heats them to their melting point, yielding glassy powders that can replace 25 to 40 percent of the Portland cement needed for different mixes of concrete. The company estimates that every ton of cement replaced by Terra’s SCM results in 70 percent lower carbon-dioxide emissions compared to pure Portland cement.

CarbonCure, a Canadian startup that injects carbon dioxide captured from other emitting sources into concrete, which both strengthens the concrete and stores the carbon, preventing it from entering the atmosphere. This practice can reduce the carbon footprint of concrete by roughly 5 to 15 percent, and it is relatively simple to integrate into how concrete is produced today.

Low-carbon SCMs tend to be the next step for cement-makers trying to cut their carbon emissions.

Fly ash is already supply-distressed now. By the end of the decade, it’s going to go away. And blast furnace slag is going away as well.

Cement makers such as Heidelberg Materials and Hoffman Green Cement Technologies are pursuing one SCM option known as calcined clays. This material is widely available in Asia and Africa, but not as much in North America and Europe, making it less suitable for those markets.

Other approaches that promise a completely zero-carbon replacement for Portland cement are in a more experimental phase and would require retooling the cement industry to bring to scale. Some examples include startups such as Sublime Systems and Chement, which are developing electrochemical processes to replace the high-heat methods used to make cement. More esoteric concepts include using living organisms to ​“grow” cement.

St. John (2023) Terra CO2 says its Texas factory will cut carbon and cost from cement