Set in Stone

Article by Amanda Doyle

Amanda Doyle speaks to Rahul Shendure about innovations for lower-emissions cement and storing CO2 in concrete

Prize winner: One of 10,000 concrete blocks hardened with CO2 from a coal-fired plant in Wyoming

CEMENT production is responsible for 8% of emissions worldwide, but as CO2 is a byproduct of the process, decarbonising the industry will need to involve some innovative solutions. I spoke to Rahul Shendure, CEO of CarbonBuilt, about how the company is aiming to improve the industry in two ways: by reducing emissions from cement production and by sequestering industrial CO2 emissions in concrete.

In other industrial sectors, reductions in CO2 emissions can be made by making changes such as using renewable energy or green hydrogen. However, in cement production CO2 is emitted during the process itself, not just from the combustion of fuels.

The most common type of cement used is known as ordinary Portland cement (OPC). It is made from Portland clinker, a nodular material made by heating limestone and clay in a rotating kiln at around 1,450°C. The clinker is ground to a fine powder to make the cement. The cement then acts as a binder when mixed with aggregate and water to make concrete – the final product that’s used in construction.

More than half of the emissions come from calcination – the chemical breakdown of limestone to produce clinker. The remaining emissions come mainly from the heat needed for the kilns, with the rest being associated with mining and transporting materials.

CarbonBuilt is tackling emissions in the cement industry by reducing the amount of Portland cement in concrete and using waste CO2 emissions from industrial flue gas to cure (harden) the concrete.

Company origin

CarbonBuilt – originally known as CO2Concrete – was spun out of the Samueli School of Engineering at the University of California, Los Angeles (UCLA) in 2014, led by Gaurav Sant, a Professor at the Department of Civil and Environmental Engineering. Sant is a third-generation civil engineer who started looking for a way to reduce the expensive materials and processes used in concrete production. The work involved looking at including alternative materials in cement that were already large-scale commodities, as well as developing a process amenable to carbonation where the uptake of CO2 is used as a “binder” to harden the concrete.

Shendure joined CarbonBuilt in 2021 as CEO. Shendure is a chemical engineer who began his career in the plastics business at General Electric, before moving on to focus on the commercial side of clean energy technology, including fuel cells, advanced biofuels, and renewable fuels and chemicals. He took a break from the energy sector to co-found a cancer diagnostics company with his brother, who is a professor in that area. Shendure then focussed on investing in early-stage clean technology before getting involved with CarbonBuilt.


“Concrete is typically made with ordinary Portland cement, often with a small amount of fly ash or other fillers like slag, and then the vast majority of it is aggregate rock and sand,” explained Shendure. “To that mix we add one new component which is hydrated lime, otherwise known as calcium hydroxide, or portlandite, in its natural form.”

While portlandite still needs to be processed like Portland clinker, it can be done at a much lower temperature of around 800°C, which lowers the carbon footprint of the process. Adding portlandite to the mix allows CarbonBuilt to reduce the amount of Portland cement needed by 60–90%. Other low-cost and low-carbon components such as fly ash and slag are also added to the concrete mix in higher quantities.

Using portlandite (Ca(OH)2) in cement has another advantage in that it has the capability to uptake CO2 to form calcium carbonate (CaCO3) when the concrete is being hardened. “Portlandite converts back into limestone at a molecular level when exposed to CO2,” said Shendure. “This conversion of portlandite to limestone happens at low temperature, ambient pressure, and does not require concentrated CO2 so the conversion process can happen at much lower operating expense and capital expense.”

Work is also ongoing for improving the process, as Shendure explained. “It’s not part of our core process, but we have a number of work streams underway to reduce the carbon footprint, for example alternate ways of producing calcium hydroxide that are not based on the conventional heating up of limestone.”

Carbon capture with a difference

Typically, carbon capture from an industrial emissions source means using a solvent to capture the CO2, and then pressurising the CO2 so that it can be transported and stored. CarbonBuilt’s process can use this waste CO2 without actually capturing it.

“One of the benefits of our process is that we don’t actually have to capture it,” said Shendure. “All you’re doing is taking the flue gas as is and using it without any changes of the concentration of the CO2 or to the pressure. You’re not screening out any impurities.”

The system basically involves running a pipe from the facility to the curing rooms. This reduces the cost significantly compared to carbon capture systems as the main operating expense is the electricity needed to run the fans. It also means that it is easy to retrofit to facilities.

“The simplest and cheapest way to do this is to just retrofit existing curing rooms in concrete production facilities. The target spots for us are concrete producers that are located very nearby to a CO2 source where we can run a pipe from the source directly to the concrete facility. Typically, most of the precast products we’re focussed on use steam curing so they’ve already got a process where they’re curing with a gas.”

The curing process enables 2% or more of the weight of the concrete to come from the absorbed CO2 and there is the potential to increase the amount of CO2 absorbed. Different compositions in flue gas also don’t make a difference to the finished concrete product. The first pilot project used coal flue gas and CarbonBuilt has also tested the system on natural gas flue gas. It is looking to partner with a cement plant for another technology demonstration later this year or early next year. While it doesn’t matter what the source of CO2 is, using emissions from a cement plant is another step towards the circular economy. “It is an interesting synergy with cement plants,” said Shendure. “Often you have cement plants that will have concrete block operations or precast concrete operations that are located either on site or very nearby. So concrete producers will set up their facilities very close to the cement plant to minimise transportation cost. The piping of CO2 can be handled quite easily from a cement plant.”

Winning the Carbon XPRIZE

The development of the process was driven by funding from the US Department of Energy, which paid for the construction of the pilot system, and also by the Carbon XPRIZE.

The Carbon XPRIZE was launched in 2015 as a five-year competition, with ten finalists announced in 2018. The competition has a two-track structure, with one focussing on technology tested at a coal-fired plant in Wyoming, US, and the other at a natural gas power plant in Alberta, Canada. The aim of the competition was to convert the most CO2 into the highest value products at each track. CarbonBuilt was one of two winners of the Carbon XPrize, having produced 10,000 concrete blocks using the coal flue gas at the Wyoming Integrated Test Center. The US$7.5m prize fund will go to UCLA.

Shendure spoke on the impact of the XRPIZE for the company. “For us we were quite under the radar screen as a company, so the XPRIZE has been big just in terms of expanding profile. Having both of those things together, the DoE funding and then the XPRIZE funding, really was critical to us moving the technology at the pace that we were able to.”

Scaling up

CarbonBuilt is currently at pilot scale using a curing chamber made out of a shipping container. The gas processing equipment is at sub-scale, but it is easy to scale it up as it is off-the-shelf equipment. “Scaleup is really just a question of ordering larger pieces of equipment, or more of the same equipment,” said Shendure.

“The next step for us after our pilot demonstrations is to do our first full-scale conversion or integration. That would be taking traditional sized concrete block lines – something of the order of 200–300 t/d of concrete – and integrating our technology into that scale of plant. We’ll do multiple versions of those – let’s say 5–10 of those – and then once we’ve proven it at that scale we will be in a position to secure debt financing for further growth.”

He explained that the benefits of the CarbonBuilt process are two-fold in terms of environmental benefit and cost.

“When you translate the material changes and the carbon benefits into revenue and cost, and then look at that relative to the capital investments for the retrofits, they’re attractive investments either for the concrete producers or for the third-party investors who are looking to invest in sustainable infrastructure of any kind.”

Working towards low emissions cement is an interdisciplinary endeavour, and Shendure added that while concrete plants are not typically thought of as chemical plants, chemical engineering is an important part of improving the processes involved with cement and concrete production.

“I definitely think there is an opportunity for chemical engineers to pair up with our fellow engineers on the civil side together to drive some real change.”

Article by Amanda Doyle

Staff Reporter, The Chemical Engineer

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