Two researchers at Tokyo University develop a new type of concrete

A new type of concrete, called calcium carbonate concrete, made of concrete wastes and the carbon dioxide emitted from the air or industrial exhaust gases, has recently been developed by two researchers at Tokyo University.

The new material is a step toward reducing construction industry emissions. In Japan, the construction industry accounts for nearly 20% of CO2 emissions and consumes a vast number of natural resources for cement production.

The new material requires less time and money to produce than cement. It also reduces the need for limestone that is used to manufacture cement.

The calcium carbonate concrete is made from a mixture of calcium hydroxide (Ca (OH)2), which is extracted from concrete wastes, with CO2 in an autoclave — a vessel that can withstand extremely high pressures — to form calcium carbonate (CaCO3). The process takes only days to complete, compared with the months needed to produce cement.

The new material is lighter than normal concrete and has better insulating properties. It also reduces the need for limestone that is used to manufacture cement.

Global warming is one of the most serious problems today. There are many sources that lead to global warming, such as the use of cars, burning fuels, and so on. In fact, estimates refer to that about 7% of the carbon dioxide emissions come from the manufacture and use of cement, the main component of concrete. A sizeable part of the 7% is due to the necessary use of calcium which is usually extracted by burning limestone.

Concrete is used in many of today's global construction processes, especially of tall buildings, where durable various materials are used to give it the necessary strength. In all construction processes, there is a need for strong and durable materials. The most widely used material is concrete. Concrete consists mainly of cement and aggregates (sand and gravel) which are produced by nature or recycled materials. Cement consists of calcium which must be produced by burning limestone and clay together at 1400 degrees Celsius. This process produces substantial amounts of carbon dioxide emissions (CO2).

Professor Ibe Maruyama and Professor Takafumi Noguchi, both from the Architecture Department of Tokyo University, developed the "Recycling calcium carbonate for construction" system (C4S) to reduce these emissions by combining recycled concrete with carbon dioxide (CO2) in a way that could be used in construction.

The researchers discovered a method to extract concrete wastes and the carbon dioxide trapped within it and combined it in a new process as a usable form, called "calcium carbonate concrete."

In the C4S process, crushed concrete waste is mixed with freshly processed calcium oxide. The calcium oxide turns calcium hydroxide into calcium carbonate when combined with water – or CO2 – creating a binding agent as strong as conventional cement-based concrete.

The C4S system is inspired by the way some aquatic organisms are solidified in fossils over time. As skeletal remains of creatures like mussels or corals become buried, they’re subjected to high pressure and heat — conditions that cause calcium carbonate to form around them, solidifying them into fossils.

Professor Maruyama thought about the possibility of applying this method by forming solid calcium carbonates deposited on concrete from dead organic materials. Given that calcium is necessary for the reaction between cement and water to form concrete, he took the opportunity to develop a method to achieve less intensive carbon emissions while doing the same job.

Now, researchers at Tokyo University have developed this type of concrete that offers not simply better environmental impact but also more tensile strength than its traditional counterparts. The secret? It's made from calcium extracted from recycled concrete waste.

The new cement is made by extracting calcium from recycled concrete waste and combining it with carbon dioxide — which can be extracted from exhaust fumes or the air — at low temperatures. And so far, its results have been promising. The resultant material produced over twice as much compression strength as conventional concretes, and it's highly resistant to water, according to senior researcher Masashi Maruyama.

"The objective is to extract calcium from concrete wastes which would otherwise be lost" Maruyama commented. And he explained that "We combine that with the carbon dioxide extracted from industrial exhaust or even from the air, we do it under very lower temperatures than that used to extract calcium from limestone nowadays."

In their research, the scientists studied different parameters of calcium carbonate concrete, such as compressive strength and durability. Their experiments showed that calcium carbonate concrete has lower compressive strength than standard concrete, but it is completely comparable with already known bio concrete.

The developers note that this material has high environmental friendliness since its components are natural. The main advantage is the possibility of recycling enormous amounts of materials and waste. In addition, calcium carbonate concrete has the same level of durability as normal concrete. This makes it more environmentally friendly, but at the same time allows it to be used in various fields of construction.

"Achieving a progress in this area is exciting, but we are still having many challenges to overcome" Professor Noguchi confirmed referring to that it would be better to increase the strength and volume of calcium carbonate concrete if the energy used in the production process is reduced.

In summary, the researchers at Tokyo University have developed a new type of concrete to mitigate the effects of climate change. It remains to be seen if their formula will become a widespread practice in concrete manufacturing, but it does show that our old understanding of cement is not as complete as we once thought it was.

Noguchi concluded by hoping that calcium carbonate concrete could become the dominant type of concrete, in addition to that it is one of the climate change solutions.