## 2 Materials and Methods  
### Raw Material Characterization and Mortar Preparation  
A synthetic granular slag was produced with a chemical composition that mimics the expected inert vitrified material produced during the thermochemical conversion of combustible components of municipal and mined solid wastes. The detailed description of its production process is provided by [PERSON] et al. [1; 2]. X-ray fluorescence (Bruker AXS S8 Tiger spectrometer, Billerica, MA, USA) was used to determine the bulk chemical composition of the slag, which can be generally described as a CaO-FeO\({}_{\text{x}}\)-Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\) quaternary system. The complete chemical composition of the slag is given in Table 1. The Fe oxidation state was quantified as described by [PERSON] et al. [17] and the iron phases were found to be predominantly in the bivalent oxidation state (92%). Aiming to deliver sustainable materials, the use of ordinary Portland cement and natural aggregates was completely avoided in the mortars' mix design; the vitreous slag was used both as the main precursor and aggregate. For this purpose, the granular slag was subjected to distinct pre-processing procedures to achieve different particle size distributions \begin{table}
\begin{tabular}{l c c c c c c c c c c} \hline \hline  & **LOI** & **SiO\({}_{\text{2}}\)** & **CaO** & **FeO\({}_{\text{x}}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **MgO** & **TiO\({}_{\text{2}}\)** & **K\({}_{2}\)O** & **Na\({}_{2}\)O** & **MnO** & **Fe\({}^{3+}\)/Fe\({}_{\text{total}}\)** \\ \hline \multirow{2}{*}{
\begin{tabular}{c} **Synthetic** \\ **slag** \\ \end{tabular} } & \multirow{2}{*}{1.9} & \multirow{2}{*}{34.9} & \multirow{2}{*}{23.1} & \multirow{2}{*}{22.8} & \multirow{2}{*}{16.2} & \multirow{2}{*}{1.3} & \multirow{2}{*}{0.6} & \multirow{2}{*}{0.5} & \multirow{2}{*}{0.3} & \multirow{2}{*}{0.1} & \multirow{2}{*}{0.08} \\  & & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the synthetic slag used as main raw material (% by mass). according to the envisioned usage (Figure 1). Particle size distribution was determined using a laser particle analyzer (Malvern Mastersizer 3000, Malvern Panalytical, Malvern, UK). Finely powdered slag with a specific surface area of 1120 m\({}^{2}\)/kg (determined by the Brunauer-Emmett-Teller method, BET) and density of 2.97 g/cm\({}^{3}\) was used as the binders' main precursor (94.5 wt.% of binders' solid load). Silica fume (SF, Elkem, >95% SiO\({}_{2}\)) with a BET specific surface area of approximately 22,200 m\({}^{2}\)/kg was used as an admixture to increase the availability of Si species and support strength development. Increasing the Si/Al ratio was also expected to promote the formation of highly thermally stable phases [14] and produce compact microstructures with low porosity levels after thermal exposure [18], thus improving the high-temperature resistance and residual strength of the produced materials. A potassium-based activating solution was used following a mix design previously optimized. Theoretical predictions [10] and empirical experimentations [6] in the available literature confirm the superior thermal performance of potassium-based AAMs relative to their sodium-based counterparts. Potassium hydroxide (14 M) and potassium silicate solutions (23.8 wt.% SiO\({}_{2}\), 9.5 wt.% K\({}_{2}\)O, and 66.7 wt.% H\({}_{2}\)O) were prepared by dissolving potassium hydroxide beads and anhydrous potassium silicate in demineralized water. The water content in the activating solution was adjusted (0.68 SiO\({}_{2}\)/K\({}_{2}\)O molar ratio and 83.4 wt.% H\({}_{2}\)O) to achieve the pre-defined solid over liquid ratio (S/L) of 2.85 and molar ratios in the binder composition. Based on our previous laboratory experimentation, the mortar mixture was designed for a binder-to-aggregate mass ratio of 0.88. Fuller distribution was applied to optimize aggregate particle size distribution (PSD).