On the theoretical carbon storage and carbon sequestration potential of hempcrete

Hempcrete is a natural insulation material that is well known for exhibiting favorable thermal properties and low manufacturing emissions. Hempcrete is a biocomposite, consisting of hemp shiv and a lime-based binder composed of hydrated lime and either a hydraulic (e.g., natural hydraulic lime and ordinary portland cement) or pozzolanic binder (e.g., metakaolin). While long-term biogenic carbon storage can be achieved via utilization of hemp shiv in hempcrete, additional carbon storage can be achieved via carbonation of the binder. This study advances previous carbonation modeling approaches by deriving a theoretical model based on the fundamentals of cement hydration and carbonation chemistry to quantify the total theoretical in situ CO2e sequestration potential of hempcrete binders. To estimate the percentage of manufacturing CO2e emissions that can be recovered through in situ binder carbonation, the model is implemented in life cycle assessments of 36 hempcrete formulations of various binder contents and densities using an equivalent functional unit (FU) of a 1 m2 wall assembly with a U-value of 0.27 W/(m2K). Our model estimates between 18.5% and 38.4% of initial carbon emissions associated with binder production can be sequestered through in situ carbonation. Considering biogenic carbon storage, we predict that the total life cycle CO2e emissions of hempcrete can be negative, with a minimum of −16.0 kg CO2e/FU for the hempcrete mixture formulations considered herein. However, we estimate that some hempcrete formulations can exhibit net-positive emissions, especially high-density mixes (>300 kg/m3) containing portland cement, thereby illustrating the importance of materials selection and proportioning in designing carbon-storing hempcrete.