Biogenic Limestone: A Revolutionary Pathway to Carbon-Negative Construction
The global construction industry has a sustainability problem. Because cement production requires the addition of burnt limestone – a finely ground powder known as portland cement – as binder, it causes huge amounts of greenhouse gas emissions, which contribute to the industry’s role as a leading source of atmospheric CO2.
Overall, the use of cement by the building sector accounts for about 7 percent of carbon discharges worldwide. Increasing awareness of this problem on the part of regulators and the general public are driving efforts to decarbonize cement production and bring global construction in line with the UN’s Sustainable Development Goals.
In June 2022, a research group in the US involving scientists from the University of Colorado Boulder, the University of North Carolina Wilmington (UNCW), and the National Renewable Energy Laboratory (NREL) received a US$3.2 million grant from the US Department of Energy’s Advanced Research Projects Agency – Energy (ARPA-E) to develop an alternative way of manufacturing portland cement from limestone made by microalgae called coccolithophores. These single-cell plankton organisms build up shells of calcium carbonate from sunlight, seawater, and dissolved carbon dioxide using the same process that results in the growth of coral reefs.
Net negative carbon emissions
Not only does this method avoid the laborious, energy- and time-consuming, and costly process of mining limestone from quarries; because the microalgae extract and store atmospheric CO2 through photosynthesis, the resulting biogenic limestone has a neutral carbon footprint, since the amount of carbon released during calcining (heating) is equal to that stored by the microorganisms. If additional quantities of biogenic limestone are used as a filler material, the portland cement produced through this novel method would ultimately result in net carbon negative emissions by permanently storing CO2 in concrete.
“This neutral-CO2 cement would be used to replace normal cement in concrete structures, which would contribute significantly to reducing the carbon footprint of the cement industry.”
Catharina Alves-de-Souza, University of North Carolina Wilmington
“This neutral-CO2 cement would be used to replace normal cement in concrete structures, which would contribute significantly to reducing the carbon footprint of the cement industry,” said Catharina Alves-de-Souza, who heads the Algal Resources Collection at UNCW’s Center for Marine Science. She added that ARPA-E’s “Harnessing Emissions into Structures Taking Inputs from the Atmosphere” (HESTIA) program was one of the more innovative and exciting examples of microalgal biotechnological applications: “The proposed biotechnological approach offers a revolutionary pathway to produce, for the first time, CO2-neutral portland cement using microalgae. Nothing like that has ever been attempted before.”
Commercial viability of biogenic limestone
Alves-de-Souza is optimistic that the biogenic limestone would not only be good for the environment, but also generate revenue: “We will also obtain other high-value products from the microalgae, such as lipids and proteins, which will make the project economically viable.” Moreover, the industry could adapt the novel manufacturing process very rapidly, according to Dr. Wil Srubar, the lead principal investigator on the stone project and associate professor in Civil, Environmental and Architectural Engineering and CU Boulder’s Materials Science and Engineering Program.
“Because biogenic limestone is plug-and-play with traditional portland cement manufacturing, the only real challenge for us is achieving economies of scale.”
Wil Srubar, CEO, Minus Materials
Srubar is also the CEO of Minus Materials, a pre-seed startup spin-off company of UC Boulder that handles the commercialization of the climate-friendly building material. He said the biogenic lime could be taken up almost immediately because it can be seamlessly integrated with state-of-the-art cement production: “Because biogenic limestone is plug-and-play with traditional portland cement manufacturing, the only real challenge for us is achieving economies of scale. Minus Materials has developed multiple viable pathways to get there. All we need to do now is execute,” Srubar told Supertrends.
Denver-based Minus Materials describes its mission as “developing new science and technology to accelerate decarbonization of the cement and concrete industry while strengthening their global value chains through advanced biotechnology”. In a June 2022 funding round, the company received backing from SOSV, a multi-stage venture capital firm with US$1.3 billion in assets under management that focuses on human and planetary health, and from SOSV’s biotech startup accelerator IndiBio.
The researchers believe that producing all the cement required by the US construction industry would require a space of 1 to 2 million acres (4.000 to 8.000 km2) of open ponds for cultivation of the calcium-producing algae. This amounts to just 0.05 to 0.10 percent of the country’s land area. For comparison, a hundred times more land is used to grow corn in the US. The effect would be even greater if scaled up globally: Replacing all cement-based construction around the world with biogenic limestone cement would prevent two gigatonnes of CO2 emissions annually, while extracting and storing another 250 million additional tonnes of atmospheric CO2.
According to the International Energy Agency (IEA), the direct CO2 intensity of cement production saw a 1.8 percent annual increase during the period from 2015 to 2020. However, achieving the goals set in the IEA’s Net Zero Emissions by 2050 Scenario for the global energy sector would require that this value decrease by 3 percent per year.
The intergovernmental organization says that a realistic path toward achieving these climate targets would require a reduction of the clinker-to-cement ratio, since the amount of clinker – the main ingredient in cement – used in production is directly proportional to the volume of CO2 emissions caused during this process. The IEA has also said that carbon capture, usage, and storage (CCUS) technologies will be “crucial” in preventing carbon emissions during limestone calcination in cement-making.
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