Concrete as CO2 trap – right at the plant

After their production, which emits a lot of carbon dioxide, cement-based building materials such as concrete absorb the climate gas again – a process that takes decades and can hardly be controlled. What if it could be accelerated to protect the climate? Empa researchers are collaborating with numerous partners in an elaborate project to find a solution that already takes place in the concrete production plant. Initial results are encouraging.

Even the biggest sinners can improve: This is the hope that experts have for "concrete as a climate polluter". 6 to 8 percent of man-made CO2-emissions worldwide are due to cement, the indispensable binder for the hard and versatile building material - but at the same time it is able to chemically bind the climate gas emitted during cement production again after manufacture; at least partially: between 11 and 30 percent, depending on the recipe and conditions.

"Carbonation" is the name given to this process, in which limestone is formed from calcium hydroxide in concrete with CO2 - a leisurely reaction that takes years and whose pace depends on numerous factors. For a long time, experts have been thinking about using it to improve the carbon footprint of concrete - and now, in the "DemoUpCARMA" project, Empa experts are working with partners under the leadership of ETH Zurich to test whether and how the process can be used and, above all, accelerated in a real concrete plant.

Elaborate analyses in the Empa laboratories

Specifically: in a specially installed plant of the company Kästli Bau AG in Rubigen in the canton of Bern - and with recycled material from deconstructed concrete structures. The carbon dioxide used to "feed" this material comes from the nearby wastewater treatment plant and is delivered in a liquefied state. At the plant, it is then stored in a special silo from where the recycling granulate is continuously "flooded" using a process developed by the Bern-based company neustark AG - in other words, exposing to pure CO2 gas and precisely measuring the uptake. The team of experts researched the processes that take place in detail during the CO2 uptake of the recycled granulate. In addition, research was carried out into how so-called "recycled water" from water, cement and sand, which is produced for example during the cleaning of concrete trucks and mixing plants, can be used to bind carbon dioxide.

Andreas Leemann and Frank Winnefeld from Empa's "Concrete & Asphalt" department have conducted numerous tests to find out how well and efficiently the recycled granulate absorbs CO2 under which conditions - with surprising results. Samples of the treated material showed distinct changes under the microscope: Smaller particles had patches of dark and light portions on the surface, where the original cement stone had changed.

Analysis with a scanning electron microscope showed that the light portions are calcium carbonate, while the dark phases consist mainly of calcium silicate hydrate - C-S-H for short - the main product of cement hydration that gives concrete its strength. This C-S-H has been deprived of part of its calcium by carbonation: it is thus lower in calcium and can in turn react with newly formed cement compounds in the recycled concrete - with the result that its compressive strength increases.

Practical tests with various types of concrete commonly used in Switzerland confirmed these insights. The recycled products with carbonated concrete granules achieved higher strengths than comparative concretes with untreated recycled material. "So a reactive phase that is newly formed in the granules and produces higher strength in the recycled concrete," Leemann says, "that did surprise us."

Furthermore, the analyses showed that the moisture factor plays an important role in CO2 accumulation: Drier recycling mixtures showed a significantly faster uptake of the greenhouse gas than material that is too moist. And because the outdoor storage of recycled pellets in Switzerland does not exactly promote drying, Empa experts say the question arises as to whether this process should be technically optimized at all.

Significant improvement within reach

The positive results show that the process can make concretes more climate-friendly in two ways. Firstly, by absorbing CO2 in order to "relieve" the atmosphere: in the case of the novel building materials, it reaches up to 10 percent of the emissions that were released into the atmosphere during the production of the cement for the original concrete. Secondly, with the opportunity to reduce the cement content in recycled concretes - by 5 to 7 percent - thanks to the higher strength. According to the Empa experts, the bottom line is that the potential for CO2 savings is a good 15 percent.

And: CO2 treatment of the "recycled water" showed further potential. Samples were able to bind a considerable amount of the gas in analyses: about 120 grams per kilogram of dried material. The use of this material also led to an increase, albeit small, in the strength of concretes to which it was added.

The extent to which these results can be implemented in practice is, of course, still an open question. For example, because of the question of how well and with what technical and financial effort the process can be implemented in concrete plants. And how the carbonation of recycled granules affects different concretes in the long term, i.e. over the entire "service life" of several decades.

Great potential over the entire service life

An initial assessment of this is provided by so-called "life cycle analyses" by Romain Sacchi and Christian Bauer of the "Technology Assessment Group" at the Paul Scherrer Institute (PSI) in Villigen, together with Empa expert Andreas Leemann. Taking into account all the influences on CO22-emissions from the production, use and disposal of "classic" concretes as well as variants with untreated and CO22-enriched recycled granules, it turns out that the carbonated material can reduce the net greenhouse gas effect by around 13 percent - compared to concrete with conventional cement and no recycled material. For concrete with recycled material, the effect is still 9 percent; a considerable potential.

Ambitious pilot project with many stakeholders

DemoUpCARMA stands for "Demonstration and Upscaling of CARbon dioxide MAnagement solutions for a net-zero Switzerland". Under the leadership of ETH Zurich, experts from Empa, Eawag, PSI and the companies Kästli Bau AG and neustark AG are working with 18 other partners from research and industry to identify and investigate pathways for generating negative emissions. In addition to the storage of CO2 in new and recycled concrete in Switzerland, this also includes a study on the transport and permanent storage of CO2 in a geological reservoir in Iceland in the spin-off project "DemoUpStorage". Both projects are funded and supported by the Swiss Federal Office of Energy (SFOE) and the Swiss Federal Office for the Environment (FOEN).

View full article