By Diego Giuliani

As resources grow scarcer and mountains of construction waste pile up, Europe is rediscovering an old idea: the city itself can become a source of materials. And experts say this is no longer optional: “The cities we live in will have to become the biggest mines of the coming decades.”

In Rome’s Basilica of San Lorenzo fuori le mura, the past is literally built into the present. The early medieval church was erected using ancient columns — shafts and capitals of different kinds, taken from older Roman buildings. Centuries later, when more space was needed, another nave was added, this time with grey granite columns of different diameters and Ionic capitals. “So the old church from the sixth century becomes a kind of monumental spolium itself, incorporated into a new structure built around 1200,” explains Maria Fabricius Hansen, Professor of Art History at the University of Copenhagen. “You have a first building made of spolia, combined with a second building also made of spolia, in this eclectic combination of two different eras.” The practice Fabricius Hansen refers to as spolia consists in taking architectural elements — columns, capitals, sculptures — from one context and reusing them in another. After the fall of the Roman Empire, shrinking populations and the transition to Christianity left temples and other buildings obsolete, while a new type of building was suddenly in demand: the church. Materials were there, so they were reused. Then the logic gradually reversed. Industrialisation brought new technologies and materials, while the idea of progress became increasingly associated with growing wealth and consumption. Fabricius Hansen compares it to the Futurists’ desire to discard the culture of the past and create something new. Recycling, as she puts it, “became old-fashioned.”

The mine beneath our feet
Two centuries later, that equation is changing again — out of necessity. In 2022, construction and demolition accounted for 38.4% of all waste generated in the EU, making construction by far its largest waste-generating economic activity. Meanwhile, environmental pressures and Europe’s dependence on imported resources are forcing us to reconsider something medieval builders understood instinctively: what we need may already be around us. “The concept of the urban mine is precisely about looking at the entire anthropogenic stock around us — our cars, homes, appliances, televisions, and all the batteries contained in the objects we own — as a potential resource,” explains Giulia Iattoni of UNITAR. “We practically have the whole periodic table available to us in the products around us.” The potential is considerable. The EU-funded FutuRaM project assessed critical raw materials across several waste streams and developed three scenarios up to 2050. Under “Business as Usual”, secondary raw materials recovered from the urban waste streams assessed by the project could replace about 33% of primary raw materials; only the most ambitious “Circularity Scenario” — combining improved recovery with reduced consumption and demand — reaches 56%.

A matter of choice — and necessity
But Iattoni, a co-author of FutuRaM’s final report, insists on a crucial distinction: “Our work is based on estimates, data and scenarios. What we have tried to show is what could happen under certain conditions — and those conditions depend largely on the choices we make.” For Jeroen Poppe of Vrije Universiteit Brussel (VUB), a RECONSTRUCT project partner, the direction is nevertheless clear: “The cities we live in will have to become the biggest mines of the coming decades. They will have to do so, both for environmental and geopolitical reasons, because Europe simply does not have enough resources of its own.” “In the past, there was already a lot of research into modularity, prefabrication and similar approaches to construction,” Poppe notes. What has changed is the motivation: once primarily about building faster and more efficiently, these approaches are now being revisited because “another factor has become increasingly important: environmental impact.”

Knowing what is in the mine
But finding a mine is one thing. Knowing what is inside — and being able to extract it — is another. One of FutuRaM’s main outcomes, the Urban Mine Platform, helps make this hidden stock visible. The publicly accessible database provides data on critical raw materials that could be recovered from waste up to 2050. For construction and demolition, it shows material demand, existing stocks in residential and non-residential buildings, and when those materials are expected to become available at end of life. José Mogollón, Associate Professor at Leiden University and co-author of the construction and demolition section of FutuRaM’s final report, points to one possible application. A follow-up study is comparing where materials for new concrete are currently quarried with areas where the platform predicts buildings and concrete will reach their end of life. “If we develop technologies that can recover old concrete for use as aggregate in new concrete, rather than simply downcycling it into roads, could some of that urban material replace the need for continued quarrying?”. Reported recycling rates are very high, but the problem is that much of this is actually downcycling. So, the material is not being recycled in the same functional way it was originally intended to be used. “As a result, we still need large quantities of virgin materials to continue producing concrete and other construction products,” Mogollón adds. “One of the important opportunities we discuss is therefore moving towards more functional recycling.”

From isolated experiments to a systemic change
So how close is Europe to turning its cities into functioning mines? As part of the EU-funded INBUILT project, a survey identified around 60 initiatives broadly associated with circularity and urban mining in European construction. The picture is encouraging but fragmented, says Marco Mori, Principal Investigator on Energy Efficiency at LEITAT, which led the survey: “There are plenty of successful examples, but they have not yet developed into a coherent ecosystem.” And having enough material is only the beginning. “The quantity and quality of materials are there. The first challenge is knowing exactly what we have,” he says. More difficult still is preserving that value through the whole process: “The challenge is therefore not simply to extract materials, but to extract their value and then put it back into circulation.” That means careful deconstruction, storage, matching supply with demand and overcoming regulatory barriers. “From a scientific and technological perspective, we already have some very promising examples,” notes Mogollón, yet “the biggest challenge is not necessarily whether we can recycle these materials, but whether we can build the infrastructure and economic conditions to do it at scale.” Which is why urban mining cannot simply be reduced to better recycling technology. “Closing the material loop is a far more complex challenge than achieving energy efficiency,” Poppe sums up. “It is not only a technical challenge, but a systemic one.”

Building the building after the building
This is where the question shifts from mining yesterday’s city to designing tomorrow’s. Buildings can be conceived from the outset to change function and eventually come apart. RECONSTRUCT follows this logic, combining reclaimed components and reusable building systems with design for disassembly and local low-carbon alternatives to conventional materials. The project is also developing AI-based tools using satellite, CCTV and drone imagery to locate construction and demolition waste, identify materials and estimate their volumes, helping determine what could potentially be recovered and reused. At VUB’s university campus, Poppe has explored similar principles of adaptability through the Circular Retrofit Lab. Prefabricated student housing modules from the 1970s had been destined for demolition. Instead, they were retained and transformed: one floor became a multi-use space, while another tested the possibility of switching between offices and residential guest houses. But technology is only part of the experiment. “It’s a different beast, this one,” he says of the systemic challenge: “What we see is that architects are debating a lot about the circular economy among themselves. Manufacturers are doing the same, constructors are doing the same, and so are the big developers. But what is often missing is bringing all these people around the same table and getting them to start working together.” That collaborative logic is also built into RECONSTRUCT. Alongside its two demonstration buildings in Barcelona and Brussels, the project has established separate Circular Territorial Hubs in both regions to connect stakeholders across the local construction value chain and explore opportunities for industrial symbiosis.

Learning from the past, rethinking value
And if resource scarcity and environmental pressures are forcing us to rethink how much we build, rather than simply how we build, the past may again offer a more radical lesson. “The challenges are enormous, so I don’t see any quick fix,” Fabricius Hansen says. “But we ought to stop building completely and simply stick with what we have already built, or transform it in other ways.” Transformation, she argues, need not mean losing the past. “Being ready to transform is a way of preserving the past. If we turn all our cultural heritage into museums, it dies rather than being kept alive.” Making such approaches mainstream, however, may also require changing the way value itself is calculated. “One of the main lessons that we can draw from our report is that we need to move beyond a strictly economic assessment,” stresses Iattoni. “It is not simply a question of comparing the cost of recycling with the cost of extracting primary raw materials. We need to look at the entire value chain and also take into account security of supply and broader environmental benefits: reducing CO2 and other emissions, land and resource consumption, and the additional waste generated by primary extraction.”

Making urban mining the new norm
Despite the many challenges, Mori sees demolition and construction sites becoming increasingly connected, with specialised actors helping match supply with demand. “Urban mining is a growing trend, and ultimately, this way of working will become increasingly normal across the construction sector: urban development will no longer be planned through isolated projects, but as a more interconnected system in which materials can move from one building to another,” he predicts. For Poppe, normalisation will ultimately depend on finding a tangible benefit for everyone involved. Adaptable buildings can retain value for owners; prefabrication can help constructors build faster; accessible building services can reduce maintenance costs. Regulation can raise minimum requirements, but circularity has a better chance of scaling if it creates value rather than merely demanding sacrifice. “There has to be a win for everyone for it to work,” he insists. And that may be what finally turns urban mining from an environmental ambition into an ordinary way of building. As Poppe puts it, “If they can create long-term value for building users and professionals, then we have a much broader range of arguments than simply saying we have to do it to save the polar bears.”