Double benefit: Old concrete from demolished buildings can help bind the greenhouse gas carbon dioxide and at the same time become new bricks. This is made possible by a method in which old concrete is crushed, soaked in carbon dioxide and dried in layers with CO2. The material absorbs the CO2 and forms a strong, resilient stone. This can be installed and recycled multiple times, as Japanese researchers report.
Cement and concrete are among humanity’s most important building materials. But producing cement from limestone releases huge amounts of CO2 – both through the chemical conversion of calcium carbonate (CaCo3) into calcium oxide (CaO) as well as the kilns needed to burn the lime. That is why scientists have been looking for some time for methods to save cement, for example by adding it wasteplastic waste or Volcanic ash.
Accelerated aging of concrete
But there is another way to make cement and concrete more climate-friendly: through upcycling. A team led by Ippei Maruyama from the University of Tokyo has developed a method to create new, stable building blocks from old concrete waste. This saves cement and thus also the greenhouse gases released when it is used. “Concrete from urban buildings is an urban mine of calcium oxide resources,” the team explains.
But the main thing is that this recycling of concrete retains CO2 from the air or from industrial exhaust gases – and thus acts as a greenhouse gas sink. This is possible because the researchers use a natural process for their recovery: the chemical aging of concrete. If it is exposed to humid air and CO2 for a long time, carbonation occurs – calcium hydroxide (Ca(OH)2) slaked lime from concrete reacts with CO2 from the air to form calcium carbonate.
This natural aging of mortars and concretes can at least almost half of CO2 emitted during cement production – although this typically takes several decades. Maruyama and his team have now accelerated this process and optimized it to create a resilient material.
This is how upcycling works
For concrete recycling, concrete waste – in the test, it was building material from a demolished school building – is first finely ground. Then this mass is repeatedly moistened and exposed to air for at least a month. The large surface area of the powder promotes carbonation. “After just seven days, the calcium hydroxide and amorphous phases of the material are reduced and the proportion of calcium carbonate increases to 35 percent,” report Maruyama and his colleagues.
Then comes the next step: The carbonate concrete powder is poured into molds in layers, each dipped in a hydrogen carbonate solution and then dried at about 80 to 100 degrees under high pressure. The result is a solid block that can withstand compressive pressure of up to 20 megapascals, the researchers determined. “We can make these calcium carbonate concrete blocks large enough and strong enough to build normal houses or sidewalks,” Maruyama says.
Double advantage
According to the researchers, their method opens up the possibility of recycling concrete waste and thus saving raw materials, energy and greenhouse gases. At the same time, this accelerated carbonation removes CO2 from the air or industrial exhaust gases, which also benefits climate protection. Another advantage: the recycled concrete can then be crushed again and processed into new blocks, as the team explains.
“We are trying to develop systems that contribute to a circular economy and carbon neutrality,” Maruyama says. “In the next few years, we would like to use and test the method in a pilot plant to optimize the efficiency and industrial applicability of production.” The team is also already working on producing larger components from its recycled concrete. The first two-story building made from the material is expected to be built by 2030. (Journal of Advanced Concrete Technology, 2024; is that i: 10.3151/jact.22.406)
Source: University of Tokyo
August 13, 2024 – Nadja Podbregar