Research brings circularity to concrete blocks

The process involved grinding the old concrete into powder, reacting it with CO2 from the air, pressurising it in layers in a mould and heating it to form the new block.

This technique could offer a way to recycle old materials while also trapping carbon dioxide. The blocks could theoretically be remade repeatedly through the same process.

In 2021 researchers developed a new kind of concrete, which had the potential to reduce greenhouse gases and reuse waste from the construction industry.

The C4S project (Calcium Carbonate Circulation System for Construction) was led by Professor Takafumi Noguchi and included Professor Ippei Maruyama, who led on material development. They developed a method to combine old concrete with carbon dioxide, taken from the air or industrial exhaust, to create calcium carbonate concrete pieces that were only a few centimetres long.

They have now taken this technology to another level.

“We can make calcium carbonate concrete bricks large and strong enough to build regular houses and pavements,” Maruyama said in a statement. “These blocks can theoretically be used semi-permanently through repeated crushing and remaking, a process which requires relatively low energy consumption. Now, concrete in old buildings can be thought of as a kind of urban mine for creating new buildings.”

Limestone is a key ingredient in Portland cement, used to make concrete. The rock provides durability and strength while improving workability. However, limestone reserves are limited, and in some countries more than others, such as Japan.

“We are trying to develop systems that can contribute to a circular economy and carbon neutrality” said Maruyama. “In Japan, the current demand for construction material is less than in the past, so it is a good time to develop a new type of construction business, while also improving our understanding of this vital material through our research.”

To make their new building blocks, demolished concrete from a school building was crushed into a fine powder, sieved and carbonated for three months.

Carbonation is usually a slow, natural process which occurs when compounds in concrete, such as portlandite and calcium silicate hydrate, react with CO2 in the air to form calcium carbonate.

The researchers performed a sped-up version of this process to recreate the same kind of concrete found in older buildings. This was to test that they could still make strong new blocks, even from older concrete.

The carbonated powder was then pressurised with a calcium bicarbonate solution and dried. In their previous experiment, the team created calcium carbonate concrete by pouring a bicarbonate solution through carbonated concrete powder and heating it.

In this new version, as well as heating the material, the team built the concrete up in layers in a mould, which compacted it under pressure. They found this enhanced the strength of the blocks.

“As part of the C4S project, we intend to construct a real two-story house by 2030,” said 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.”

The team’s work is detailed in the Journal of Advanced Concrete Technology.