*2.1. Stability of Alkali-Activated Materials*

Table 1 lists the conductivity values of aqueous solutions after 4 h at 150 ◦C under an N<sup>2</sup> atmosphere of 2 MPa and an AAM concentration of 4 g/dm<sup>3</sup> .


**Table 1.** Conductivity of aqueous solutions after 4 h at 150 ◦C under an N<sup>2</sup> atmosphere of 2 MPa and AAM concentration of 4 g/dm<sup>3</sup> .

The conductivity values of aqueous solutions after 4 h of experiments were 200–300 µS/cm; according to these values, the alkali activation of BFS stabilized the material. With the increase in the amount of NaOH in the sample, the conductivity of aqueous solutions decreased slightly. In addition, the curing temperature affected the conductivity, i.e., samples that were first cured at 60 ◦C for 24 h exhibited lower conductivity than those cured at room temperature.

Table 2 lists the concentrations of Ca, Si, and Al in aqueous solutions after 4 h at 150 ◦C, under an N<sup>2</sup> atmosphere of 2 MPa and an AAM concentration of 4 g/dm<sup>3</sup> .

**Table 2.** Ca, Si, and Al concentrations of aqueous solutions after 4 h at 150 ◦C, under an N<sup>2</sup> atmosphere of 2 MPa and an AAM concentration of 4 g/dm<sup>3</sup> .


In addition to those of Ca, Si, and Al, Mg and Na concentrations also were analyzed from water samples by inductively coupled plasma–optical emission spectroscopy (ICP-OES). However, the magnesium concentration was less than the detection limit (≤0.1 mg/dm<sup>3</sup> ), and the maximum sodium concentration was 1 mg/dm<sup>3</sup> after 4 h at 150 ◦C and an N<sup>2</sup> atmosphere of 2 MPa. All of the samples exhibited almost the same Ca and Si concentrations. However, with the increase in the amount of NaOH in the samples, the leaching of aluminum increased. Clearly, alkali activation immobilized Al in the inorganic matrix, but basicity enhanced its dissolution [41]. Furthermore, curing at room temperature led to the enhanced dissolution of Ca and Al from AAMs. A curing temperature of 60 ◦C has been found to be favorable for geopolymer preparation. Muñiz-Villarreal et al. [42] have reported that the optimum dissolution and formation of hydroxy species and oligomers, as well as further polymerization or condensation, occur at 60 ◦C. Therefore, with the increase in the curing temperature of the BFS-based AAMs, the leaching of Ca, Si, and Al decreased. Thus, based on these stability tests, AAMs that are first cured at 60 ◦C for 24 h are further characterized by XRD, diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), field emission scanning electron microscope with energydispersive X-ray spectroscopy (FESEM-EDS), ICP-OES, and surface area techniques, as well as being examined for the CWPO of a BPA aqueous solution.
