Consequential Life-Cycle-Assessment as policy-support tool to promote green concretes

The manufacturing of Ordinary Portland Cement (OPC), normally used as a binder in concrete production, is responsible for 5% of global carbon emissions. Potential sustainable alternatives, namely “green concretes”, are currently studied in scientific literature. In green concretes, OPC is replaced with by-products coming from other industries. At present, however, the economic and environmental consequences of the introduction of green concretes into the market are not clear yet. This paper aims at filling this gap by using a Consequential Life Cycle Assessment (C-LCA) for the case of green concrete made from Stainless Steel Slag (SSS), with a focus on the Flemish market (in Belgium) for construction blocks. For the present study, a green concrete block was produced using SSS as OPC replacement. Today, SSS is considered as an hazardous waste, therefore stabilization is needed before its recycling as low-quality aggregate. Alternatively, SSS can be also chemically activated to produce a new binder for green concrete, hence foregoing the need of SSS stabilization. Therefore, the potential switch in the recycling of SSS, from low quality aggregates to green concrete, affects both the construction blocks and the low quality aggregates markets. In order to analyse the environmental consequences of these changes in both markets, a C-LCA was performed including all processes that are significantly affected by the changes, namely the production of traditional concrete, the production of natural/secondary low quality aggregates and their transports. In order to understand how mechanisms of revenue maximization and price equilibrium are affected by the changes, market information and economic models are also considered during the Life Cycle Inventory phase. Finally, the recycling of SSS as a binder for building materials will reduce the emission of CO2 and NOx , thanks to the avoided use of OPC. At the same time the change in the final use of SSS causes an increase use of natural aggregates for low value applications, unless alternatives from other by-products are considered. The C-LCA approach indicates that replacing a material by a more sustainable option does not lead necessarily to a better overall sustainability. Policy makers should therefore consider interactions between different markets when analysing the effectiveness of policy-support action, like the increase of the tax on the CO2 emissions or subsidies for the use of SSS as a binder.status: publishe