Look across any city and concrete is everywhere—in roads, bridges, levees, tunnels, apartment buildings, and office towers.
Concrete has supported the safety and convenience of modern society. It has enabled us to build disaster-resistant structures, extend cities upward, and connect people and goods across mountains and seas.
As more buildings and infrastructure are demolished, efforts are also expanding to crush used concrete and reuse it as recycled aggregate.
By reducing waste and limiting the extraction of natural sand, gravel, and crushed stone, recycled aggregate plays an important role in the circular use of construction resources.
But one question remains.
Does reusing aggregate really make concrete an environmentally friendly material?
The environmental impact of concrete cannot be understood by looking at aggregate alone.
We must also examine the material that binds concrete together: cement.
Most of Concrete Is Aggregate
Concrete is primarily made from:
- Cement
- Water
- Sand
- Gravel or crushed stone
- Supplementary cementitious materials and chemical admixtures, when required
Most of its volume consists of aggregate such as sand and gravel.
Recovering aggregate from demolished concrete and using it in new concrete can therefore reduce demand for virgin natural resources. It can also decrease the amount of waste sent to final disposal sites and limit the damage caused by new quarrying to landscapes and ecosystems.
But aggregate alone cannot harden into concrete.
Cement binds sand and gravel together and gives a structure its strength. Even when recycled aggregate is used, new cement is still required for most applications.
Aggregate may circulate, but the manufacture of cement continues to generate carbon dioxide.
Why Cement Production Emits So Much CO₂
Clinker, the main intermediate product used to manufacture cement, is produced by heating limestone, clay, and other raw materials in a rotary kiln.
Temperatures inside the kiln reach approximately 1,450°C.
There are two major sources of CO₂ emissions in this process.
The first is energy-related emissions. Coal and other fuels are burned to maintain the extremely high temperatures required inside the kiln.
The second source is the chemical reaction that occurs when limestone is heated.
Calcium carbonate, the main component of limestone, breaks down into calcium oxide and carbon dioxide:
CaCO₃ → CaO + CO₂
This process is known as calcination.
According to materials presented by Japan’s Ministry of the Environment, approximately 55% of CO₂ emissions from cement manufacturing in Japan come from the calcination of raw materials. Energy use accounts for around 35%, while electricity accounts for the remaining 10%.
Data from fiscal year 2018 also indicated that producing one tonne of cement generated approximately 700 kilograms of CO₂.
The critical point is that switching from fossil fuels to renewable energy or lower-carbon fuels cannot eliminate the emissions released directly from the raw materials.
Decarbonising cement is difficult not only because production requires an enormous amount of heat.
Under conventional manufacturing methods, CO₂ is released by the chemical reaction that transforms limestone into cement.
What Recycled Aggregate Can—and Cannot—Solve
Recycled aggregate has clear environmental value.
It can reduce the extraction of natural aggregate, decrease the disposal of demolition concrete, and potentially shorten transportation distances when materials are processed and reused within the same region.
But recycled aggregate is not a universal solution.
| What recycled aggregate can reduce | What recycled aggregate alone cannot eliminate |
|---|---|
| Extraction of natural sand, gravel, and crushed stone | Production of new cement |
| Final disposal of demolished concrete | CO₂ released through limestone calcination |
| Environmental damage caused by quarrying and landfilling | Energy required for high-temperature kiln operation |
| Transportation impacts under suitable local conditions | Total emissions from demolition and reconstruction |
The fact that a concrete product contains recycled material does not automatically make it low-carbon.
A meaningful environmental assessment must consider not only the proportion of recycled aggregate, but also the amount of cement used, the type of supplementary materials, energy consumed during production, transportation distances, durability, and the expected service life of the structure.
Reducing the Amount of Cement
One of the main strategies for lowering the carbon footprint of concrete is to replace part of the clinker or cement with other materials.
Common alternatives include ground granulated blast-furnace slag, a by-product of steel production, and fly ash from coal-fired power stations.
Using these industrial by-products can reduce the amount of newly manufactured clinker required and lower CO₂ emissions per cubic metre of concrete.
However, a higher replacement rate is not always better.
Required strength, curing speed, durability, and construction conditions vary according to the type and purpose of each building or infrastructure project.
Blast-furnace slag and fly ash are also by-products of other industries. If the structures of steelmaking and power generation change, the same quantities may not remain consistently available in the future.
Low-carbon construction must therefore consider not only material performance but also supply volumes, regional availability, and long-term changes in industry.
Concrete That Stores CO₂
Although concrete releases CO₂ during production, hardened concrete can later react with carbon dioxide in the atmosphere and absorb part of it.
This process is known as carbonation.
New technologies are being developed to use this reaction deliberately by introducing captured CO₂ into concrete during manufacturing or curing.
The aim is to reduce cement use while locking CO₂ inside the material in a stable form, such as calcium carbonate.
Materials published by Japan’s Ministry of the Environment describe several forms of lower-impact concrete that use coal ash, blast-furnace slag, and materials derived from captured CO₂.
Japan’s New Energy and Industrial Technology Development Organization is also supporting research and demonstration projects involving the storage of CO₂ in concrete, cement, and carbonate-based materials. These technologies may have the potential to deliver emissions reductions at a large scale.
However, it would be premature to conclude that concrete is environmentally harmless because it can absorb CO₂.
The amount absorbed through natural carbonation varies according to the concrete mixture, exposed surface area, humidity, service environment, and the particle size of concrete after demolition.
In reinforced concrete, carbonation can also reduce the alkalinity that protects steel reinforcement, potentially increasing the risk of corrosion.
The amount of CO₂ absorbed and the amount emitted must therefore be calculated under consistent conditions and assessed across the entire life cycle.
Can We Use Buildings Longer Before Demolishing Them?
The development of low-carbon concrete is important.
But there is an option that comes before recycling.
Can we avoid demolishing buildings that are still usable and keep them in service for longer?
When a building is demolished, its concrete can be processed into recycled aggregate. Constructing its replacement, however, requires new cement, steel, glass, aluminium, and other materials to be manufactured and transported to the site.
In some cases, retaining an existing structural frame or foundation and extending the building’s life through renovation or conversion may produce fewer emissions than demolition and reconstruction.
Japan’s Ministry of Land, Infrastructure, Transport and Tourism identifies several approaches to reducing the life-cycle carbon of buildings, including:
- Reusing existing buildings and foundations
- Selecting lower-carbon materials
- Reducing the quantity of materials used
- Extending building lifespans
- Designing structures for easier disassembly
Japan is also working toward the introduction of a system intended to encourage building life-cycle assessments from fiscal year 2028.
The environmental performance of future architecture should not be judged only by how energy-efficient a building is after completion.
Assessment must cover the entire life cycle—from raw-material production and construction to use, renovation, demolition, and reuse.
Circularity Does Not Mean Rebuilding Again and Again
Recycled aggregate is an important technology for returning construction waste to productive use.
But it does not eliminate the full environmental impact of concrete.
Behind the aggregate are other burdens: limestone quarrying, high-temperature firing, calcination, transportation, construction, and eventual demolition.
We should therefore avoid isolating a single technology and labelling it environmentally friendly.
A genuinely circular and lower-carbon construction system must combine multiple approaches:
- Reduce the extraction of virgin natural resources
- Minimise the amount of cement used
- Use low-carbon materials where they are technically appropriate
- Capture and store CO₂ generated during manufacturing
- Maintain and repair structures so they remain useful for longer
- Plan for reuse and recycling from the design stage
- Evaluate emissions across the entire building life cycle
Only by combining these measures can circular construction become a practical reality.
Making cities circular does not mean repeatedly demolishing buildings and replacing them with new ones.
It means using what already exists, building only what is necessary, and passing materials forward when their original use comes to an end.
The decarbonisation of concrete is not only a challenge of material technology.
It also asks society to reconsider how we define urban newness—and what we mean by prosperity.
Main References
- Ministry of the Environment, Japan: Examination of Carbon Dioxide Capture Technologies in the Cement Industry
- Ministry of the Environment, Japan: Quantifying CO₂ Reductions from Environmentally Conscious Concrete
- NEDO Green Innovation Fund: Development of Technologies for Manufacturing Concrete and Other Products Using CO₂
- Ministry of Land, Infrastructure, Transport and Tourism: Building Energy Efficiency and Life-Cycle Carbon
