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Further information

CONCRETE – FROM CLIMATE KILLER TO CLIMATE SAVIOUR?

Concrete consists of a mixture of cement (binder), aggregates (sand and gravel) and water. The chemical reaction between the cement and water creates a composite that envelops and fixes the aggregates, giving concrete its enormous compressive strength. Depending on where it is used – e.g., in load-bearing underwater bridge piers vs. non-load-bearing walls in a house – the mixture is adjusted and, if necessary, additional substances are added that can change the concrete’s flowability, heat resistance or colour, among other things.

Concrete is a heavyweight in terms of greenhouse gas emissions, as industrial cement production releases a lot of CO₂ – both through chemical processes and high energy consumption. Concrete production emits around twice as much greenhouse gases as all global air traffic.

It is not always possible to replace concrete as a building material. It is therefore important to find solutions that make concrete more climate-friendly. In general, this can be achieved in two ways:

In Switzerland, there are numerous pioneering projects in which research institutions and companies are working together to develop innovative approaches to concrete and bring them to market.

CLAY CONCRETE

Locally generated construction waste such as clay-containing excavation material can be used as an ingredient for cement-free binders. Such binders can replace conventional cement and are used, for example, in clay-based rammed concrete. By replacing cement and recycling construction waste, CO₂ emissions can be reduced by up to 80%. Clay concrete is suitable for the construction of floors, attics, interior and exterior elements, but not for load-bearing walls.

ULTRA GREEN CONCRETE (UGC)

Ultra green concrete uses low-carbon binders based on clay and limestone (calcium carbonate) instead of conventional cement to reduce emissions from cement production. It also reduces the proportion of cement or binder used in concrete.

It is estimated that if UGC were used worldwide, up to 2% of global greenhouse gas emissions could be saved.

EXHIBIT: CALCIUM CARBONATE IN ­CONCRETE

The production of cement for use in concrete generates large amounts of greenhouse gases. By adding finely ground calcium carbonate (CaCO₃), the cement content in concrete can be reduced by 15 % without affecting its strength. Although this only reduces CO₂ emissions by around 10 %, it still saves considerable amounts of emissions in large construction projects.

In modern tunnel construction, prefabricated, curved concrete elements, known as tubbings, are used to line the tunnel tube. Tubbings are made of steel-reinforced concrete and are installed continuously by the tunnel ­boring machine. The precise manufacture of the tubbings ensures a dense, resistant and durable tunnel lining.

Around 600 000 tonnes of concrete will be needed to manufacture the ­tubbings for the construction of the second Gotthard road tunnel (2025–2030). This will generate approx. 50 000 tonnes of CO₂. The use of CaCO₃ in the concrete for the tubbings would allow around 6 000 tonnes of CO₂ to be saved during the construction of the tunnel.

If CaCO₃ could be used in concrete throughout Switzerland, approx. 200 000 tonnes of CO₂ emissions could be avoided annually. This corresponds to about one-fifth of the greenhouse gases emitted each year in the city of Zurich.

Numbers: Omya International AG (pers. comm.)

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CONCRETE WITH MINERALISED CALCITE

Demolition concrete can be enriched with gaseous CO₂. The CO₂ comes, for example, from a biomass incineration plant, where it was captured. When it reacts with the cement in the demolition concrete, the CO₂ forms tiny calcite crystals (mineralisation) inside the pores of the demolition material. Thus, CO₂ is stored permanently in solid, mineral form.

BIOCHAR IN CONCRETE

Biochar can be added to concrete in the form of pellets or powder as a substitute for aggregates (gravel, sand). Depending on the mixing ratio, concrete mixed with biochar can have similar properties to standard concrete, but at the same time store large amounts of CO₂ over the long term.

ARTIFICIAL LIMESTONE IN CONCRETE

Artificial limestone (calcium carbonate), which can be produced from captured CO₂ emissions, can be used in powder or granulate form as a substitute for aggregates or as an additive in concrete. Thus, the limestone can store CO₂ in solid form in the long term.

EXHIBIT: ARTIFICAL CALCIUM ­CARBONATE

Calcium carbonate in powder and granulate form

Capturing CO₂ in industry before it enters the atmosphere – for example during cement production – prevents emissions. But what to do with the ­captured CO₂?

It can be used to produce limestone (calcium carbonate: CaCO₃), which – in the form of granules or powder – can be used, for example, for coating paper or in concrete as a substitute for cement or filler. In this way, paper and concrete can store the captured CO₂. Industrially produced calcium carbonate can also replace natural calcium carbonate, which would otherwise have to be extracted in an energy-intensive process. This saves even more CO₂.

  1. Here you can see industrially produced calcium carbonate in powder and granulate form. These are the raw materials for a wide range of products.
  2. Watch the video to see how industrial calcium carbonate is ­produced.

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LIGHTWEIGHT CONCRETE

Lightweight concrete contains air pores or particularly porous aggregates (e.g., porous rock such as pumice or foamed rock). It is mainly used in structures with lower static loads, supports the thermal insulation of buildings, and reduces energy consumption and thus emissions both during transport and during building operation.

  • Distribution: large construction companies

INTERACTIVE EXHIBIT: WHICH DUMBBELL IS MADE OF LIGHTWEIGHT CONCRETE?

Photo: Nicola Pitaro

Like any concrete, lightweight concrete is a mixture of cement and aggregates such as sand and gravel, water and fillers. One of the dumbbells contains fillers made of a volcanic rock called perlite. Through expansion, the perlite was inflated to create a lightweight material with many pores. This gives lightweight concrete a much lower density (1 600 kg/m3) than conventional concrete (2 600 kg/m3), making it lighter.

Although lightweight concrete carries less than conventional concrete, it has properties that are better for the environment. Its porous structure traps air, which slows down heat transfer and thus provides good thermal insulation. This reduces the energy requirements of buildings. And be­cau­se it weighs less, larger quantities can be transported at once, which significantly reduces greenhouse gas emissions during transport.

One dumbbell is made of lightweight concrete.
Find out in the exhibition: Which one is it?

Further information about the exhibit