Researchers from the University of Tokyo’s Department of Architecture have developed a promising new kind of concrete that has the potential to reduce emissions from the construction industry. 

Approximately 7% of the world’s carbon dioxide emissions come from the manufacture and use of cement, which is the main component of concrete. In addition, a large proportion of these emissions is due to the necessary use of calcium, which is primarily obtained by burning limestone. Professor Ippei Maruyama and Calcium Carbonate Circulation System for Construction (C4S) project manager Professor Takafumi Noguchi investigated a less carbon-intensive method of capturing calcium.

They subsequently found a way to take concrete waste and captured carbon dioxide and combine them in a process to create a usable form of concrete called calcium carbonate concrete. As noted in the research team’s report, Japan is estimated to have accumulated around 100 billion tons of concrete, with the annual amount of concrete waste generated in the country at 40 to 100 million tons. This was identified as being a potential source for the reuse of calcium in concrete.

This process was inspired by the way some aquatic organisms harden into fossils over time, in a process that forms hard calcium carbonate deposits from dead organic matter. 

"Our concept is to acquire calcium from discarded concrete, which is otherwise going to waste," said Maruyama. "We combine this with carbon dioxide from industrial exhaust or even from the air. And we do this at much lower temperatures than those used to extract calcium from limestone at present."

Other sources of calcium mentioned in the study include calcium carbonate rocks and calcium-containing industrial waste, such as gypsum. 

While this new form of concrete has shown the potential to be a stable and ecologically friendly material, according to the researchers, the calcium carbonate concrete cannot replace traditional concrete at this moment. It’s not yet as strong as typical concrete, and currently, only small blocks a few centimeters in length have been made. 

"It is exciting to make progress in this area, but there are still many challenges to overcome," said Noguchi. "As well as 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 hope that in the coming decades, carbon-neutral calcium carbonate concrete will become the mainstream type of concrete and will be one of the solutions to climate change."