Using Bayer red mud as an alternative activator in hybrid alkali-activated cement for cemented paste backfill: Experimental investigation and life cycle assessments

Ordinary Portland Cement (OPC) is the most widely used cementitious material in cemented paste backfill at present. However, OPC production consumes massive energy and is one of the main contributors to greenhouse gas emissions. Hybrid alkali-activated cement (HAAC) is a promising alternative to Ordinary Portland Cement (OPC), mainly consisting of supplementary cementitious materials (SCMs), OPC, and alkali activators. This innovative solution not only reduces energy consumption but also mitigates carbon footprint compared to OPC production. Moreover, it is worth noting that the production process of widely used commercial alkali activators, including sodium hydroxide and sodium silicate, is associated with notable energy consumption and environmental pollution. Therefore, addressing these concerns is essential for enhancing the sustainability of HAAC and making it an environmentally friendly choice in cemented paste backfill. Red mud (RM) is a by-product generated by the Bayer process during the production of alumina from bauxite, which is a potential alternative to commercial alkali activators due to its high alkalinity. The experimental results show that the cemented paste backfill with RM-NaOH activated slag cement exhibits higher compressive strength than that with OPC or NaOH activated slag cement as binders in different curing ages. By comparing the Life Cycle Assessment (LCA) results, the RM-NaOH activated slag cement shows reductions mainly in the environmental impact categories of terrestrial acidification potential (TAP), fossil depletion potential (FDP), ozone depletion potential (ODP), and global warming potential (GWP). The total environmental impact of RM-NaOH activated slag cement is reduced by 13.67% compared to OPC, indicating that the production of HAAC with RM as the primary activator would significantly decrease the environmental effect of OPC manufacturing.