## 3 Materials and Methods
### Synthesizing Iron Silicate Slags  
The approach to synthesizing iron silicate slags suitable for evaluating inherent reactivity in SCM applications was based on the logic presented in Section 2. Three main characteristics of the synthesis can be identified: using iron crucibles; remelting in an inert atmosphere; and granulating in water jets.  
In the present work, the FeO-SiO\({}_{2}\)-CaO-Al\({}_{2}\)O\({}_{3}\)-MgO-Cr\({}_{2}\)O\({}_{3}\)-system was synthesized, but components can be included or removed if deemed appropriate. The idea of including chromium(III) oxide was based on its characteristics in the industrial process. Chromium, coming from, e.g., refractories and electronic scrap, is a strong nucleation agent in industrial slags [44], leading to the crystallization of chromium-rich spinel phases [45]. Due to the importance of the amorphous content in SCM applications [28; 46; 47] and the lower activation energy for heterogeneous compared to homogeneous crystallization [48], chromium was considered significant to include for the relevance of the experimental approach.  
This methodology includes dry mixing powder chemical reagents in amounts according to the chosen synthetic composition and its total mass. As the current description of this methodology is on a general level, the ratios of the chemicals for a specific composition are omitted. Nonetheless, for the abovementioned slag system, the chosen chemicals were iron (99% Fe), iron(III) oxide (98% Fe\({}_{2}\)O\({}_{3}\)), silica (99.5% SiO\({}_{2}\)), calcium carbonate (99.5% CaCO\({}_{3}\)), alumina (99.9% Al\({}_{2}\)O\({}_{3}\)), magnesia (99.95% MgO), and chromium(III) oxide (99.6% Cr\({}_{2}\)O\({}_{3}\)). The two iron phases were mixed in proportions resulting in iron(II) oxide. The approach of using the two iron compounds, as well as including calcium carbonate rather than calcium oxide, originates from the instabilities of reagent grade iron(II) oxide and calcium oxide in ambient conditions.  
After mixing the chemicals, the blend was transferred to an iron crucible (>99.82% Fe) and melted under an inert gas atmosphere in a graphite resistance-heated furnace (Ruhstrat, Gottingen, Germany), Figure 6. The atmosphere was attained by injecting argon (99.999% Ar) and nitrogen (99.996% N\({}_{2}\)) at flow rates of 3 L/min and 12 L/min ambient temperature and pressure, respectively. In the experiments, a fixed heating rate of 10 \({}^{\circ}\)C/min was used, and the final temperature was based on superheating of 100 \({}^{\circ}\)C above the calculated liquidus temperature for the theoretical synthetic slag composition. In the present work, the liquidus temperature was calculated using the Equilib module of FactSage 8.2 with the GTOx database [37; 38; 39]. After reaching the final temperature, the slag was homogenized isothermally for 120 min.  
Following the homogenization, the crucible was removed manually from the furnace, and the slag was tapped into water jets operating with cold tap water with a flow rate of 1.1 L/s. Information on the granulation equipment used in the present study can be found in a previous publication [19]. Since the crucible was removed directly after the isothermal hold, the experimental layout required careful placement of the thermocouple and the gas injection lances. In the experiments, the time between the removal of the crucible from the furnace to the first contact with the water jets was kept within 15 s. This time necessitates the superheating of the slag to avoid crystallization during handling.  
After drying the granulated slag, it can be subjected to analyses relevant to evaluating the success of the synthesis and experiments for assessing properties, such as inherent reactivity.