Cities are built from countless particles carried from mountains.
Homes, office towers, schools, bridges, roads, and underground structures may be covered in steel and glass, but much of their weight is supported by vast quantities of concrete.
Concrete requires more than cement and water. It also depends on sand as fine aggregate and gravel or crushed stone as coarse aggregate.
Every time we build a new section of a city, mountains are quarried and sand is extracted from rivers, coastlines, and seabeds.
At the same time, ageing buildings are being demolished within cities, producing enormous volumes of concrete waste.
New resources are brought in from outside, while old materials are carried away from within.
Can we bring this one-way flow to an end?
The World Uses 50 Billion Tonnes of Sand and Gravel Every Year
Sand appears to be an abundant resource.
It can be found on beaches, in rivers, and across deserts. However, sand with the particle size, shape, and quality required for concrete is not available everywhere.
According to the United Nations Environment Programme, approximately 50 billion tonnes of sand and gravel are used worldwide each year. They are the most heavily consumed natural resources after water, and demand from the building sector alone could increase by as much as 45% by 2060.
Sand extraction is not simply a matter of digging material out of the ground.
When riverbeds are lowered, river flows and groundwater levels can change. Removing sand from coastlines and seabeds can accelerate coastal erosion and destroy ecosystems. Sediment plumes generated during extraction may also damage aquatic habitats.
Sand is not a resource that quickly returns after it is used.
It takes long periods of time for rivers to erode rock, carry fine particles downstream, and deposit them along coastlines.
Humanity is consuming sand faster than natural systems can replenish it.
Cities Already Contain Vast Reserves of Aggregate
At the same time, enormous quantities of concrete have already accumulated within cities.
The sand and gravel locked inside buildings, bridges, and roads can be seen as part of an “urban mine”—a stock of materials that could become available for future use.
When a building is demolished, its concrete is removed in large pieces. By crushing the concrete, separating reinforcing steel and other contaminants, and sorting the remaining material by particle size, it can be converted into recycled aggregate.
Instead of excavating another mountain, we can mine the old city for materials.
In Japan, the Construction Material Recycling Act requires the selective dismantling and recycling of concrete, asphalt concrete, wood, and other designated materials in construction projects above certain sizes.
By fiscal year 2018, the recycling and volume-reduction rate for construction waste in Japan had exceeded 97%.
At first glance, this suggests that construction recycling has already reached a highly advanced level.
But there is an important catch.
A 97% Recycling Rate Does Not Necessarily Mean Circularity
Crushed concrete reused as road base or backfilling material is counted as recycled material under existing systems.
Using demolition waste for another purpose rather than sending it to landfill is certainly valuable.
However, if concrete from a building does not return to a new building and is instead used for an application with lower quality requirements, the material loop is not truly closed.
This process is often described as downcycling.
The material is reused, but its value and future range of applications are reduced.
If demand for road construction and land development declines, the market for recycled crushed stone may also shrink. When the supply of recycled material exceeds demand, a high recycling rate alone cannot guarantee a sustainable circular system.
This is why Japan’s Ministry of Land, Infrastructure, Transport and Tourism has called for a transition from recycling focused on quantity to recycling focused on quality.
The central question is not how much waste has been processed.
It is what kind of material—and what level of value—the demolished building becomes next.
Recycled Aggregate Comes in Different Quality Grades
Recycled aggregate is broadly classified according to its quality and the level of processing it has undergone.
| Classification | Characteristics | Typical approach |
|---|---|---|
| Recycled Aggregate H | High-quality aggregate from which old mortar and other attached materials have been extensively removed | More suitable for consideration in structural concrete |
| Recycled Aggregate M | Medium-quality aggregate containing a controlled amount of attached mortar | Used with careful quality and application controls |
| Recycled Aggregate L | Lower-quality aggregate containing a relatively high proportion of old mortar | Generally limited to non-structural components and other restricted applications |
Concrete recovered from demolished structures still has old cement paste and mortar attached to the original aggregate.
As a result, recycled aggregate tends to absorb more water than natural aggregate. Its strength, drying shrinkage, and durability can also vary more widely.
Producing high-quality recycled aggregate requires multiple processes, such as crushing, sorting, grinding, and washing.
Additional processing can improve quality, but it also increases energy consumption and cost.
The process of recovering aggregate also generates fine concrete powder. Unless a suitable use is found for this material, the system cannot be considered fully circular.
Recycled aggregate is not a simple product created by crushing demolition waste.
The entire process—from demolition and manufacturing to quality control, structural design, and construction—must be reorganized as a connected system.
Why Recycled Aggregate Is Not Used More Widely
Technology is not the only barrier preventing the wider adoption of recycled aggregate.
The first challenge is uncertainty about quality.
If the age of a building, the materials used in its construction, its salt content, chemical exposure, and level of deterioration are unknown, it becomes difficult to determine whether the recovered concrete can safely be used in a new structure.
The second challenge is cost.
In some regions, purchasing newly extracted sand or crushed stone is cheaper than producing highly processed recycled aggregate.
When the environmental costs of resource extraction are not fully reflected in market prices, project owners have a strong incentive to select conventional materials.
The third challenge is distance.
Concrete is heavy.
If demolition sites, recycling facilities, ready-mixed concrete plants, and new construction sites are far apart, transportation costs and carbon dioxide emissions increase.
Beyond a certain distance, transportation can significantly reduce the environmental benefits of recycling.
Circular construction is therefore not only a question of material technology.
It is also a logistics challenge: where waste is generated, where it is processed, and where it can be reused within the urban region.
Circularity Must Begin Before Recycling
Expanding the use of recycled aggregate is important.
However, demolishing a building, crushing its concrete, processing the material, and turning it back into construction products requires a considerable amount of energy.
Circular construction therefore needs a clear order of priorities.
First, existing buildings should remain in use for as long as possible.
Next, structures and components should be reused without being destroyed.
Only when direct reuse is impossible should they be broken down and recycled as raw materials.
This requires renovation that extends a building’s service life, structures that can be adapted to new uses, replaceable equipment, and connections that allow components to be disassembled more easily.
Material passports will also become increasingly important.
A material passport records what materials are used in a building, where they are located, how much is present, and how those materials might be recovered or reused in the future.
Buildings should not be evaluated only at the moment of completion.
They must also be designed around their eventual exit: how they will be dismantled in 50 or 100 years, and which materials can be transferred to the next generation of buildings.
From Final Destination to Temporary Material Bank
At present, building every city without using any new sand is not realistic.
Recycled aggregate cannot yet meet every requirement for quality or supply. There will continue to be situations in which newly extracted sand and crushed stone are necessary.
Even so, dependence on virgin natural resources can be reduced.
Possible measures include:
- Extending building lifespans and reducing unnecessary demolition
- Separating construction materials more carefully during demolition
- Returning recycled aggregate to concrete production
- Connecting demolition sites, recycling facilities, and construction projects within the same region
- Creating demand for recycled materials through public works and large-scale development
- Recording the history, location, and quality of construction materials
- Designing buildings for adaptation, disassembly, and future reuse
If these systems are established, cities will no longer function only as places where resources are consumed and discarded.
They can become temporary material banks—places that hold resources for several decades before passing them to the next generation.
A building may not be a finished product.
It may simply be a temporary arrangement of materials that will one day form another building.
Can Cities Mine Themselves?
Cities have historically grown by gathering resources from mountains, rivers, and seas.
But now that humanity has already constructed enormous urban environments, continuing to grow through the same linear model will require ever-increasing quantities of material from nature.
The resources needed to build the next city are not found only in distant mountains.
They are already present in the walls of the buildings around us, in ageing bridges, and beneath roads that have reached the end of their service life.
The real challenge is not simply developing technology to process waste.
It is creating a philosophy and a system that allow an old city to become the material foundation of the next one.
Will cities continue to cut into mountains?
Or will they learn to mine themselves?
