Kelly Sempels’ dad was a builder, most of her five brothers are builders, and, until leaving her last job, she too was a builder. Now, after a career fixing roofs and laying bricks Sempels is plying a new trade: “urban mining”. She and her crew of six still dress and talk like builders, but their focus now revolves less around construction than dismantling.

“I love it when I can go to work and learn something new,” says the 43-year-old, pointing to a pile of laminate floor tiles in the corner of the terrace house that she is helping to strip. “Like how to break wooden floors without actually breaking them.”

Sempels is one of hundreds of workers and volunteers across the historic Belgian city of Leuven involved in an ambitious project to cut waste and create a “circular economy”. The plan, encapsulated by the 28 “action points” in the city’s official circular strategy, aims to reduce material consumption and be “more mindful of the limits of the planet”.

The idea’s chief protagonist is Thomas Van Oppens, a member of Belgium’s Green party and the city’s deputy mayor. From his office, looking out over a rippling tide of rooftops towards the city’s medieval centre, he maps out a glowing vision of Leuven’s future, when everything from kitchen waste to surplus factory heat will be captured, recycled and endlessly reused. His mantra runs: “What enters the city stays in the city.”

Behind Van Oppens’ eco-tinged optimism lies a more sober realisation that the city’s circular ambitions require a pragmatic footing. Hence, the decision to give initial priority to the city’s built environment. “Our ambition as a city is to become carbon neutral [by 2050],” he says. “And if you purely look at the impact on climate, buildings play a very large part.” According to the European Commission, about 40% of energy consumed in the EU is used in buildings while more than a third of the EU’s energy-related greenhouse gas emissions come from buildings.

This process of looking starts even before Sempels and her urban mining squad turn up with their wrenches, hacksaws and crowbars. Working closely with architects and developers, the city’s urban planning department sets out to identify which properties are scheduled for demolition and assess what might be salvageable.

The commission has funded various pilots to identify reusable materials using 3D scanners and other digital wizardry, but for now the process essentially amounts to a quick once-over by a materials recovery expert. In Leuven’s case, that job falls to Materialenbank, a non-profit initiative that promotes the recovery and resale of building waste that would otherwise head for the skip.

Standing next to Sempels in the now half-dismantled living room in Leuven’s Kessel-Lo district is Materialenbank’s logistics and urban mining manager Ward Verstappen. Pointing to the hallway and two ground floor rooms, he runs through the most in-demand items: bricks, stone, roof tiles, steel beams, wooden panels and flooring, plus carpentry items such as doors and window frames.

With some quick sanding-down or a fresh dab of lacquer, some of the salvaged objects can be reused like-for-like, Verstappen explains. When the items are too deteriorated, or where safety regulations don’t allow it, then “downcycling” is the next best option, he says: think steel girders being repurposed as scaffolding or roof tiles as a material for internal walls.

The two-up-two-down homes are part of a prewar residential block close to Leuven’s rail station that the city council has earmarked for demolition to add green space and ease traffic congestion. They comprise some of the 30 or so houses and garages that Materialenbank anticipates “mining” in the year ahead.

In Leuven’s circular plans, rehabilitating people is as important as reusing materials. Across Belgium, initiatives exist to promote a “social economy” that puts people over profits. Sempels is a beneficiary of the approach: she took the job after a lengthy stint without work, as did the other Belgian national in her crew. The five other members are from Iraq, Palestine, Ethiopia, Mali and the Caucasus.

All are employed by Wonen en Werken (Living and Working), a social enterprise that delivers a variety of public-oriented services, including park maintenance, cleaning, renovation and horticulture. It has 200 or so people on its books, all on minimum wage or just above, with salaries partly subsidised by the Flemish government. “Mostly, we work with people who are longtime unemployed, lower-schooled, and have a lot of issues outside work … Sure, many have their problems, but they also have their capabilities,” says Patrick Wauters, employment coordinator at Wonen en Werken.

A similar interplay between social and environmental goals plays out at the edge-of-town warehouse where Materialenbank is based. The hangar-like facility has a large space for cleaning up and storing the demolition materials, but also workshop areas that local craftspeople and entrepreneurs can use to manufacture objects from the rescued materials.

Among the entrepreneurs is Bram de Ridder, a 22-year-old climbing enthusiast, who is setting up a micro-enterprise manufacturing climbing-wall holds from leftover wood. After a friend told him about Materialenbank, he now comes regularly to hone his skills and kickstart his business. “I used to use old wood my dad had lying around or I’d go dumpster diving for old furniture, but now I come here. I get to use the equipment and I use material that would otherwise be really expensive,” he says.

Materialenbank’s main function, however, is as an intermediary between the suppliers of Leuven’s building waste and property developers and builders. In the three years since it started, the initiative has seen its annual sales increase more than tenfold, from 20 tonnes to more than 250 tonnes, and it is seeking bigger premises, says coordinator August Smessaert: “It’s difficult for trucks with heavy loads to come and go, and the gates in aren’t that high … It’s possible that we could be [recovering] more than 5,000 tonnes by 2030.”

One answer is not to cart everything back to the warehouse and instead persuade buyers to pick it up direct from from where it’s being “mined”. That’s relatively easy for bricks, steel and iron, where a secondhand market already exists, says Smessaert, but it’s harder for other materials where more restoration work is needed or where buying new is as cheap or cheaper.

So Materialenbank tends to prioritise collecting wood, as the demand is high and margins reasonable. Even so, finding buyers isn’t easy, Smessaert admits. Not only are building contractors suspicious of new ideas, he says, but they also like the certainty that comes from buying new. With recycled materials, supply can often be patchy, volumes and quality variable.

To try to improve matters, the city council has struck up a deal with three of Leuven’s largest institutions: the 600-year-old KU Leuven university, University Hospitals Leuven (Belgium’s largest hospital), and the semiconductor specialist Imec. All three have committed not only to work with Materialenbank to provide waste materials, but also to incorporate recovered items into future construction and renovation.

So, when the university teaching hospital decided to build a student “chillout zone” and admin area, its works division looked at the reuse market for options. The results are two recently completed facilities built mostly from materials destined for the dump.

In the view of Paul Lodewijckx, the hospital’s head for new builds and labs, items with a previous life bring character to a place. He points to the staircase and wall bench, both sourced from beech trees felled by a storm. The pine beams in the mezzanine come from dismantled pallets, he adds, while the kitchen units are made from fibreboard cast-offs from Imec’s offices. Lodewijckx’s favourite is the long walnut and MDF table in the chill-out area. It was part of an exhibition at the art museum.

Leuven boasts other examples. In the historic centre is a quirky, multi-level wooden structure used for urban sports and cultural events; it had served as part of the city’s velodrome. Meanwhile, three recently renovated social housing flats close to the city’s De Bruul Park are equipped with new beds, floors, kitchen units and cupboards made from recycled wood.

As full of potential as Leuven’s various circular construction initiatives are, most are effectively pilot projects. So, what will it take to make good Van Oppens’ vision of a fully circular utopia? Tougher legislation would help. The commission’s proposed circular economy act could feasibly place reuse obligations on all builders. And other European cities have set their own circular requirements. Zurich, for example, has mandated that all public buildings be built with at least 25% recycled concrete or other aggregates.

For urban mining to really take off, it will require a shift in mindset among all the city’s residents – not just its builders. People’s natural instinct needs to shift from throwing away their old stuff to thinking how they or others might find a use for it, says Kobe Vaes, an engineering graduate who works as coordinator at Maakleerplek, a multi-use centre dedicated to repair and reuse. It’s located in the shadow of two former grain silos owned by brewer Stella Artois.

At the heart of the centre is a large maker-space facility full of 3D printers, lathes, presses and other DIY equipment, plus racks of carpentry tools and boxes of broken Perspex, scraps of plywood and strips of fabric. Amid the organised chaos are the key fobs, phone cases, cheeseboards and handiwork of the hundreds of schoolchildren who regularly use the centre. “Every week in the summer, we take kids canoeing on the canal to collect plastic trash, and we use what we find back here in the workshop,” Vaes says.

Maakleerplek is also home to a small clothing repair venture staffed by immigrants, a textile room with knitting machines for local people to learn basic tailoring, and a tools “library” where, for €40 (£34) a year, residents can rent everything from ladders to power drills. The centre is just one of a crop of repair cafes, thrift shops and similar reuse initiatives now cropping up around the city. These schemes are slowly but surely changing public perceptions of the reuse economy, from something that carried “a bit of stigma” to something “kind of cool”.

That, at least, is the view of Sofie de Brouwer, a cleaner at a nursing home in Leuven, who regularly pops into the Kringwinkel secondhand store in downtown Leuven when doing her weekly supermarket shop. Part of a chain of four such stores in the city operated by the social enterprise ViTeS, it feels every bit like a department store – in which all the stock is preloved.

“I tell all my friends to come here,” says de Brouwer, who is wearing a winter jacket (€8) and leopard-print handbag (€4) bought on previous visits. “If you go to a high-street store, it’s all chosen for you and it’s all the same. Here, everything is vintage and different, which really appeals to me.”

Back in the soon-to-be-demolished terrace house in Kessel-Lo, Sempels is similarly won over by the possibilities of a circular approach to waste. She pulls out her phone and proudly shows pictures of a new council-installed unit for storing dustbins on the street. It’s made from wooden planks that she helped salvage, she explains. “It makes me happy when I see what they [the salvage materials] become,” she adds. “It’s beautiful, don’t you think?”