Recycled concrete

Recycled concrete and captured CO2 make new building material

Image credit: 2021 Maruyama et al.

Researchers from the University of Tokyo have developed a new building material with considerably lower carbon emissions than conventional concrete. The material shows promise as a construction material of the future, especially in places with limited natural resources.

Concrete is the most-used artificial material. The concrete industry is vast and it is estimated that around seven per cent of CO2 emissions come from the manufacture and use of cement (the main component of concrete) alone. A large proportion of this is due to the use of calcium, which is normally obtained by burning limestone and which is essential for the reaction between cement and water to form concrete.

Considerable research efforts are already underway to find alternative ways of making concrete or similar construction materials more sustainable.

Now, the University of Tokyo researchers have demonstrated a method for combining waste concrete and captured CO2 to create a usable form of concrete called calcium carbonate concrete.

Inspired by the way some aquatic organisms harden into fossils over time, Professor Ippei Maruyama wondered if the same process for forming hard calcium carbonate deposits from dead organic matter could be applied to concrete. Maruyama saw this as an opportunity to investigate a less carbon-intensive way of performing the same function as in the formation of concrete from cement and water.

“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.”

Calcium carbonate concrete cannot replace typical concrete at present; it is not quite as strong as typical concrete, though for some construction projects, such as small houses, this would not be a problem. At present, only small blocks a few centimetres in length have been made.

“It is exciting to make progress in this area, but there are still many challenges to overcome,” said project manager Professor Takafumi 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.”

Recently, another group of researchers from the University of Tokyo demonstrated a new method of producing concrete without cement. Their technique offers a means for the construction industry to reduce its carbon emissions, as well as offering potential for building on the Moon and Mars. 

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