Biogenic carbon storage in concrete: how permanent is it?

When a concrete mix claims a lower carbon footprint, that number usually blends two very different things: emissions you avoided and carbon you stored. They aren't the same, and the difference matters, because stored carbon only counts if it actually stays put.

Avoided vs. stored: two different levers

• Avoided emissions come from using less of the carbon-intensive ingredient, mainly cement. Use less clinker, emit less CO₂. The reduction is real, but it's a reduction relative to what you would have emitted.
• Stored carbon is CO₂ that has been pulled out of the atmosphere and locked into the material. It's not a smaller number; it's a negative one.

A biochar admixture like ΔC uses both levers at once: it lets producers reduce cement (avoided) and carries biogenic carbon into the concrete (stored).

Where the stored carbon comes from

Trees pull CO₂ from the air through photosynthesis and store it as biomass. Sawmill residues from that biomass are converted into biochar, a well-established, stable form of carbon. ΔC brings that biochar into concrete as a liquid admixture, so the carbon ends up embedded in the hardened matrix.

Typical stored carbon lands in the range of 30–50 kg CO₂e per cubic metre of concrete, depending on dose and mix.

How permanent is "permanent"?

This is the question that carbon accounting actually turns on. Biochar can be a long-duration store, but permanence is not automatic:

1. Stability depends on the feedstock. How the biochar is made, and from what, affects how resistant its carbon structure is to breaking back down into CO₂. Not every biochar is equal.
2. Application matters. The same biochar behaves differently depending on where it ends up. In agriculture, degradation can be minor for some biochars and significant for others, so soil use is not the same long-duration store.
3. Concrete delivers ~1,000-year permanence. Once cast, the biochar is physically bound inside hardened concrete for the service life of the structure and beyond. That encapsulation is how you achieve long-duration storage on the order of ~1,000 years.

That is why storage in concrete is treated as long-duration removal rather than a temporary offset: the application, not biochar alone, makes the permanence claim hold.

What it means for producers claiming attributes

When you report a mix's footprint, that usually means an EPD. You are not expected to split avoided and stored into two custom reports. Cement reduction shows up as a lower footprint because there is less of the carbon-intensive ingredient. Biogenic storage from biochar shows on the EPD as a negative attribute. With ΔC, all environmental attributes transfer to the customer. alterBiota does not claim or resell them as offsets, so the producer and their client keep the full carbon benefit.

• See the headline proof on the ΔC page.
• See how the two mix strategies move the number in Sand Replacement vs Cement Replacement.
• Model your own mix with the ΔC estimator.