Quantifying the Carbon Uptake of Hempcrete

Hempcrete is often though of as a carbon “negative” material, or a construction material that stores more carbon than it emits. Yet, how much carbon can it store? Well, that depends on a numerous factors, primarily the type of binder used and the density of the mix.

I’m excited to share the results of our recent project in which we developed a simple model for quantifying the carbon uptake of hempcrete, considering both the carbon stored during photosynthesis (plant growth), and carbon stored via carbonation of the binder (considering both CH and CSH). We applied this model in a life cycle assessment of 36 hempcrete mix designs of various densities and binder compositions to compare cradle-to-gate carbon emissions with the carbon uptake for a 1 square meter wall with a U-value of 0.27 W/m2K). We found that up to 16 kg CO2/m2 of wall could be stored, yet not all mix designs we looked at resulted in net carbon storage. For example high density mixtures (425 kg/m3) using ordinary portland cement as a hydraulic binder (with hydrated lime) emitted 8.16 kg CO2/m2 of wall, even when considering the carbon uptake of carbonation and photosynthesis.

For a look at the results of all 36 mix designs and the model, check out the publication. It is available for free download for the next couple of weeks.

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Hey Jay,
thanks for sharing this article. The link to the Elsevier page appears to be broken, can you share the citation so I can look it up?
Thanks,
Steph

Of course - here’s the permanent DOI: https://doi.org/10.1016/j.jclepro.2020.121846

Thanks for sharing, @jayarehart! What are the options for hempcrete at end of life? Are there similar concerns as wood about end of life emissions that can be a setback to carbon sequestration?

Since there isn’t a lot of hempcrete out there (yet), the end-of-life hasn’t been studied extensively (others please chime in if you are familiar with sceanrios). For the biogenic part, it will decay at some point, yet the binder will hold onto the carbon until calcined again. So depending on how the hempcrete is processed at the end of its life, I expect that the decay time will be longer than just green hemp due to the alkalinity of the binder inhibiting decay.

So, even though at some point some of the carbon that was stored in the biomass will continue in the carbon cycle, the net carbon storage benefit is realized because the rotation time for hemp is less than one year. Thus a carbon that is stored today is will be replaced in a year. This quick table from Guest et al. 2013 give a nice comparison of carbon storage (GWP_bio) as a function of rotation time and period stored in buildings with the assumption that at the end-of-life the biomass is used as bioenergy. A negative number means net carbon storage over the time horizon, so with a 1 year rotation time, and storage for 60 years, 1 kg of CO2 absorbed during plant growth results in a net storage GWP of 0.5 kg, whereas if it was stored for 100 years, it would be nearly 1kg of net storage. Note that this number can be refined for specific species (i.e., hemp) and the results change meaningfully if a different midpoint indicator is used (e.g., 500 year global warming potential). So for measuring carbon storage, there’s a strong argument for not using the metric of 100-year GWP (even though its currently the standard), and a longer time horizon should be considered.

While that’s a bit long-winded of an explanation, the key takeaways are:

  • At the end-of-life, the binder will retain the carbon indefinitely, while the biomass will decay at some point.
  • Storage from hempcrete is realized no matter what happens at end-of-life as long as it is stored for more than one year.
  • The longer it is stored in the building, the more carbon storage is realized (as measured by GWP).
  • A 100 year time horizon is not the best metric for quantifying carbon storage and either longer time horizons should be used, or dynamic LCA.

There’s a good bit more research that needs to be done in this area, specific to buildings, which I am working on, so stay tuned for more updates.

I’ll also point to this other thread where we had a brief discussion about accounting for biogenic carbon.

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Thanks, Jay. That’s a really useful way of thinking about it.

Wow. Great resource Jay! Thank you.I’m also tagging this as NES… Perhaps we need a tag for key publication resources!?!

@ksimonen - I think that could be a great way to keep them so they are all easy to find.