## 3 Results and Discussions
### Early Hydration Results via Calorimetry  
Figure 4a,b show, respectively, the heat flow rate and the cumulative heat release during the activation of the BOF slag. The alkali-activated BOF slag pastes present different behavior depending on the activation parameters. In general, the hydration process of alkali-activated SS may be divided into five stages similar to cement-based materials and GGBFS-based AAM: the rapid initial dissolution, induction/reorganization, acceleration, deceleration and stabilization periods [33; 47; 43]. However, the changes in both  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Heat Release (J/g SCM)** &
\begin{tabular}{c} **Calcium Hydroxide** \\ **Consumption (J/g SCM)** \\ \end{tabular} \\ \hline BOF slag & 76 & 16 \\ Another BOF slag [26] & 210 & 9 \\ LFS [26] & 490 & \(-\)11.5 \\ GGBFS [26; 57] & 500 & 32 \\ Limestone [57] & 20 & \(-\)5 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Results of modified R\({}^{3}\) reactivity test.  
the silica modulus and Na\({}_{2}\)O concentration in the activator clearly modified the BOF slag activation kinetics.  
The rapid dissolution of the slag particles (calcium silicate and calcium aluminate) led to the first exothermic peak observed in all mixtures despite differences in intensity, as shown in Figure 4a. This stage is characterized by the formation of the initial dissolved silicate and aluminate units [43]. The type and concentration of the alkali activator are known to influence the duration and intensity of each stage during the reaction [63]. Figure 4 shows that the intensity of the first peak varied between 13 and 38 mW/g when the Na\({}_{2}\)O concentration was 4%. Conversely, the values were much higher for the 6% concentration, varying between 80 and 130 mW/g. Those results indicated that the higher the alkalinity in the solution, the faster the initial dissolution and the higher the initial heat release rate [43]. The effect of the silica modulus on the first stage was also evident for a fixed Na\({}_{2}\)O percentage. The rise in the silica modulus from 0.75 to 2.22 also increased the intensity and speed of the heat release [44].  
The induction period was characterized by low reactivity and shortened when NaOH + Na\({}_{2}\)SiO\({}_{3}\) were used as activators. Moreover, these activators are thought to make a small contribution to the total heat [63]. Since the aluminum phase in the BOF slag does not form  
Figure 4: Hydration kinetics of the pastes: (**a**) heat flow and (**b**) cumulative hydration heat.  
crystalline products, such as ettrinigite, no delayed reaction occurs and the induction period is short [47]. The induction period was only identified for the 0.75M-4% and 1.50M-4% curves. The acceleration and deceleration periods (second peak) are related to the formation of hydration products, including reaction gels and Ca(OH)\({}_{2}\), that result from the hydration of the calcium silicate present in the slag [33]. Similarly, regarding the induction period, the second peak was only identified for 0.75M-4% and 1.50M-4%. These results do not necessarily indicate the absence of Ca(OH)\({}_{2}\) or reaction gels in the other formulations. Actually, the acceleration start point was significantly delayed and the reaction rate was reduced as the silica modulus increased, which indicated that Na\({}_{2}\)SiO\({}_{3}\) had a retarding effect on the early reaction of the BOF slag [64].  
As shown in Figure 4b, the cumulative heat output of the formulations varied between 13.19 and 21.51 J/g, which was a slight difference between matrices. Those values are considered very small compared with other cementitious materials, such as GGBFS-based AAM (80-150 J/g) [65] and cement-based materials (250-350 J/g) [66]. These results from our study are in line with the findings of the R\({}^{3}\) reactivity test; the lower heat generation of the studied SS was probably associated with the slight presence of active components on the BOF slag mineralogical composition (Figure 2).