Life cycle assessment of SCM substitution in various CDW-based geopolymer concretes and sensitivity analyses on allocation methods

Construction & demolition is one of the 5 target fields prioritized by the EU action plan for the circular economy, posing unique potentials due to the massive amount of materials mobilised by this sector and facing particular challenges on reducing its environmental footprint. This paper presents a Life cycle assessment (LCA) to evaluate the potential environmental impacts of three novel mixture formulas for recycling construction and demolition waste (CDW) into geopolymer concretes and compares them with the impacts of Portland cement concrete. Additionally, it includes sensitivity analysis through various allocation methods for supplementary cementitious materials (SCMs) such as slag and silica fume, which are partially substituted in geopolymer recipes. The findings indicated that recycling scenarios demonstrated a reduction in emissions to the atmosphere for global warming potential (GWP) between 9% and 35% compared to the baseline scenario, highlighting their advantage in mitigating carbon emissions from Portland cement. However, recycling scenarios exhibited higher environmental impacts in categories other than GWP, primarily due to high dosages of alkali activators. Sensitivity analysis on allocation methods showed that if no burdens were allocated to SCMs, lower environmental impacts in GWP and eutrophication (EP) were obtained for the recycling scenarios, while mass-based allocation led to worse performance of recycling across all categories. Contribution analysis show the hotspots of environmental sustainability in recycling CDW for geopolymer concretes were SCMs and alkali activators, suggesting that future development of the mixture formula should aim at avoiding or minimizing SCM and alkali usage, while enhancing CDW reactivity.