Cement primarily consists of limestone and is the main ingredient used to make concrete. Global cement production is estimated to be the cause of 5 per cent of the total anthropogenic carbon emission, and prognoses predict that the global need for cement and concrete will be twice as large in 2050 as it was in 2010. This has led to an increased interest in finding ways to limit carbon emissions from cement and concrete production. One company that has noticed this interest is Aalborg Portland, Denmark’s only cement manufacturer, where Jesper Sand Damtoft is Development Director:
“At Aalborg Portland, we’re seeing a growing customer interest in the environmental impact of the materials, which is evident from the fact that more and more people want to see an environmental product declaration on the products. It’s a trend we see with workmen and contractors as well as architects and clients,” says Jesper Sand Damtoft in a press release issued by the company.
Jesper Sand Damtoft was Chairman of the Steering Committee behind the innovation consortium Grøn Beton II (Green Concrete II), which managed to invent a new and greener recipe for cement in just five years and with a budget of DKK 29 million. The new cement, called FutureCem, can reduce global carbon emissions from cement production by up to 30 per cent.
Energy-intensive production
In standard cement production, carbon emissions are caused by two things: Firstly, the limestone needs to be heated to very high temperatures in large cement ovens, and high temperatures result in an energy-intensive production, which is costly in the CO2 accounts. Secondly, limestone produces CO2 in the degradation process that occurs when the material is heated.
Consequently, work is being done all over the world to find solutions that make it possible to produce cement at lower temperatures and with other raw materials, says Henrik Stang, Vice Director of DTU Civil Engineering, who was a partner in the Grøn Beton II project.
“By changing the cement recipe a little and replacing some of the limestone with calcined (i.e. burned) clay, it is possible to lower the energy consumption of cement production. While the limestone needs to be heated to 1,400°C, the clay only needs to be heated to 800°C. And the clay hardly releases any CO2 when heated. Together, these factors reduce the carbon emissions of cement production,” says Henrik Stang.
Phasing out fly ash
The new recipe also solves another problem with concrete production. According to the Danish Technological Institute, which was Project Manager on Grøn Beton II, Denmark and other countries have so far achieved a lower carbon footprint in concrete production by replacing some of the cement with fly ash.
But fly ash is a waste product from coal-fired power plants, and as coal is being phased out in the move towards a fossil-free society, there is a shortage of fly ash, and manufacturers are starting to turn their attention towards the fact that, at some point, there will be nothing left. The newly developed cement type solves this problem as well:
“With the new cement type, we have a solution that can eventually make fly ash redundant in concrete production,” says Henrik Stang.
Testing green concrete
"Med den nye cementtype har vi også en løsning, der på sigt kan gøre flyveasken overflødig i betonproduktionen."
HENRIK STANG, VICEDIREKTØR, DTU BYG
The green concrete has already been tested in four demonstration projects, one of which is based at DTU. In March 2019, the cement was used in a DTU building for the first time for walls and flooring. In the other three projects, the green concrete has been used for bridges built by the Danish Road Directorate and Banedanmark.
“The demonstrations have been extremely important to the project and the faith of our stakeholders, and the results are very promising. It’s also no coincidence that we’re testing the new cement and concrete in bridge construction. These are very demanding constructions that help emphasize the strength and durability of the green building materials,” says Jesper Sand Damtoft.
Researchers calculating durability
In fact, the durability of the materials is what has been the focus of the researchers at DTU Civil Engineering in the Grøn Beton II project. It is inherently difficult to gain knowledge of the long-term durability of the green concrete in only four to five years. Consequently, DTU Civil Engineering has developed a new calculation model that can predict the durability of the new concrete—and of other concrete types in the future. Among other things, the model considers the roles of water, gases, and vapours in the concrete degradation processes.
“Our calculation model takes into account the physical and chemical processes that occur when concrete reacts to the environment. Thus, the model gives an improved understanding of the concrete degradation processes and can quickly provide an indication of the durability of new, more eco-friendly concrete materials,” says Mouadh Addassi, who is a Postdoc at DTU Civil Engineering and has written about the new calculation model in his PhD dissertation.
The new model can provide a better understanding of the long-term consequences of replacing the traditional cement with other materials. The calculation model has been tested in laboratory experiments with ten different cement mixtures from Aalborg Portland.
“The results of the experiments confirm that the model can contribute to the durability assessment of concrete with new and more eco-friendly cement, thus reducing the costs and time of screening new concrete mixtures,” says Mouadh Addassi.
Concrete part of teaching
Researchers at DTU Civil Engineering continue to develop and test the model’s ability to predict the long-term durability of different concrete mixtures. At the DTU Civil Engineering laboratories, accelerated tests on the degradation of concrete are carried out to validate and improve the conclusions of the mathematical model. In addition, the researchers further develop and test the model’s ability to predict the durability of different concrete mixtures in harsh environments with high temperatures and pressure conditions.
The building containing the green concrete at DTU Lyngby Campus is already part of the teaching of the university students. Through an app, students can monitor the hardening of both the green concrete and the other concrete types in the building, as temperature-sensitive sensors are embedded in the concrete. This makes it possible to extract data that can be used directly in the classroom, and when the building is put into use in 2020, new students will be able to extract data from the process for many years to come.