Concrete for the future

Mineral-based construction materials are coming under criticism, with concrete – the building material of the century – of particular concern in terms of climate impact. Science and industry are working hard to decarbonise this key construction material. By Elke Hildebrandt

We will have to fundamentally rethink concrete as a construction material. The cement it contains is a real “climate killer”. This powdery material that binds together sand, gravel and water to form concrete is responsible for 6 to 8 percent of worldwide greenhouse gas emissions. It’s a dilemma for the construction industry because concrete is not only readily available, it is also affordable, locally obtainable and can be used in countless different ways. 

According to the Global Cement and Concrete Association (GCCA), concrete is the most widely used building material in the world. In fact, it is the second most used substance in the world after water. If global greenhouse gas emissions have to be cut to net zero to achieve the 1.5 degree target by 2050, this much-loved building material will need to be pretty much reinvented.

Review of the entire value chain required

“Over the next few years, we must urgently focus on greenhouse gas emissions from the construction industry,” says the German Sustainable Building Council (DGNB). According to the DGNB, a good third of all greenhouse gas emissions from a building are created before it actually enters use, i.e. during manufacture and construction. To reduce built-in emissions, known as grey energy, a reassessment of construction methods, high-mass components and of the useful lifetime of construction materials is required. All parties along the entire value chain need to be involved, starting with planners and architects. But how can net zero construction be achieved with concrete?

This “bone concrete” is characterised by its varying density, thus reducing the amount of material required.
Universität Stuttgart

Decarbonisation of concrete to reduce its carbon footprint

“If we’re going to rethink concrete, we need to start with cement,” says Nicolas Schnabel, press spokesman for construction material manufacturer Holcim Deutschland. Central to this endeavour are CO2-efficient recipes and sustainable production. Schnabel points out that clinker, an intermediate product in cement manufacturing, is especially problematic. Burning clinker unavoidably releases CO2 as part of the chemical process. This accounts for around two thirds of emissions, with a further third generated by the fuel used to heat the clinker to temperatures of around 1,450 °C. Every tonne of cement produced in Germany generates an estimated 600 kilograms of CO2, while the figure for a cubic metre of concrete is 200 kilograms.

Energy efficiency measures, the use of more climate-friendly fuels and new types of cement have enabled the German cement industry to reduce emissions per tonne of cement by around 22 percent since 1990. In particular, innovative cement mixtures with a lower clinker content have a significantly smaller carbon footprint. Concrete with the Concrete Sustainability Council logo has been recognised in the DGNB certification system as beneficial since 2018, which highlights its contribution to sustainable buildings.

Pilot schemes are trialling the production of net zero concrete

In the CO2 roadmap drawn up by the German cement industry, capturing emissions is another important factor in delivering decarbonisation of concrete. The plan is that in future, the CO2 captured in cement works will be used to manufacture new products or put into storage to protect the atmosphere. The first pilot carbon capture plants are already being commissioned by Heidelberg Cement and Holcim Deutschland. 

Both companies are signatories to the international GCCA Roadmap for Net Zero Concrete and have committed to producing net zero concrete by 2050. “Carbon capture is a key cornerstone if we are to achieve national and international climate targets,” says Dominik von Achten, Chairman of the Managing Board of Heidelberg Cement. 

Looking at the establishment of a circular economy, concrete itself could actually be part of the solution. The carbonisation principle is already being applied in a pilot scheme by Neustark, a spin-off from ETH Zurich University. This involves the use of recycled concrete granules to make new concrete. The additional concentration and capture of environmentally damaging carbon dioxide in recycled concrete granules essentially reverses the chemical process involved in the cement manufacturing process. “In the medium term, we want to develop our technology to the point where the captured CO2 added to the concrete fully offsets the emissions generated during production of the cement. The result would be net zero concrete,” explains Valentin Gutknecht, CEO of Neustark.

Topping out took place in February 2022: The Cube in Dresden is the world’s first carbon concrete building. Non-metallic reinforcement means that carbon concrete avoids the normal negative impact of concrete. It allows for slimmer and freer designs with much-reduced CO2 emissions.
Iurii Vakaliuk/ TU Dresden/ IMB

Less is more: new “concrete light” construction methods

Alternative types of construction – such as skeleton frame structures – mean that buildings can be slimmed down and thus have a reduced carbon footprint. Pre-stressed flat slabs and false ceilings, for instance, can be produced with up to 50 percent less concrete and as much as 75 percent less steel. Similarly, infra-lightweight concrete scores highly when it comes to climate protection. It is used for monolithic components for residential buildings, among other applications, with its porous aggregate making it significantly lighter, with correspondingly lower CO2 emissions. The air bubbles trapped inside the material also offer an additional benefit in terms of thermal insulation. 

Builders and planners would be well advised to keep an eye on various forward-looking pilot projects that are deploying climate-friendly concrete innovations. The Cube in Dresden is one such prototype, the world’s first carbon concrete building. This significantly “leaner” type of construction is made possible by non-corrodible carbon reinforcement that replaces conventional steel reinforcement. Thorsten Hahn, CEO of Holcim Deutschland, explains: “Carbon concrete can reduce the carbon footprint of a structural element by up to 75 percent.” To take the concept further, carbon fibres could be manufactured from natural waste products in a net zero production process. 

A new building method featuring functionally graded concrete takes a rather different approach. Werner Sobek, professor at the Institute for Lightweight Structures and Conceptual Design (ILEK) at the University of Stuttgart, is researching the technology, which involves concrete that varies in density, rather like the structure of a bone. The carefully engineered porosity means that considerable savings can be made in both materials and weight within components, which in turn has a positive impact on energy and emissions. This “bone concrete” is set to be used for the first time in a building in Hamburg’s HafenCity development.

Bacteria and fungi can replace finely ground cement

Surprisingly, living organisms are also worth considering for making net zero construction materials and boosting resource efficiency. Research into organic concrete has shown that “bacteria set a process in motion which finishes with the precipitation of calcium carbonate crystals that form a bond with the aggregate,” explains Christoph Nething, who works at ILEK at the University of Stuttgart. He adds, “The formation of these crystals binds CO2, rather than releasing it.”

Fungal organisms likewise offer exciting potential when it comes to developing new building materials. When a mycelium is used, its threads grow very quickly through a material until a more solid substance is created in just a few days. Water is removed from the end product by heating. The fungal organism then dies, leaving a compact structure that can be made into building blocks or insulation material and which is fully recyclable. Martin Ostermann, professor at the University of Stuttgart’s Institute of Building Construction, believes that the benefits are compelling. He predicts that “this material will replace concrete in the future.” Exactly how, is still unclear: “We’re still just at the beginning, but I hope that a much broader and more varied mix of materials will be available in future.”

By Elke Hildebrandt

Title image: ILEK

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