Lightweight concrete CO₂-neutral – practical approaches

Decarbonizing concrete construction is a key task for construction research and the construction industry. One promising approach is the use of biochar as a multifunctional component in concrete. In the laboratory, lightweight concretes have been produced in which the mineral aggregate has been completely replaced by biochar-based aggregates. The porous structure of biochar not only leads to a significant reduction in bulk density, but also has a positive effect on strength development. Due to its high water absorption capacity, biochar can support internal water supply during hydration, reducing autogenous shrinkage and improving the microstructure.

At the same time, studies show that biochar can also be used as a partial cement replacement. Depending on the raw material used, the pyrolysis conditions, and the post- or pre-treatment (e.g., grinding, pre-saturation), the optimal replacement rates are in the range of 1 to 10% by mass. The key to practical applicability is an adapted mixing strategy that coordinates water addition, superplasticizer content, and mixing intensity to ensure good workability and mechanical properties.

Since the carbon in biochar is of biogenic origin and is stabilized in the long term by pyrolysis, lightweight concrete with biochar also acts as a permanent carbon sink. Life cycle assessments show that, taking into account the CO₂ storage in biochar, CO₂-neutral and, in the future, CO₂-negative lightweight concrete formulations are already possible today.

Data-driven optimization approaches are promising for future applications in order to systematically control the material-related fluctuations of biochar. This opens up new avenues for resource-efficient and greenhouse gas-reduced lightweight concrete as a building block of sustainable construction.