Concrete production is responsible for 6–9% of global anthropogenic CO2 emissions. “The largest contribution to that comes from the production of cement, a precursor product,” says Peter Stemmermann from the Institute for Technical Chemistry (ITC) at the Karlsruhe Institute of Technology (KIT; www.kit.edu). “An important component of typical Portland cement is clinker, which is produced in an energy-intensive process by heating limestone.” Stemmermann’s research group has developed a clinker that is made instead from recycled concrete and only partly from limestone.
To produce belite cement clinker, the pilot plant (photo) uses an all-electric heating system powered by renewable energy and a carbon dioxide atmosphere, reducing the energy required for the process. “We can manage with a process temperature of 1,000°C instead of 1,400°C in the rotary kiln,” Stemmermann says. Compared with conventional clinker production, the overall energy consumption is 40% lower. “The CO2 unavoidably emitted as a result of the limestone reaction in the kiln is captured and then bound to the recycled concrete in the second step of the process,” Stemmermann explains. This second step is to be integrated in a future expansion of the pilot plant, which is currently capable of producing 100 kg/d of clinker.
Source: H. Reis, KIT
Circular concrete from KIT’s plant is climate-neutral thanks to the direct link between the production of clinker and the production of carbonated aggregate (the coarse fill material in concrete). The CO2 from clinker production is used directly in the carbonation hardening of coarse-grained concrete waste, where it is permanently bound. The heat generated during clinker production is also used in this step, which takes place in an autoclave, making it particularly energy-efficient. “The result is a high-quality aggregate,” Stemmermann says. “We were able to prove beyond doubt that carbonation improves the microstructure of the aggregate by reducing its porosity.” The resulting aggregate is then processed with belite cement to produce circular concrete with a balanced overall carbon footprint.