After water, concrete is the most widely used resource in the world. There is almost no construction site today that can do without concrete. It is used, for example, in high-rise buildings, tunnels, bridges and dams. Even the smallest construction projects cannot do without concrete in the foundation components. In terms of climate, concrete is devastating because it is used so extensively worldwide. Otherwise, the CO2 footprint of concrete is on a par with that of steel or brick.

The huge quantities of concrete produced worldwide emit around 2.5 to 3 billion tons of CO2 annually. More precisely, the production of cement, which, in addition to water and gravel, is the essential element of the building material concrete as a binder. Cement is obtained from limestone, which is first crushed and then burned in a rotary kiln at 1450 °C to form clinker. After cooling, it is mixed with gypsum and ground to make cement. 40% of CO2 emissions are caused by heating the rotary kiln, although alternative fuels are already being used and emissions can no longer be significantly reduced. 60% of emissions result from the firing process, during which the CO2 bound in the limestone is released. This proportion cannot be further optimized either.

For years, granulated blast furnace slag from steel production and fly ash from coal combustion have been used as substitutes for limestone. Granulated blast furnace slag could replace up to 70% of the limestone in cement production, but is only available for up to 8% of global cement production. Quite simply, there is not enough of it to meet the high demand. Fly ash is a by-product of coal combustion in power plants and industrial boilers. This type of energy generation is known to be phased out in pursuit of the goal of climate neutrality by 2050.

The solution is to use raw clay, which is available worldwide in large quantities and particularly in the emerging markets and future markets in Africa. It is the most widely available raw material in the world. The cement production process from raw clay is exactly the same as for limestone. However, raw clay only needs to be heated to 800°C in the rotary kiln, compared to 1450°C for limestone. The result is calcined clay clinker, which is then ground into calcined clay.

Green cement consists of 50 percent clinker, 30 percent calcined clay, 15 percent untreated limestone and 5 percent gypsum. Compared to the production of cement purely from limestone, green cement can reduce CO2 emissions by 30 to 40 percent because the rotary kiln only needs to be heated to 800 °C and no CO2 is released when raw clay is burned.

The researchers at Empa in Düdendorf are a few steps further in the laboratory. They are now producing climate-neutral concrete. They add carbon dioxide in solid form to the green concrete in the form of granules. The proportion of carbon dioxide in the granules corresponds to the proportion of carbon dioxide emitted during the entire concrete production process. But that's not all. Empa's ambitious goal is to produce concrete with a negative carbon footprint. To do this, carbon dioxide is filtered out of the air and then bound in methane, which is then split into hydrogen and solid carbon particles by pyrolysis. The carbon powder can then be added to the concrete. The disadvantage is the high energy demand of the process. To extract one cubic meter of carbon dioxide from the air, approximately 2 million cubic meters of air must be filtered.

For the mass-produced product concrete, the question of the scalability of the new achievements remains. It will probably take more than just the widespread production of green concrete to achieve the serial production of concrete with a negative CO2 balance. According to initial estimates, the use of raw clay in cement production is already possible in over 50% of the world's plants. This step would save around 500 million tons of CO2 per year.