**3. Results and Discussion**

### *3.1. Compressive Strength Development*

The compressive strength results are presented in Figure 2. NaOH-activated BFFS demonstrated a good activating effect. The lowest 28-d compressive strength was observed for 12 M activated BFFS. The highest compressive strength was observed for 8 M activated BFFS. As the concentration increased, the strength of the mortar first increased and then decreased, indicating that when the concentration of the activator is too high, the mechanical properties of the alkali-activated BFFS are decreased. This is consistent with the results of previous research [31,32]. A high concentration of NaOH increases the alkalinity of the activator and the pore solution, which is critical for BFFS dissolution. However, a higher pH in the pore solution also promotes the formation of crystal products in the paste such as portlandite and hydrotalcite, which decrease the strength. According to Bondar [33] higher concentration of NaOH results in a higher viscosity of the solution. The pastes require more time for excess water to evaporate; this is the key step in the development of a network of aluminosilicates in which strength is developed.

**Figure 2.** Compressive strength of AA–BFFS mortar at different concentrations.
