Using lithium refining residue and highly reactive metakaolin to optimise cement replacement in ternary and quaternary blended systems: Taguchi-Grey relational analysis method

Abstract The declining availability of conventional supplementary cementitious materials (SCMs), particularly fly ash (FA) and ground granulated blast furnace slag (GGBFS), has created a need to identify alternative low-carbon binders for cement-based materials. In this context, emerging SCMs such as lithium refining residue, known as delithiated beta spodumene (DBS), and highly reactive metakaolin (HRM) offer potential for reducing cement consumption while maintaining engineering performance. This study investigates the optimisation of blended binder systems based on a reference binary blend of 75% ordinary Portland cement (OPC) and 25% FA, aimed at reducing embodied carbon (EC) while maintaining satisfactory flowability and compressive strength. Two series of experiments were conducted: the first incorporated FA, DBS, and HRM, and the second incorporated FA, GGBFS, and HRM. A Taguchi L9 orthogonal array was adopted to design the experimental programme and evaluate the effects of the selected parameters on flowability and compressive strength. Grey relational analysis was then used to determine the optimal combination of responses, while analysis of variance (ANOVA) was performed to assess the relative influence of the individual parameters. The results indicate that HRM had a strong influence on early-age strength development, while the DBS material investigated in this study showed promising performance as a partial replacement material within the selected mortar-scale blended binder system. The optimum mix within the selected experimental domain in the first series comprised 63.75% cement, 21.25% FA, 10% DBS, and 5% HRM, while the optimum mix in the second series comprised 60% cement, 20% FA, 15% GGBFS, and 5% HRM. The embodied carbon of the optimum binder mixes was 264 and 261 kgCO 2 e per 450 kg of binder, respectively, representing reductions of 35% and 36% compared with the pure GP cement system.