Experts: Ueli Angst (ETH Zürich), Barbara Lothenbach (Empa)
The cement and concrete industry is responsible for around 8 percent of global greenhouse gas emissions. The need and potential for further decarbonisation in the construction sector are correspondingly great.
Picture: Uve Sanchez, Unsplash
By weight, concrete is the most commonly used artificially produced material. Hardly any new buildings can make do without concrete – and thus without cement. Greenhouse gas emissions occur at various points in the process of manufacturing concrete, a material that contains a significant proportion of cement. Around half of the CO2 generated is a result of the chemical reaction that takes place when the raw materials limestone and clay are converted into Portland cement. The rest comes from the energy needed to heat the blast furnaces to over a thousand degrees, from the mills’ electricity consumption and from transport. The cement and concrete industry thus causes around 8 percent of global CO2 emissions, about three times as much as international air traffic.
The Swiss cement and concrete industry has set itself the goal of becoming climate-neutral by 2050. There are various strategies for achieving this goal. Perhaps the most obvious is to modify the concrete with admixtures, so that it contains less cement; this would mean fewer CO2 emissions for the same amount of concrete. Countries with major steel industry or coal mining industries have been adding other materials such as slag or fly ash – industrial waste products from these two industries – to concrete for many years. Today, researchers are trying to develop materials that have similar properties to cement but are not made with calcinated limestone. Another approach to reducing CO2 emissions is to sequester the CO2 generated during cement production. The sequestered CO2 is then used for other industrial processes or (as another research topic) reincorporated into concrete.
Another possible way to reduce CO2 emissions is to develop cements or cement-like construction materials that have a different chemical composition and do not release CO2 during the burning process. One promising candidate in this regard, which is the subject of research at Empa, is magnesium silicate. This is used to produce a magnesium oxide, which is then enriched with water and CO2 during cement manufacturing. As a result, the construction material even absorbs and stores CO2. One general challenge with such cement-like construction materials is that they do not have the same alkalinity as cement, so they are less effective at protecting steel against corrosion.
Emissions from the calcination process will remain unavoidable for as long as cement continues to be used. To prevent these emissions from directly entering the atmosphere and having a damaging impact on the climate, the released CO2 can be sequestered from the exhaust gases for storage or use in other industrial processes.
Usually, concrete material from demolition is placed in landfills and not reused. However, there have now been successful attempts to mill demolished concrete and reuse it as a substitute for gravel in new concrete. Other experiments are adding CO2 to cement after milling so the concrete becomes a limestone-like resource again and can then be mixed with other materials. The aim of this is to store the CO2 generated during cement manufacturing to a certain extent. Cement processed in this way can be used as filler in the synthetic resin industry, for example.
Research into reducing greenhouse gas emissions from cement and concrete is also addressing the question of how cement and concrete in the built environment could be turned into a CO2 reservoir. Pilot tests have shown such CO2 absorption by concrete and cement to work. However, there is still a need for better means of controlling the framework parameters, such as moisture, so that steel built into the concrete does not corrode.
Switzerland is a leader in the use of recycled concrete. Nevertheless, as the construction industry tends to be conservative, it is hesitant in adopting innovations such as concrete that stores CO2. Until the relevant standards mandate use, the industry will do little to help such new materials or procedures achieve a breakthrough.
If the relevant standards and laws are amended and the storage of certain quantities of CO2 becomes a requirement, Switzerland could establish a pioneering role for itself. As soon as such concrete can also be shown to be economically worthwhile, for instance by increasing CO2 incentive levies, such procedures and products will automatically gain acceptance; but until then, there is still a long way to go in research, development and certification.
Switzerland’s cement and concrete industry is very strong both domestically and internationally. The large number of universities and research institutions investigating the issue also provide a strong framework for further decarbonisation of the construction sector. Efficient funding and stimulation could occur, for instance, via the industry association or via corresponding standards that, on one hand, ensure certainty and, on the other hand, regulate the use of cement and concrete, and possibly even specify targets. This would provide legal certainty.