Concrete is the most widely used product in the world. Though cement is indispensable it is a reality that cement and concrete have become a talking substance in ongoing climate debate. Mixed with water, sand and coarse aggregate, it results in concrete, on which our modern world is built. However, less costly material is in the limelight primarily because of another property. The production of one ton of cement causes around 700 kg of carbon dioxide (CO2) that is emitted in the atmosphere. This is less when compared to other industries like, say, steel or aluminium. Every year, we produce around twelve cubic kilometres of concrete worldwide.

The share of global CO2 emissions caused by the cement industry is currently around seven percent. However, this is likely to increase in future, as demand is growing in Asia and increasingly also in Africa, while production in Europe is more or less stable. So, it is high time to look for a cement that offers people housing and infrastructure, but still takes environmental aspects into account and can be produced in line with our climate targets. The United Nations Environment Program (UNEP) also calls for the immediate development and use of new cement-based materials that are more climate-friendly and at the same time cost-effective.

Cement production is highly energy intensive and is traditionally burnt in a rotary kiln at around 1450 degrees Celsius. We need to find out a cement which needs less firing temperature and serves the purpose eco friendliness. In this line, most promising cement would be calcium sulpho aluminate cement (CSA). It requires a firing temperature that is 200 degrees lower and emits around 200 kg less CO2 per ton of cement. But the reduction in greenhouse gas emissions is not only due to the lower firing temperature. A large proportion of the climate advantage of CSA cement is due to the lower amount of limestone in the raw material mix.

Demand

Limestone is responsible for CO2 emissions through a chemical reaction during cement production. Thus, reducing the proportion of limestone is an interesting proposition in developing eco-cement. In addition to CSA cement, researchers look at substitute constituents that accumulate as waste materials in other industries.

These include slag from blast furnaces used in the production of pig iron and fly ash left over from coal combustion. Both products can be mixed with cement to help reduce CO2 emissions.

The type of additives in cement could even be changed in such a way that the burning process could be completely eliminated. In so-called alkali-activated cement, the components such as slag, ash or calcined clay are animated to the desired chemical reaction by strong alkaline solutions such as sodium silicates. The products of this reaction then combine to form a material whose compressive strength corresponds to that of burnt, conventional cement.

GreenGas caught up in Concrete

The ability to bind CO2 in concrete instead of releasing is also an ingenious feature. A CO2-negative concrete would be a true climate friend. Researchers are working on a magnesium-based cement that will provide the basis for this eco-concrete. These cements are still unexplored.

To ensure that such approaches do not end up as niche products, but can be produced industrially and costeffectively, meticulous analyses must show that eco-cement meets the same requirements as conventional products. Many alternative types of cement currently lack the simple recipes for adding new constituents or modifying manufacturing processes without compromising the coveted properties of traditional cement. For as long as the at least equivalent performance of eco-cement cannot be demonstrated beyond doubt, the classic Portland cement, a low-cost and well-characterized building material, will remain the material of choice for civil engineers.

Cement researchers are currently analysing chemical mixing ratios and conformity criteria such as the strength and durability of new types of cement, paving the way for approvals that comply with standards. These include investigations on a small and gigantic scale. In addition to chemical investigations, microscopic analyses and thermodynamic modelling, with which the reactions inside cement are investigated, the load-bearing capacity of large components made of different types of cement is also compared. Industrial processes will have to be optimized, as they are too expensive in many cases. Many researches show that, alternative types of cement can be used to produce concrete with a comparable or even better durability.

Conclusion

It is sure that, one development is already emerging. The variety of cement and concrete products will increase in the future. For cement producers, this diversity leads to increased requirements.

Concrete production is responsible for around 6% of man-made CO2 emissions globally. For example, 300 kg of cement, 180 l of water and 1890 kg of aggregate produce a cubic metre of concrete. The CO2 emission of the concrete comes largely from the cement content: cement must be burnt at 1450 degrees, whereby mineral-bound CO2 dissolves from the limestone.