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Limestone grown from algae can be used to make carbon-neutral cement

Words:
Stephen Cousins

The algae-derived biogenic limestone draws down the same amount of CO2 from the atmosphere as that emitted during the cement's production, according to research by the University of Colorado

Wil Srubar, professor at the University of Colorado Boulder, holding a sample cube of concrete containing biogenic limestone produced by calcifying algae.
Wil Srubar, professor at the University of Colorado Boulder, holding a sample cube of concrete containing biogenic limestone produced by calcifying algae. Credit: Photo: Glenn Asakawa/University of Colorado

A team of engineers at the University of Colorado Boulder has developed a biogenic limestone, grown by microalgae, which they claim can be used to produce carbon-neutral – or potentially even carbon-negative – portland cement.

Conventional cement production, responsible for 7 per cent of annual global greenhouse gas emissions, is energy intensive due largely to the burning of quarried limestone and clay at high temperatures.

The algae-derived biogenic limestone draws down the same amount of CO2 from the atmosphere during the algae growing process as that released into the atmosphere when the material is burned in a kiln.

It has been developed by Colorado Boulder in collaboration with the Algal Resources Collection at the University of North Carolina Wilmington and the National Renewable Energy Laboratory.

A product-based life cycle assessment found that using biogenic limestone alone to make portland cement would reduce its embodied carbon by around 60 per cent. Furthermore, if ground biogenic limestone is also used as a filler material in Portland cement, replacing quarried limestone that often comprises 15 per cent of the mixture, it could enable a reduction in embodied carbon of up to 70 per cent.

When this process is used in combination with the electrification of kilns, powered by renewable energy at scale, the team claims it would be possible to create carbon-neutral or even carbon-negative Portland cement.

Calcifying microalgae can slowly pull carbon dioxide out of the atmosphere to produce produce limestone, enabling the production of a carbon-neutral cement.
Calcifying microalgae can slowly pull carbon dioxide out of the atmosphere to produce produce limestone, enabling the production of a carbon-neutral cement. Credit: Photo: Glenn Asakawa/University of Colorado.

Biogenic limestone is formed through the cultivation of coccolithophores, cloudy white microalgae that sequester and store carbon dioxide in mineral form through photosynthesis. According to researchers, the tiny organisms produce the largest amount of new calcium carbonate on the planet, in the form of limestone shells, and at a faster pace than coral reefs.

Microalgae thrive in both warm, cold, salt and fresh waters, making them ideal candidates for cultivation. Principal investigator and head of CU Boulder's Living Materials Laboratory Wil Srubar tells RIBAJ: 'We estimate that we would need around 1-2 million acres of land area to meet 100 per cent of the demand for cement production in the United States. That’s only around 0.1-0.2 per cent of the total land area and 1-2 per cent of the land area we currently use to grow corn. This is only if the algae are grown in open ponds, but there are other options, including photobioreactors, vertical farms, and continuous offshore cultivation, etc.'

The project explored what would happen if global cement-based construction were replaced with biogenic limestone cement. Calculations revealed that 2 gigatonnes of carbon dioxide would no longer be pumped into the atmosphere each year, and over 250 million tonnes of carbon dioxide would be sucked from the atmosphere and stored in the material.

'We see a world in which using concrete as we know it is a mechanism to heal the planet,' says Srubar. 'We have the tools and the technology to do this today.'

Previous research at the University of Colorado Boulder explored the use of microalgae to grow bricks.
Previous research at the University of Colorado Boulder explored the use of microalgae to grow bricks. Credit: Photo: Glenn Asakawa/University of Colorado.

The project was recently awarded a $3.2 million grant from the US Department of Energy and was recently selected by the programme Harnessing Emissions into Structures Taking Inputs from the Atmosphere, to develop and scale up the manufacture of biogenic limestone-based portland cement.

'The only challenges that we face are cost and scale,' says Srubar. 'We are addressing this through the $3.2 million project. We have a clear pathway to deliver biogenic limestone at cost parity to traditional limestone and at a scale that can meet the demand of US cement production.'

RIBAJ has reported on various other projects investigating the viability of algae-based building materials. They include a ‘living’ brick developed by the same team at the University of Colorado Boulder and a high-performance carbon fibre stone, developed by Technical University of Munich.

See more on the technology.

 

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