researchers recycle concrete from demolished school and carbon dioxide in air into bricks

Making recycled concrete blocks with carbon dioxide

Researchers led by a team at the University of Tokyo have recycled concrete from a demolished school building and carbon dioxide in the air into new bricks that are strong enough to be used in building houses. In collaboration with Tokyo University of Science and Taiheiyo Cement Corporation, they pulverize the old concrete from the demolished school first, and when it has turned into powder, they mix it with the carbon dioxide from the air.

After that, they pressurize the substance in layers using molds before heating it so it becomes a concrete block. The researchers believe that with this method, it’s easier and feasible to recycle old materials while trapping carbon dioxide at the same time. On top of this, these ‘refreshed’ blocks can be broken into pieces and be remade again into new sets of blocks, should the building they’re used in is demolished.

Two samples of calcium carbonate concrete, one using hardened cement paste (left) and the other using silica sand. Both raw materials are common construction and demolition waste products | image © 2021 Maruyama et al.

pressurizing carbonated powder to become concrete blocks

The researchers led by the University of Tokyo detail that the recycled concrete blocks have been carbonated for over three months. Normally, concrete slowly reacts with carbon dioxide in the air over many years. This carbonation process changes some of the compounds in the concrete, like portlandite and calcium silicate hydrate, into calcium carbonate. It also helps strengthen the concrete over time. But the researchers want to see if they could speed up the process, all the while still manufacturing strong recycled concrete blocks with carbon dioxide. The process has the researchers pressurizing the carbonated powder with a calcium bicarbonate solution.

After that, they proceed to dry it in order to solidify the concrete blocks. This isn’t the first experiment of the researchers when it comes to recycling concrete with carbon dioxide. In an earlier experiment, they created calcium carbonate concrete by letting a bicarbonate solution flow through carbonated concrete powder and then heating it. In the recent testing, they made some changes by building the concrete in layers inside a mold, rather than just mixing it, and applying pressure to compact the layers. The latter means they made the material denser, resulting in blocks stronger compared to the previous method.

Calcium carbonate brick (on the right) compared to a regular brick (on the left) | image © I. Maruyama, N.K. Bui, A. Meawad et al.

Plans to construct a real two-story house by 2030

The latest study is part of the researchers’ C4S project, which stands for Calcium Carbonate Circulation System for Construction. It is led by Professor Takafumi Noguchi, as project manager, with Professor Ippei Maruyama leading material development. Their aim is to develop a technique where they can create sturdy recycled concrete blocks called ‘calcium carbonate concrete’ mixed with carbon dioxide from air or industrial exhaust.

‘As part of the C4S project, we intend to construct a real two-story house by 2030,’ says Professor Ippei Maruyama. ‘Over the next few years, we also plan to move to a pilot plant, where we can improve production efficiency and industrial application, and work on creating much larger building elements, as we move towards making this material commercially available.’

image by Aniston Grace, via Unsplash