New carbon capture method turns up the heat

A centuries-old technique for making cement may be the key to large-scale carbon capture, according to US chemists.

The first step in making cement involves converting limestone to calcium oxide, inside a kiln heated to 1400 degrees Celsius. Calcium oxide is then mixed with sand to produce a vital ingredient for cement.

Inspired by this technique, Stanford University researchers used a conventional kiln to transform common minerals into reactive materials with the ability to pull carbon from the atmosphere – and sequester it permanently.

“The Earth has an inexhaustible supply of minerals that are capable of removing CO₂ from the atmosphere, but they just don’t react fast enough on their own to counteract human greenhouse gas emissions,” says Matthew Kanan, a professor of chemistry at Stanford and an author of the new study, published in Nature.

“Our work solves this problem in a way that we think is uniquely scalable.” 

Carbon capture recap

Most global and national targets of net zero emissions require not just immediately cutting fossil fuel production, but also removing carbon dioxide (CO₂) from the atmosphere. In fact, these targets rely on carbon capture and storage technology (CCS).

In the IPCC’s 2018 special report on pathways to limiting global heating to 1.5°C above pre-industrial levels, nearly all modelled pathways involved overshooting the 1.5°C limit – then returning to it later, by using CCS to bring the temperature back down.

But a Nature study from October 2024 showed that crossing the threshold and then returning to it would not result in the same world as simply staying below the temperature limit. Many climate impacts are irreversible, and others will take decades to undo.

In addition, although many CCS technologies are in development, so far they are limited. They are either too costly, too energy-intensive, or unproven at a large scale.

Confronting fossil fuel interests remains the most vital step in curbing the climate crisis – but carbon capture technology may still be useful, and research continues apace.

From slow to quick weathering

The new paper from Stanford University promises big results.

The method essentially speeds up the natural process of silicate weathering. In this process, CO₂ in the atmosphere dissolves in rainwater to form a weak acid. This reacts with common minerals in rocks called silicates, breaking them down into other compounds such as bicarbonate ions (HCO₃^–), which flow into the ocean and stably store carbon for thousands of years.

However, the weathering process itself can take hundreds or thousands of years. This new research instead converts slow-weathering silicates into much more reactive minerals, thus capturing and storing carbon quicker.

“We envisioned a new chemistry to activate the inert silicate minerals through a simple ion-exchange reaction,” explains Stanford postdoctoral researcher Yuxuan Chen, lead author of the study.

They used a similar method to producing cement in a kiln, but instead of mixing calcium oxide with sand, they mixed calcium oxide with another mineral composed of magnesium and silicate ions. The heat catalysed an exchange of ions, forming magnesium oxide and calcium silicate: alkaline minerals that react quickly with acidic CO₂ in the atmosphere.

In a lab test, Kanan and team exposed the two minerals to water and pure CO₂. Within two hours, the materials had reacted with the CO₂ to form new carbonate minerals. A more realistic test involved exposing the minerals to atmospheric CO₂. Although the carbonation process took weeks to months to occur, it was still thousands of times quicker than natural processes.

The team is planning how this approach could be used at an industrial scale. They estimate that it would require one ton of reactive material in order to remove one ton of CO₂ from the atmosphere. However, their lab can currently only produce 15 kilograms of material per week, and in 2024 alone, more than 37 billion tons of carbon dioxide were emitted by burning fossil fuels.

It is estimated that 4 billion tons of cement are manufactured each year.  

Scaling up the carbonation process to meaningfully slow global heating would require millions – perhaps billions – of tons of magnesium oxide and calcium silicate. The team suggests that they could use mining waste, which contains suitable silicates, as a large source of raw material.

“Society has already figured out how to produce billions of tons of cement per year, and cement kilns run for decades,” Kanan says. “If we use those learnings and designs, there is a clear path for how to go from lab discovery to carbon removal on a meaningful scale.”

He adds: “You can imagine spreading magnesium oxide and calcium silicate over large land areas to remove CO₂ from ambient air.

“One exciting application that we’re testing now is adding them to agricultural soil. As they weather, the minerals transform into bicarbonates that can move through the soil and end up permanently stored in the ocean.”