Sustainable construction

From construction waste to new concrete

A new method allows waste stone wool and concrete to be used as new building materials in a process that also removes CO2 from a point source gas stream. The result is concrete with a smaller carbon footprint, thereby addressing two problems at the same time.

Man holding up handfuls of shredded stone wool. Photo: Sine Fiig — Technical coordinator Petros Kanelis is preparing discarded stone wool which will be placed in reactors where it will be transformed into a product that can be used instead of some of the cement needed to make concrete. Photo: Sine Fiig

Wednesday 09 April 2025

Miriam Meister

Reactors of various sizes spread across five laboratories at DTU Lyngby Campus might hold the secret for saving nonrecyclable building materials from being sent to landfill, turning them into new building materials instead. The method is the brainchild of DTU Professor Susan Stipp.

In a nutshell, it uses waste stone wool (fact box below) and crushed waste concrete in water, in containers, where CO₂ and a patented mix of specially developed, natural molecules are added.

The waste building materials, which are alkaline, are converted into a fine-grained product containing calcium carbonate and other minerals.

The product collects as sediment in the containers—much like the scale that collects at the bottom of a tea kettle when you boil water. This solid material has properties that make it suitable as an ingredient in concrete. It can partially replace cement, which has a large carbon footprint (fact box below).

“Concrete made using this product will probably not be strong enough for constructing tall buildings, but could be used to make paving stones with a smaller carbon footprint,” Susan Stipp says.

Facts

Stone wool—popular as insulation and growth medium

Many people are familiar with stone wool as the insulating material that keeps our buildings nice and warm in the winter and cool in the summer.

However, stone wool is also a popular growth medium for tomatoes and the like in greenhouses. It allows gardeners to precisely control fertilizer and pesticide applications.

Stone wool is 98 per cent air but even when it is crushed before being deposited as landfill, waste stone wool still takes up a lot of space.

Focus on upscaling

With a Grand Solutions grant from Innovation Fund Denmark, Susan Stipp and her colleagues have been working on the idea for months in the CO2Fix project and they have demonstrated “proof of concept”. In trials run in small containers (called reactors), they have shown that CO₂ is immobilised as a component in the new material.

Work in the laboratory is now focussed on adjusting the process and the additives so production could be scaled up with the same good results. The researchers have gone from working with tiny 50 ml reactors, to 1 and 20 litre reactors and now to a 4-metre-high reactor that can hold 300 litres.

“It’s like baking a cake. You might be able to double the recipe and get a good cake—but you can’t just multiply it by 10 or 100 and be sure it works. You might need to adjust the ingredients or the way you mix things or perhaps bake it at a different temperature,” the professor explains.

Once the researchers have mastered the process in the large reactor in the laboratory, they aim to build a pilot facility at the ARGO waste-to-energy company in Roskilde, which is one of the partners in the project. The plan is for CO₂ captured from the flue gas from waste incineration to be fed into the CO2Fix reactor, where it will be “mineralised”—converted into materials that can be used in new concrete.

The potential of the project excites ARGO’s Deputy Director, Klaus W. Hansen, who is working towards climate improvements and the circular economy:

“If we succeed in the project, we will help the climate by reducing CO₂ while also taking a material out of landfill and making it into a new valuable building material.”

Man adjusting equipment in a lab where several reactors are placed. Photo: Sine Fiig — In the project, the researchers have worked with reactors of various sizes to scale up the process so that it can eventually be used for commercial purposes around Denmark's CO2-producing companies. Photo: Sine Fiig

Facts

The CO2 emissions related to cement production

Concrete used by the construction industry is produced using cement, which contains a large amount of burnt chalk or limestone. Burning limestone requires heat and the limestone itself emits CO₂. Cement production currently accounts for around 8 per cent of global CO₂ emissions, according to figures from World Economic Forum.

In addition to DTU and ARGO, other partners in the CO2Fix project include ROCKWOOL, that manufactures stone wool, and IBF, Denmark’s largest concrete producer.

Multiple benefits

It is not just the environment that benefits from reduction in construction waste in landfills, or the climate that benefits from captured CO₂ being stored in the new concrete products.

“We know that parts of the material we produce can be used as filler in cement, and we’re working intensively towards it being able to replace some of the cement because this will increase the value of the product we make. Removing CO₂ from the flue gas also leads to considerable savings, because it avoids the need to pay carbon tax on emissions,” Susan Stipp emphasizes.

The aim is for reactors to be installed in companies around Denmark that need to remove CO₂ from their flue gas. Making a solid out of CO₂ will make Denmark less dependent on having to ship captured CO₂ to Norway, to be stored underground, says Susan Stipp, and it will help create Danish jobs for the production and maintenance of the reactors and to the creation of the molecules needed to speed the process.

Facts

Sustainable construction

Around 22 per cent of Denmark's carbon footprint comes from the building and construction sector. By building more sustainably, we can reduce energy consumption and thus greenhouse gas emissions.

Contact

S.L. Svane Stipp Professor Mobile: +45 28750202 stipp@fysik.dtu.dk