Biochar for Concrete
Stable carbon for lower-carbon construction
Concrete is one of the most useful construction materials ever developed — and one of the most widely used.
It also presents a major decarbonisation challenge.
This is why researchers are increasingly investigating whether biochar can play a role in the next generation of lower-carbon cementitious materials.
Biochar introduces something unusual into concrete: carbon originally captured from the atmosphere by plants, converted through pyrolysis into a much more stable form, and then locked into a long-lived construction material.
Research is exploring biochar as a partial cement replacement, fine-aggregate replacement and functional additive. Recent reviews suggest that, at carefully controlled inclusion rates, biochar can potentially be incorporated without sacrificing performance and may in some formulations improve particular mechanical or durability characteristics. But results depend heavily on biochar properties, particle size, dosage and the concrete formulation. (ScienceDirect)
For concrete manufacturers, precast businesses, material developers, architects and contractors interested in lower-carbon materials, we think that's worth investigating.
Why Put Biochar Into Concrete?
The environmental opportunity is particularly interesting.
Trees and other plants remove carbon dioxide from the atmosphere as they grow, storing some of that carbon within their biomass.
If woody biomass decomposes or is completely combusted, much of that carbon eventually returns to the atmosphere.
Pyrolysis changes that pathway.
Heating biomass under oxygen-limited conditions converts a proportion of its carbon into a stable, carbon-rich material: biochar.
In agricultural applications, that biochar is incorporated into soil.
But it doesn't necessarily have to go into soil.
It can potentially go into buildings.
Research into biochar-containing cementitious materials is investigating the possibility of using construction materials themselves as long-term stores of biogenic carbon. (ScienceDirect)
That creates an intriguing proposition:
What if part of the carbon captured by a tree could end up permanently incorporated into the buildings and infrastructure around us?
Biochar as a Concrete Additive
There isn't one single way of making "biochar concrete".
Research has investigated biochar as:
a partial replacement for cement or other binder components
a partial replacement for fine aggregate
a relatively low-dose additive
a component within mortars and other cementitious materials.
These approaches aren't interchangeable.
Biochar has very different physical properties from cement or sand, so replacing a percentage of one material with biochar changes the behaviour of the overall mixture.
The appropriate approach depends upon the intended product and performance requirements.
For professional concrete applications, biochar should therefore be treated as a technical material requiring formulation and testing, not simply something that can be added by the bag to an existing concrete recipe.
Can Biochar Replace Some Cement?
Potentially — and this is one of the most researched approaches.
Cement production has a substantial carbon footprint, so reducing the quantity of Portland cement required while maintaining the necessary concrete performance is an important area of materials research.
A 2024 meta-analysis covering 606 paired observations found that biochar dosage, feedstock, pyrolysis conditions, particle size, pretreatment and curing all influenced compressive strength. It found that biochar additions below 2.5% of binder weight could enhance strength in cement composites under some conditions, although the effect became more complex in concrete and mortar containing aggregates. (Springer Link)
A 2025 review reported that much of the literature points towards relatively modest biochar dosages, with performance potentially deteriorating when excessive amounts are incorporated. (ScienceDirect)
The message for commercial producers is therefore important:
More biochar does not automatically mean better concrete.
Optimisation matters.
Biochar and Concrete Strength
It's understandable that one of the first questions from a concrete producer will be:
What does it do to compressive strength?
The answer is: it depends.
Research shows that the effect is strongly influenced by biochar characteristics and mixture design.
Particle size appears particularly important. The 2024 meta-analysis found that smaller biochar particles were associated with improved compressive strength in Portland cement composites, while feedstock and pyrolysis conditions also influenced performance. (Springer Link)
Other research has demonstrated that carefully characterised biochars can be incorporated at considerably higher rates under specific conditions without necessarily reducing ultimate mortar strength. (Springer Link)
But these results shouldn't be interpreted as proof that any biochar can be added to any concrete at the same percentage.
They demonstrate something more useful:
Biochar-concrete performance can be engineered.
Particle Size Matters
For construction applications, the biochar specification becomes critical.
The biochar we might supply to a gardener is not necessarily the same grade that should be incorporated into a cementitious material.
For concrete trials, factors such as:
particle-size distribution,
moisture content,
porosity,
density,
ash content,
carbon content,
feedstock,
and production conditions
may all be relevant.
Research increasingly suggests that these characteristics help determine how biochar interacts with the cement matrix. (Springer Link)
This is why we're particularly interested in working with concrete businesses on specific grades and controlled trials, rather than presenting biochar as a generic construction additive.
Water Demand Needs Careful Attention
One of biochar's defining characteristics is its porosity.
That's useful — but it also creates a challenge.
Biochar can absorb water.
In concrete, where water-to-binder ratio and workability are carefully controlled, introducing a porous material can change the behaviour of the mix.
The moisture state of the biochar before mixing therefore matters.
A dry porous biochar may interact with mixing water differently from a preconditioned or saturated biochar.
Research into biochar concrete has consequently examined effects on rheology, hydration and internal curing as well as hardened performance. (ScienceDirect)
For commercial trials, we'd recommend treating biochar moisture condition as a controlled variable rather than simply using whatever material happens to come out of the bag.
Biochar and Internal Curing
That ability to hold water may also be useful.
Water held within porous biochar can potentially behave as an internal reservoir within cementitious materials, influencing hydration and internal curing.
Recent reviews identify internal curing as one of the potentially useful functions of biochar within cement-based materials. (ScienceDirect)
This is another example of why biochar shouldn't necessarily be thought of simply as a "filler".
Its pore structure can actively influence the behaviour of the surrounding cementitious matrix.
Carbon Storage Within Concrete
This is where biochar becomes particularly compelling from an environmental perspective.
Imagine a building product containing biochar produced from responsibly managed woody biomass.
The tree captured atmospheric carbon dioxide while growing.
Some of that carbon was converted through pyrolysis into stable biochar.
The biochar was incorporated into concrete.
The concrete became part of a building.
Instead of rapidly returning to the atmosphere, a proportion of that biogenic carbon is now physically contained within a long-lived material.
Research reviews increasingly examine this carbon-sequestration potential of biochar-containing construction materials, including the possibility that appropriately formulated systems could achieve very low or even net-negative lifecycle emissions. (ScienceDirect)
But we'd make an important distinction:
Biochar contains stored biogenic carbon. That does not automatically make the concrete carbon negative.
To make a carbon-negative claim about a finished concrete product, the entire relevant lifecycle would need to be assessed — including cement, aggregates, biochar production, transport, batching and other emissions.
That's a much stronger and more defensible approach than simply subtracting the carbon in the biochar and declaring the product carbon negative.
Could Biochar Reduce the Carbon Footprint in Two Ways?
Potentially.
There are two conceptually different benefits.
1. Storing biogenic carbon
The biochar itself contains carbon derived from atmospheric CO₂ captured during plant growth.
2. Potentially reducing conventional material requirements
Where an appropriately designed formulation allows biochar to replace a proportion of a higher-emission component while maintaining required performance, there may be an additional benefit.
These need to be calculated carefully.
But it creates an exciting possibility:
A construction material that doesn't merely emit less carbon, but physically incorporates stable biogenic carbon within it.
Biochar for Precast Products
Precast manufacturing may provide a particularly interesting environment for early commercial trials.
Production is controlled.
Mix formulations are repeatable.
Raw materials can be measured accurately.
Products can be tested before widespread deployment.
Potential areas for investigation could include non-structural and appropriate precast applications such as blocks, pavers and other cementitious products, subject to the relevant testing and standards.
A manufacturer could produce:
Control: existing mix
Trial A: low biochar inclusion
Trial B: higher biochar inclusion
and then compare the properties that actually matter for the product.
That provides evidence rather than assumptions.
What Should Concrete Businesses Measure?
A professional biochar-concrete trial should consider much more than whether the finished sample sets.
Depending upon the intended product, relevant measurements may include:
workability and slump,
water demand,
density,
setting behaviour,
7-, 28- and longer-term compressive strength,
flexural and tensile performance where relevant,
shrinkage,
water absorption,
permeability,
freeze-thaw performance where relevant,
durability,
thermal properties,
and ultimately whole-life carbon performance.
For structural or regulated construction applications, all applicable standards, certification, engineering and product-testing requirements still apply.
Biochar doesn't provide a shortcut around them.
Start Small and Test Properly
For concrete applications, this is probably our strongest recommendation.
Don't buy a bulk load of biochar and start adding it to production.
Start in the laboratory.
Establish your existing mix as the control.
Characterise the biochar.
Determine an appropriate particle size.
Control its moisture condition.
Trial low inclusion rates.
Measure fresh properties.
Cure the specimens properly.
Test them.
Then adjust.
If the results justify it, scale up.
Formulate. Test. Measure. Optimise.
That's how biochar becomes a credible construction material rather than an environmental novelty.
Consistency Is Particularly Important
Concrete producers require predictable raw materials.
Biochar, however, can vary significantly according to:
feedstock,
pyrolysis temperature,
production conditions,
particle size,
ash content,
moisture,
and post-production processing.
Research confirms that these variables can materially affect the performance of biochar-containing cement composites. (Springer Link)
If biochar is going to move from research into commercial concrete production, specification and quality control will therefore be essential.
This is also one reason why traceable feedstock becomes particularly valuable.
Why Sweethill Biochar?
Traceable wood biochar produced by us in Devon
For concrete manufacturers experimenting with biochar, knowing where the material came from isn't simply part of the environmental story.
It's part of understanding the raw material.
Sweethill Biochar is produced by us in Devon from timber arising from the management of our own woodland.
Much of our woodland is ancient woodland that was historically planted with commercial conifers.
We're gradually removing those plantation trees as part of our long-term project to restore a more diverse native woodland ecosystem.
Some of that timber becomes our biochar.
This gives us unusually direct traceability.
We know the feedstock.
We know where it grew.
We know why the trees were felled.
We produce the biochar ourselves.
And we're directly involved in restoring the woodland from which it originated.
For businesses investigating biochar as a serious construction material, that traceability provides a much stronger starting point than working with an unknown or continually changing feedstock.
From Woodland Carbon to the Built Environment
There's a fascinating potential journey behind biochar concrete.
A tree grows and captures atmospheric carbon.
It is removed as part of the management and restoration of our woodland.
Some of the timber is pyrolysed.
A proportion of its carbon becomes stable biochar.
That biochar is processed to an appropriate specification.
It enters a concrete mix.
The concrete becomes a block, paver, panel or other construction product.
That product becomes part of a building or landscape.
And the carbon that once formed part of a tree in our Devon woodland is now physically contained within the built environment.
From woodland carbon to construction material.
That's a very different way of thinking about what concrete could contain.
Work With Us on a Biochar-Concrete Trial
We're not going to tell concrete manufacturers that they can simply add Sweethill Biochar to an existing mix and immediately produce stronger, cheaper or carbon-negative concrete.
The research doesn't justify such a blanket claim.
What it does justify is serious experimentation.
Biochar-containing concrete is now the subject of a growing body of research, with recent reviews identifying potential benefits for carbon storage and, at optimised dosages and specifications, particular mechanical and durability properties. At the same time, researchers continue to emphasise the importance of feedstock, particle size, production conditions, dosage and long-term testing. (ScienceDirect)
That's exactly where a small, traceable biochar producer can be useful.
If you're a concrete manufacturer, precast producer, construction-material developer, research organisation, architect or engineering business interested in investigating biochar, we'd be interested in working with you.
We can discuss feedstock, grading and quantities and provide material for controlled trials before moving towards larger-volume supply.
Don't take our word for it. Put it in the mix and test it.
Traceable Devon biochar. Stable biogenic carbon with the potential to become part of the buildings we leave behind.