Aluminium and calcium were predicted to stabilize primarily in spinel-group minerals, gehlenite, and related silicate phases. The stability field plots for individual phases (Figure 3) further refined this interpretation by quantifying the predicted abundance and stability range of the most relevant phases. Metallic iron was predicted to form extensively within the selected temperature range, confirming the strong thermodynamic driving force for iron metallization. Likewise, perovskite was shown to remain stable under these conditions, supporting the selective concentration of titanium into a recoverable Ti-rich phase rather than forming less favourable oxide assemblages. The persistence of spinel and gehlenite as stable hosts for aluminium and calcium also explains the observed redistribution of these elements into structurally stable residual phases following reduction. Overall, the modelling predictions agree with experimental XRD observations, which confirmed the formation of metallic iron as the dominant iron-bearing phase and perovskite as the primary titanium host phase. At lower oxygen fugacity than those investigated in this study, thermodynamic and experimental evidence shows that titanium oxides can undergo progressive reduction through a sequence of suboxide phases, including Ti₄O₇, Ti₃O₅, and Ti₂O₃, before potentially reaching lower oxidation states such as TiO or metallic titanium at sufficiently low oxygen partial pressures and high temperatures. This reduction proceeds via stepwise oxygen removal controlled by thermodynamic stability boundaries in the Ti-O system, with intermediate Magnéli phases forming as oxygen vacancies increase (Wu et al., 2023; Liu et al., 2018; Ekanayake et al., 2025). However, such transformations require strongly reducing atmospheres and elevated temperatures, typically exceeding those applied in the present experimental conditions. Consequently, titanium remains stabilized in oxidized mineral phases within the investigated processing window, consistent with the absence of reduced titanium phases in the experimental observations. The presence of larnite (β-Ca₂SiO₄) is particularly significant, as it is a well-known hydraulic phase and the principal constituent of belite-rich cements. Upon hydration, larnite reacts with water to form calcium silicate hydrate (C-S-H) and calcium hydroxide, contributing to strength development and long-term durability, albeit with slower kinetics compared to alite (Taylor, 1997; Hewlett & Liska, 2019). Its occurrence in industrial residues such as metallurgical slags and BR therefore represents an intrinsic latent binder potential that can be utilized without the need for energy-intensive clinker production (Snellings et al., 2012; Stopic et al., 2024). From a circular economy perspective, valorising larnite-bearing residues as supplementary binder components provides a direct pathway to circularity, “closing the material loop”, reducing clinker demand, lowering CO₂ emissions, and transforming metallurgical by-products into functional cementitious resources (Scrivener et al., 2018; Provis & van Deventer, 2014). The thermodynamic modelling significantly reduced the experimental design space and enabled a more efficient and targeted experimental approach. Rather than exploring a broad and largely unconstrained range of temperatures and atmospheres through empirical trial-and-error, the modelling provided clear guidance on the temperature and oxygen fugacity conditions most favourable for achieving the desired phase transformations. This allowed experimental efforts to focus directly on the predicted optimal reduction window,
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