Los científicos han cultivado cocolitóforos, que crean la mayor cantidad de nuevo carbonato de calcio en el planeta, y lo hacen más rápido que los arrecifes de coral, utilizando solo la luz solar, el agua de mar y el dióxido de carbono disuelto.
Cómo los científicos esperan usar piedra caliza de algas para construir ciudades
La quema de piedra caliza de las canteras contribuye significativamente al 7 % de las emisiones anuales de gases de efecto invernadero de la fabricación de cemento en todo el mundo. Un equipo de investigación dirigido por la Universidad de Colorado en Boulder ha descubierto un método para usar microalgas para absorber dióxido de carbono de la atmósfera, lo que hace que la producción de cemento sea neutra en carbono o incluso negativa en carbono.
La Agencia de Proyectos de Investigación Avanzada de Energía (ARPA-E) del Departamento de Energía de EE. UU. (DOE) otorgó a los ingenieros de CU Boulder y sus colegas del Laboratorio Nacional de Energía Renovable (NREL) y la Colección de Recursos de Algas de la Universidad de Carolina del Norte en Wilmington (UNCW ) $3.2 millones por su trabajo creativo. El grupo de investigación fue elegido recientemente por el programa Harnessing Emissions into Structures Taking Inputs from the Atmosphere (HESTIA) para avanzar y expandir la producción de cemento portland biogénico a base de piedra caliza y ayudar a crear un futuro sin carbono.
«Este es un momento realmente emocionante para nuestro equipo», dijo Wil Srubar, investigador principal del proyecto y profesor asociado de ingeniería civil, ambiental y arquitectónica y programa de ciencia e ingeniería de materiales en CU Boulder. “Para la industria, ahora es el momento de abordar este tema tan espinoso. Creemos que tenemos una de las mejores soluciones, si no la mejor solución, para que la industria del cemento y el concreto resuelva su problema de carbono.
Wil V. Srubar sostiene un cubo de muestra de piedra caliza biogénica (blanca) producida por la calcificación de microalgas, conocidas como cocolitóforos. Crédito: Glenn Asakawa/Universidad de Colorado
Uno de los materiales más comunes en la tierra y una base de construcción en todo el mundo es el hormigón. Comienza como una pasta hecha de agua y cemento Portland, a la que luego se le agregan componentes como arena, grava o piedra triturada. La pasta mantiene unidas las partículas y endurece la mezcla de concreto.
La forma más popular de cemento, el cemento Portland, se crea extrayendo piedra caliza de grandes canteras y quemándola a altas temperaturas, lo que produce una gran cantidad de dióxido de carbono. El equipo de estudio descubrió un método neutro en carbono neto para producir cemento Portland al reemplazar la piedra caliza extraída con piedra caliza generada biológicamente, un proceso natural que algunas especies de microalgas calcáreas complementan a través de[{» attribute=»»>photosynthesis (much like building coral reefs). In other words, the amount of carbon dioxide that is released into the atmosphere is equivalent to what the microalgae have already captured.
Another common filler material used in portland cement is ground limestone, which generally replaces 15% of the mixture. Portland cement could become not just net neutral but even carbon negative by sucking carbon dioxide out of the atmosphere and storing it permanently in concrete if biogenic limestone was used as the filler instead of quarried limestone.
A whopping 2 gigatons of carbon dioxide would no longer be pumped into the atmosphere each year and more than 250 million additional tons of carbon dioxide would be pulled out of the atmosphere and stored in these materials if all cement-based construction worldwide were replaced with biogenic limestone cement.
This could theoretically happen overnight, as biogenic limestone can “plug and play” with modern cement production processes, said Srubar.
“We see a world in which using concrete as we know it is a mechanism to heal the planet,” said Srubar. “We have the tools and the technology to do this today.”
Limestone in real-time
Srubar, who leads the Living Materials Laboratory at CU Boulder, received a National Science Foundation CAREER award in 2020 to explore how to grow limestone particles using microalgae to produce concrete with positive environmental benefits. The idea came to him while snorkeling on his honeymoon in Thailand in 2017.
He saw firsthand in coral reefs how nature grows its own durable, long-lasting structures from calcium carbonate, a main component of limestone. If nature can grow limestone, why can’t we? he thought.
“There was a lot of clarity in what I had to pursue at that moment. And everything I’ve done since then has really been building up to this,” said Srubar.
Wil V. Srubar III, Assistant Professor of Civil, Environmental and Architectural Engineering, leads the Living Materials Laboratory at the University of Colorado Boulder. Their current work utilizes calcifying microalgae, which produce limestone, to create carbon-neutral cement, as well as cement products that can slowly pull carbon dioxide out of the atmosphere and store it. Here, Mady Murphy, CU Boulder chemical and biological engineering undergraduate student, left, and Rebecca Mikofsky, CU Boulder material science Ph.D. student, hold samples of (white) biogenic limestone produced by calcifying microalgae, known as coccolithophores. Credit: Glenn Asakawa/University of Colorado
He and his team began to cultivate coccolithophores, cloudy white microalgae that sequester and store carbon dioxide in mineral form through photosynthesis. The only difference between limestone and what these organisms create in real-time is a few million years.
With only sunlight, seawater, and dissolved carbon dioxide, these tiny organisms produce the largest amounts of new calcium carbonate on the planet, and at a faster pace than coral reefs. Coccolithophore blooms in the world’s oceans are so big that they can be seen from space.
“On the surface, they create these very intricate, beautiful calcium carbonate shells. It’s basically an armor of limestone that surrounds the cells,” said Srubar.
Commercializing coccolithophores
These microalgae are hardy little creatures, living in both warm and cold, salt and fresh waters around the world, making them great candidates for cultivation almost anywhere—in cities, on land, or at sea. According to the team’s estimates, only 1 to 2 million acres of open ponds would be required to produce all of the cement that the U.S. needs—0.5% of all land area in the U.S. and only 1% of the land used to grow corn.
A scanning electron micrograph of a single coccolithophore cell, Emiliania huxleyi. Credit: Wikimedia Commons / Alison R. Taylor, University of North Carolina Wilmington Microscopy Facility
And limestone isn’t the only product microalgae can create: microalgae’s lipids, proteins, sugars, and carbohydrates can be used to produce biofuels, food, and cosmetics, meaning these microalgae could also be a source of other, more expensive co-products—helping to offset the costs of limestone production.
To create these co-products from algal biomass and to scale up limestone production as quickly as possible, the Algal Resources Collection at UNCW is assisting with strain selection and growth optimization of the microalgae. NREL is providing state-of-the-art molecular and analytical tools for conducting biochemical conversion of algal biomass to biofuels and bio-based products.
There are companies interested in buying these materials, and the limestone is already available in limited quantities.
Minus Materials, Inc., a CU startup founded in 2021 and the team’s commercialization partner, is propelling the team’s research into the commercial space with financial support from investors and corporate partnerships, according to Srubar, a co-founder and acting CEO. Minus Materials previously won the university-wide Lab Venture Challenge pitch competition and secured $125,000 in seed funding for the enterprise.
The current pace of global construction is staggering, on track to build a new New York City every month for the next 40 years. To Srubar, this global growth is not just an opportunity to convert buildings into carbon sinks, but to clean up the construction industry. He hopes that replacing quarried limestone with a homegrown version can also improve air quality, reduce environmental damage, and increase equitable access to building materials around the world.
“We make more concrete than any other material on the planet, and that means it touches everybody’s life,” said Srubar. “It’s really important for us to remember that this material must be affordable and easy to produce, and the benefits must be shared on a global scale.”
Reference: “Cities of the future may be built with algae-grown limestone” by Kelsey Simpkins, University of Colorado Boulder.