Life cycle assessment of natural building materials: the role of carbonation, mixture components and transport in the environmental impacts of hempcrete blocks

Hempcrete is a natural building material that, in recent years, has known an increased popularity in a number of European countries. Hempcrete-based construction materials are used in non-bearing walls, as finishing plasters and floor/roof insulators. In the present work, the environmental performances of a non-load-bearing wall made of hempcrete blocks were assessed via Life Cycle Assessment (LCA). The analysis encompassed the whole life cycle but the end of life, due to the lack of reliable data for this stage. The production phase of the raw materials was identified as the main source of environmental impacts, but the transport distance of raw materials, as well as the amount and composition of the binder mixture, can considerably affect the results. An experimental assessment (via X-ray Powder Diffraction analysis) of the carbonation process taking place within the binder during the use phase of the wall showed that the carbonation rate may be smaller than assumed in previous works: after 240 d, only the outermost layers of the blocks showed significant levels of carbonation, while the innermost layers experienced only a negligible increase in the amount of carbonates. Nevertheless, the overall emission balance is very favourable: thanks to biogenic CO2 uptake during hemp growth and to CO2 uptake by carbonation, hempcrete blocks have a negative carbon footprint and act therefore as effective carbon sinks.