A new type of concrete made from waste concrete and carbon dioxide from air or industrial exhaust gases could reduce emissions from the construction industry.
This calcium carbonate concrete also shows promise as a future building material, especially in places where natural resources are limited, according to scientists from the University of Tokyo who have presented it in the Journal of Advanced Concrete Technology.
The modern world is built with concrete. Every tall building in every city on Earth uses the durable and versatile material to give it shape and strength. So the concrete industry is huge, and this comes at a cost: it is estimated that around 7% of the world's carbon dioxide emissions come from the manufacture and use of cement, the main component of concrete. And a large proportion of this 7% is due to the necessary use of calcium, which is usually obtained by burning limestone.
Professor Ippei Maruyama and C4S (Calcium Carbonate Circulation System for Construction) project leader Professor Takafumi Noguchi, both from the Department of Architecture at the University of Tokyo, proposed and demonstrated a new way to reduce emission levels caused by the use of concrete.
They have found a way to take waste concrete and capture carbon dioxide, and combine them in a novel process into a usable form of this new calcium carbonate concrete.
Inspired by the way some aquatic organisms harden into fossils over time, Maruyama wondered if the same process that forms hard calcium carbonate deposits from dead organic matter could be applied to concrete. Calcium is essential for the reaction between cement and water to form concrete, and Maruyama saw this as an opportunity to investigate a less carbon-intensive way of performing the same function.
"Our concept is to acquire calcium from discarded concrete, which would otherwise go to waste," Maruyama said in a statement. We combine this with carbon dioxide from industrial exhaust gases or even air. And we do it at much lower temperatures than those used to extract calcium from limestone today.
Calcium carbonate is a very stable material, making it a durable building material. And the ability to recycle large amounts of material and waste is a great benefit. However, calcium carbonate concrete cannot replace typical concrete today. It is not as strong as typical concrete, although for some construction projects, such as small houses, this would not be a problem. Also at present, only small blocks of a few centimetres in length have been made.
"It's exciting to make progress in this area, but there are still many challenges to overcome," said Noguchi. "In addition to increasing the strength and size limits of calcium carbonate concrete, it would be even better if we could further reduce the energy use of the production process. However, we expect that in the coming decades, carbon-neutral calcium carbonate concrete will become the conventional type of concrete and will be one of the solutions to climate change."
