The “Hamburg Mix” shows that construction waste can provide high-quality raw materials for concrete production, thereby making the industry more sustainable.

Concrete is a versatile and durable building material. Several hundred million tons of it are used in construction in Germany every year. It consists of cement, water and aggregates such as sand, gravel and crushed stone. However, these are becoming increasingly scarce and expensive worldwide, which is why the industry is looking for substitutes. One option is to recycle construction and demolition waste. In an EU project, the Technical University of Hamburg, together with partners from the construction industry, has developed a promising recycled concrete – the “Hamburger Mische”. After successful tests, it has now been used in practice for the first time: parts of a new primary school have been built in Hamburg-Eilbek using the innovative concrete recipe. 
“When construction and demolition waste is recycled, it is usually only used as filler material for road construction – we call this downcycling,” says Mahsa Doostdar from CREM, the Institute of Circular Resource Engineering and Management at the TU Harburg. ”In contrast, we wanted to use this waste to produce high-quality concrete – recycled concrete.” To implement the idea, the working group led by Janus zum Brock participated in the EU project “CIRCuIT”. The concept sounds simple at first: the rubble is crushed into particles the size of pebbles or grains of sand, which are then added to the cement as an aggregate.

The Hamburg mix has prevailed

But in practice, there are various challenges – for example, the rubble must not be contaminated. And to ensure that the concrete is stable and durable later on, a good balance between fine and coarse grains must be found in the size of the particles used. Together with several companies from the Hamburg construction industry, the TU team set itself an ambitious goal: can high-quality concrete be produced whose aggregates consist entirely of recycled materials? To find out, the partners set up the “Musterbude” – a kind of research hut – on the premises of the OTTO WULFF construction company in Hamburg-Billstedt. Its walls and floor slabs were each made of different concrete formulations with varying proportions of recycled materials. In contrast, the ceiling was made of normal concrete and served as a reference point. “We achieved the best result with a recipe that we call the Hamburg mix,” says Doostdar. ”Its aggregates consist entirely of rubble, mainly clinker, brick, sand-lime brick and concrete.” The new material is in no way inferior to most conventional concretes in terms of strength, load-bearing capacity, and processing – and it doesn't look any worse either. The result was so convincing that the new recipe was used for the first time in a real building in 2024: when a three-story new building was constructed at the Richardstraße primary school in Hamburg-Eilbek, around 100 cubic meters of the new recycled concrete were used. “It was important that the construction and demolition waste for the concrete came from Hamburg,” emphasizes Doostdar. ‘We got it from the neighborhood, so to speak.’ If the rubble had to be carted in from more distant places, the long transport routes and the associated CO2 emissions would worsen the ecological balance. This has been shown by environmental impact assessments at the TU. The outlook for Hamburg is favorable: in 2025, OTTO DÖRNER – one of the project partners in CIRCuIT – will be commissioning a new, highly efficient recycling plant. It can separate construction and demolition waste into its component parts much more thoroughly than has been possible to date.

So far, only one percent recycled concrete

However, recycled material is more expensive than conventional concrete – the price premium is likely to be around ten percent. “So far, we at the TU have focused on the environmental impact assessment and see positive effects,” reports Mahsa Doostdar. “In order to estimate the life cycle costs, we have hired a colleague with a business background who can help us with these calculations.” In Germany, only around one percent of the concrete used in building construction consists of recycled material. To increase the proportion, the additional costs would have to be significantly reduced. Furthermore, there is room for technical improvement: to produce recycled concrete of the same quality and strength, significantly more water is needed than in the production of conventional concrete. Research projects are being conducted to rectify this shortcoming. For example, experts are experimenting with immersing the recycled aggregates in water before mixing them with the cement – this could mean that less water is needed for the subsequent mixing process.

Replacing the cement needed for concrete

Another research question concerns the algae growth. “About a year after the completion of the Musterbude, we noticed that some of its walls had turned green because of algae growth,” says Doostdar. To find the cause, the TU team took samples and analyzed them in the laboratory. The porosity of the recycled stone granules is a suspected factor. If they contain more pores than conventional aggregates, water could penetrate into the tiny cavities and create a breeding ground for algae growth. 
But the working group is not only concerned with sustainable alternatives for concrete aggregates. It would be even more important to replace the cement needed for concrete. Cement production releases enormous amounts of greenhouse gases – around eight percent of global CO₂ emissions are caused by it. The TU experts are already pursuing an idea: “Perhaps ash can be used as a cement substitute,” hopes Mahsa Doostdar. “This is something we want to take a closer look at in the future.”