## 2 Materials and Methods  
### BOF Slag Characterization  
The SS used in this work was a byproduct generated during the conversion of iron into steel in a basic oxygen furnace (BOF). The original BOF slag had a maximum particle size of 4.8 mm. The BOF slag was crushed and then ground in a ball mill to reduce it to a maximum particle size of 75 \(\upmu\)m. The density of the BOF slag measured using helium pycnometry was 3.18 g/cm\({}^{3}\). The particle size distribution (PSD) of the ground precursor was obtained via laser granulometry (Cilas 1090 Laser) using the Fraunhofer diffraction theory. The analyses were made using the wet method (SS does not react with water) with a 60 s ultrasonic bath to increase particle dispersion. Triplicate samples were tested. The ground precursor presented 100% of particles as lower than 75 \(\upmu\)m, with 8.11 \(\upmu\)m as the mean diameter, as shown in Figure 1.  
The chemical composition, which is presented in Table 1, was obtained via X-ray fluorescence (XRF) on a Bruker S2 Ranger X-ray spectrometer (Bruker, Billerica, MA, USA). The BOF slag mainly consisted of CaO (36.66%) and FeO (24.19%), followed by SiO\({}_{2}\) (12.10%) and MgO (10.60%). Its alkalinity was 4.14, which favored its hydraulic activity [49], but not necessarily its cementitious behavior (the latter depends on the phases present within the chemical composition) [50]. The mineralogical phases were determined via X-ray diffraction (XRD) on a Shimadzu XRD 7000 diffractometer with Cu radiation at 40 kV and 30 mA. The scanning speed was 0.30 s/step, the length of each step was 0.02\({}^{\circ}\) and the scanning range was 5-80\({}^{\circ}\). Rietveld refinement software (GSAS) was used for the quantitative analysis. The XRD pattern of the BOF slag shown in Figure 2 presents several mineral phases, including larnite (C\({}_{2}\)S), srebroolskite (C\({}_{2}\)F), brownwillerite (C\({}_{4}\)AF), portlandite (Ca(OH)\({}_{2}\)), calcite \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{8}{c}{**Chemical Composition of Oxides (\%)**} \\ \hline  & **CaO** & **FeO** & **SiO\({}_{2}\)** & **MgO** & **Al\({}_{2}\)O\({}_{3}\)** & **Cr\({}_{2}\)O\({}_{3}\)** & **MnO** & **P\({}_{2}\)O\({}_{5}\)** & **SO\({}_{3}\)** \\ \hline BOF & 36.66 & 24.19 & 12.10 & 10.60 & 3.84 & 0.10 & 4.53 & 1.11 & 0.71 \\ slag & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of BOF slag. Figure 1: Particle size distribution of the BOF slag. (CaCO\({}_{3}\)), quartz (SiO\({}_{2}\)), wustite (FeO) and periclase (MgO). The quantification of each phase is also presented. A similar complex mineralogical composition of BOF slag was previously reported [51; 52; 53]. The TGA-DTA (thermogravimetric analysis coupled with differential thermal analysis) was conducted on Hitachi STA73000 equipment to identify and quantify the variation in the solid phases. The BOF slag was heated from 25 \({}^{\circ}\)C to 1000 \({}^{\circ}\)C at a heating rate of 10 \({}^{\circ}\)C/min in a high-purity nitrogen atmosphere. Figure 3 shows the TG and DTG diagrams, which are divided into two main mass loss regions: (i) portlandite dehydroxylation between 350 \({}^{\circ}\)C and 450 \({}^{\circ}\)C and (ii) calcite decomposition between 600 \({}^{\circ}\)C and 800 \({}^{\circ}\)C.  
The standard pozzolanic activity index was determined according to the Brazilian Standard ABNT NBR 5752:2014 [54]. The pozzolanic index is the ratio between the compressive strength at 28 days of a mortar containing 25% wt. of the studied materials (herein the BOF slag) replacing the PC and the strength of the reference mortar containing 100% of PC. Composite PC type CP II-F-32 according to the Brazilian Standard NBR11578-1997 [55] and normalized sand [56] were used to produce the mortars. Figure 3: TG (dotted line) and DTG (solid line) diagrams of the BOF slag. Figure 2: XRD pattern and Rietveld refinement.