Environmental & Energy Systems

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.”

The production of biomethane from kitchen waste offers an as yet untapped potential for the energy transition. So far, only a small portion has been used for this purpose.

Of 85 kilograms of kitchen waste generated in private households per person and year, only about 21 kilograms is collected via the organic waste garbage can so far and used for further recycling into biogas and compost. "In order to generate biogas from kitchen waste, the waste must be properly separated. A large part of kitchen waste mistakenly ends up in residual waste and is thus incinerated and lost for high-quality energy and material recycling," explains Steffen Walk. He is part of the Bioresource Management (BIEM) research group at the Institute of Wastewater Management and Water Pollution Control at the Hamburg University of Technology, which has set itself the goal of recycling biowaste with a project. Up to now, in many places, biogas has not yet been obtained from the portion that goes into the organic waste garbage can, but only compost is produced. Ideally, there should be a process cascade of biogas production followed by composting of the so-called fermentation residue for efficient energy and material use of the biowaste. Biogas contains mainly methane and carbon dioxide. By removing the carbon dioxide, the biogas can be upgraded to biomethane, which has a similar calorific value to fossil natural gas.  "Only about 15 percent of Germany's 120 or so municipal biogas plants operate according to this principle and feed biomethane into the natural gas grid, while the others directly convert it into electricity. So there's still room for improvement," Walk said.

Covering gas consumption for a year

If the biowaste potential of all 83 million inhabitants in Germany were used to produce biomethane, the gas consumption of 2.8 million people could be covered for a year. We can all contribute to this by separating kitchen waste better. The use of small containers with lids is recommended for separating organic waste in the kitchen. Compared to plastic or paper bags, this saves resources and is easy on the wallet. The more that is collected, the greater the incentive for new construction and upgrading of compost and biogas plants for biomethane production. Steffen Walk demands: "Politicians should also demand kitchen waste separation more consistently. Separation rates of 65 percent, equivalent to 55 kilograms per person, are definitely possible." A start here would be the consistent installation of organic garbage cans for all households. So far, only just under 60 percent of German households have an organic waste garbage can.

Project Circular Economy

In order to give people a practical understanding of waste separation and recycling, Steffen Walk founded the "BioCycle" project. "BioCycle" describes the interrelationships of a cycle in which food becomes waste and then becomes new products, such as biogas or compost and soil fertilizer. The idea is that not all waste is avoidable. "You have to throw away a banana peel, but, disposed of properly, it can close the biocycle" explains Walk. In times of scarce energy and degradation of soils due to their over-intensive use, this problem can be counteracted with a circular economy. Biocycle is conceived as a learning opportunity, designed in six stages to show the cycle of food.

The project is also integrated into the "mudflat walks" organized by the Hamburg Open Online University (HOOU). The concept includes hiking or "driving" through places in Hamburg where renewable energies are produced.

More Information

www.tuhh.de/aww
www.hoou.de/projects/biocycle/preview
wattwanderungen.hoou.tuhh.de

Researchers                 Prof. Ralf Otterpohl

                                     Dr. Tavseef Shah

                                     Durga Prasad Babu Nasika

Institute                        Wastewater Management & Water Treatment

School of Studies         Civil Engineering

Funding                         Private, TUHH & FHH Hamburg

By 2050, the global water demand is expected to increase by 55 percent, much of which is attributed to agriculture. No wonder, since a good 40 percent of all food worldwide is grown on artificially irrigated land. Any savings in agricultural water use can free up water for other pressing needs like drinking water. A TU Hamburg-project shows how water and fertilizer use can be drastically reduced with a new cultivation concept.

Peas, beans, potatoes, and rice. Agriculture feeds us, but it pollutes groundwater with “unhealthy” nutrients especially nitrates and a variety of biocides. These are used in farming to control the growth of harmful organisms. But that also makes them potentially dangerous to humans, the environment and other organisms beneficial for plant growth. At the same time, agriculture worldwide uses about 80 percent of all freshwater withdrawals. Of this, about 40 percent is used in rice cultivation alone. This is a trend that has been going on for a long time. In the densely populated regions of South and Southeast Asia in particular, huge investments were made in additional irrigation systems between the 1960s and 1980s in order to keep increasing yields.

Dr. Tavseef Shah, with the help of his team from Hamburg University of Technology, has tried out new cultivation methods on site and in field trials in Kashmir in northern India. His idea is to significantly improve the dry rice cultivation (System of Rice Intensification, SRI) propagated mainly by Cornell University in the USA. At Hamburg University of Technology, he developed an intercropping concept that involves the simultaneous cultivation of different crops in one field. He combined SRI rice with bush beans. In this way, the nitrogen requirements of the rice plants could be supplemented with the help of the beans, which bind it to their roots. If this type of cultivation were used worldwide, it would save about 20 percent of the world's water needs as well as some of the fertilizer requirements.

And the bush beans provided an additional effect: the weeding requirement, which is otherwise very considerable for dry rice, fell by about 70 percent. Shah expanded his research even further for this purpose and founded the “Environmental Robotics” working group last year in 2021. In parallel with the development of rice cultivation in Kashmir, the group invented and built a selective weeding robot that has automatic plant recognition and is thus able to mechanically remove only the weeds that are harmful to the crops without chemicals. This development is in prototype status and is led by doctoral student Mr. Durga Nasika.

3 Questions for Dr. Tavseef Shah of the working group Environmental Robotics at the Institute of Wastewater Management and Water Protection about the project:

How does intercropping work, alternating rice and bush beans?

In intercropping, we plant rice and beans together in one plot. The beans are sown between the crop rows two weeks after the rice is sown. Since we use the dry rice cultivation method, the beans find good growing conditions. We have tested this method in fields in Kashmir with success. We observed reduced weed infestation, better and more diverse crop yields, and thus diversified income streams.

How much water can be saved with dry farming?

The SRI method can save up to 40 percent water in rice cultivation. We have observed this time and again in our trials at the TU Hamburg and in our field trials in Kashmir. We have to consider that on average 5,000 liters of fresh water are consumed to produce 1 kilogram of rice using the conventional flood rice method. Even if this method is slow to spread, the water savings will be significant. With intercropping, the dry rice cultivation method provides an added incentive to the farmers and the environment! We are currently looking at the possibility of using this methodology to grow rice in saline soils.

What has been the reaction of farmers to the weeding robot?

The farmers we talked to here in northern Germany were really excited about such an agricultural aid that eliminates weeds without the use of agrochemicals. For them, it's the kind of environmentally friendly and cost-effective solution that they're hoping for from a technical university. The idea was initiated by Prof. Otterpohl and Mr. Nasika has been working on this project since the very beginning.

Global climate targets call for rapid decarbonization of energy generation and increasing integration of renewable energies. But the wind doesn't always blow or the sun doesn't always shine. To ensure a secure supply, electricity, gas and heat grids must be coupled.

On the one hand, the energy system of the future will be determined by the increasing electrification of the consumption sectors of transport, industry, commerce and households. On the other hand, the volatility of renewable energy generation and the different dynamics of energy consumption require a high degree of flexibility and thus energy storage capacity of the overall system. This is the only way to ensure a resilient energy supply. It succeeds when the systems and networks of the energy carriers electricity, gas and heat are coupled with each other. And at the same time, intelligent networking takes place to control the plants, generators and consumers of the energy system. The idea is to convert energy flexibly between the energy carriers according to demand, for example, to convert energy from renewable electricity production into another form of energy, store it on a larger scale and convert it back into electricity when needed.

Ensuring grid stability

An example of such coupling effects is the use of cogeneration plants that feed energy into both the heat and electricity grids. For example, if there is a high demand for heat and at the same time a large supply of electrical energy, these plants cannot be down-regulated to ensure that the heat demand is met. This may result in less power being fed in from renewable energy sources such as offshore wind farms to ensure the stability of the electrical power grid. The transient, i.e. temporary, consideration also creates the possibility of creating a temporal balance through the targeted use of storage technologies. These considerations can also be used to answer the question of what type of storage, in what size, and at what location are sensibly deployed.

Reliably integrating renewables

Based on different scenarios using the model just described, we will look for ways to reliably integrate renewables into an existing energy supply structure. The final evaluation of the different scenarios is based on the CO2 emissions per year, which allows a direct conclusion on the goals of the energy transition. The results of this research contribute to securing society's growing energy needs and to the greatest possible environmental and climate compatibility.

Example projects with TU Hamburg participation:

TransiEntEE: Transient behavior of coupled energy grids with a high share of renewable energies, ResiliEntEE: Resilience of coupled energy grids with a high share of renewable energies, CyEntEE: Cyber Physical Energy Systems - Sustainability, Resilience and Econimics, funded as I³-Lab of TU Hamburg, EffiziEntEE: Efficient integration of high shares of renewable energies in technical-economic integrated energy systems, iNeP in NRL: Integrated Network Planning of the Sectors Electricity, Gas and Heat in the North German Real Lab.