## 3 Results and Discussions
### Hardened Properties  
Figure 5 presents the average results for the compressive strength of the studied pastes at 1, 3, 7 and 28 days. The error bars represent the standard deviation. Considerable development in strength from 1 to 28 days was observed for all formulations. The increases in strength from 1 to 28 days for 0.75M-4%, 1.50M-4%, 2.22M-4%, 0.75M-4%, 1.50M-4% and 2.22M-4% were 336.49%, 209.74%, 233.67%, 386.52%, 297.50% and 759.78%, respectively. These results confirmed the alkali activation process of the BOF slag and the hardening of all pastes, such as those presented by [PERSON] et al. and [PERSON] et al. [43; 44]. Figure 5: Compressive strengths of the pastes. It is possible to notice that the increase in silica modulus within the same Na\({}_{2}\)O concentration had different impacts on the compressive strength for each case (4% or 6%). For 4% Na\({}_{2}\)O, the higher the silica modulus, the higher the strength of the pastes. A rise in the modulus provided more silicate (higher Si/Al molar ratio in the matrix), which promoted strength development [67]. However, the opposite behavior occurred when 6% Na\({}_{2}\)O was employed. In the latter situation, the strength of pastes decreased for a higher silica modulus. This will be further discussed in the following section (Section 3.4). Similarly, the increase in Na\({}_{2}\)O concentration within the same modulus only enhanced the strength in the case of a low modulus (0.75M). In the other cases (1.50M and 2.22M), the increase in Na\({}_{2}\)O negatively affected the mechanical behavior. [PERSON] et al. [68] observed a threshold above which the alkali concentration compromises the strength development. [PERSON] et al. [69] also pointed out that a high silica modulus may decrease the liquid alkalinity provided by the high alkali concentration and consequently reduce the extent of activation and compressive strength. The trend of the compressive strength results is presented in Figure 6, where all variables are gathered. The figure graphically shows that the lowest strength values were found for extremes in the activator's content: a low alkali concentration combined with a low silica modulus or a high alkali concentration combined with a high silica modulus. Better strength performance could be achieved when an optimized combination of activator sources was considered. In this study, a low Na\({}_{2}\)O concentration (4%) with an intermediary silica modulus (1.50-2.22) presented the best mechanical results, increasing the compressive strength up to 20 MPa. The compressive strength of the pastes was lower than the conventional ones (based on the activation of GGBFS); however, it is essential to emphasize that BOF slag is an industrial waste that presents economic and environmental benefits. The compressive strength results are helpful for material design in future engineering projects that do not require high-strength materials. Similar results were presented by [PERSON] and [PERSON] [43] in their study, where 4% Na\({}_{2}\)O was combined with an optimal silica modulus of 2.0 to develop an alkali-activated SS paste with compressive strengths of 15 MPa and 25 MPa at 28 and 180 days, respectively. On the other hand, [PERSON] et al. [42] successfully developed an alkali-activated SS mortar with considerably high strength (22 MPa) by employing a higher Na\({}_{2}\)O concentration and an intermediate silica modulus of 6% and 2.0, respectively. Figure 7 presents the mean compressive strength at 28 days and the initial heat release found using calorimetry (Section 3.2) plotted against the silica modulus and Na\({}_{2}\)O  
Figure 6: Compressive strength distribution with respect to the formulation and curing age. concentration (4% and 6%). The graphs do not have the same trend. A rise in the silica modulus increased the initial heat release irrespective of the Na\({}_{2}\)O concentration. However, a high silica modulus combined with a high concentration of Na\({}_{2}\)O (6%) was detrimental to the strength development.