*3.2. Pore Structure of Hardened Pastes*

Pore structure is analyzed by MIP in order to understand the trend of compressive strength on the microstructure scale. Figure 3 shows the cumulative pore volumes of the alkali-activated BFFS at 3 d and 7 d. The pore structure results are consistent with the compressive strength. The 2 M activated paste had the weakest pore structure of all concentration groups. Figure 3a shows that the 2 M activated sample has many pores from 100 to 1000 nm in size, which are considered to be harmful in porous materials [34], and are mainly due to the relatively low concentration of 2 M NaOH, which slows down the early dissolution of the BFFS and reduces the production of C-A-S-H gel. The pore structure is fragile, and the compressive strength is low. The 12 M activated paste has a pore structure similar to that of the 8 M activated paste but does not match the compressive strength. Figure 3a shows that the pore structure curves are similar for 8 M and 12 M; the total pore volume for 8 M (0.249 mg/L) is slightly smaller than that for 12 M (0.265 mg/L). However, the compressive strength of the 8 M paste (24.9 MPa) is more than twice that of the 12 M paste (11.9 MPa) at 3 d. According to the section below, more crystal phases are formed at high pore concentrations. The possible reason for the optimal concentration is that although the 12 M NaOH has a pH that can dissolve the BFFS and form sufficient C-A-S-H, formation of a large amount of crystal phase diminishes the strength of AA-BFFS.

**Figure 3.** Cumulative pore volumes of alkali-activated BFFS paste at different ages: (**a**) 3 d; (**b**) 7 d.

Figure 3b shows the pore structure of AA-BFFS at 7 d, which is the same as at 3 d. Compared to the result at 3 d, the 2 M paste contains fewer pores from 100 to 1000 nm in size. The compressive strength of the 2 M paste at 7 d (17 MPa) is much greater than at 3 d (2.6 MPa), indicating that the pores between 100 and 1000 nm have a great influence on the strength of alkali-activated BFFS. The pore structures of 8 M and 12 M activated BFFS at 7 d are also improved. There are pores from 10 to 100 nm in size in the 7-d pore structure. The compressive strength of the 8 M (32.6 MPa) and 12 M (15.55 MPa) pastes at 7 d is greater than at 3 d (24.9 MPa and 11.8 MPa, respectively). The strength development of the 2 M paste is much greater than that of the 8 M and 12 M pastes, which indicates that the pores from 100 to 1000 nm have a greater impact on the compressive strength than the pores from 10 to 100 nm in AA-BFFS.

### *3.3. Phase Compositions of Hardened Pastes*

XRD was used to analyzed the phase composition in order to understand the effect of reaction production on the pore structure. Figure 4 shows the XRD spectra of the hardened pastes hydrated for 1 d, 3 d, 7 d and 28 d. The main crystal product of AA-BFFS is hydrotalcite (PDF#89-0460), which is the same as that of alkali-activated BFS [35,36]. The XRD patterns show that as the reaction progresses, calcite (CaCO3) and gypsum (CaSO4·2H2O) are gradually dissolved and consumed; spinel (MgO·Al2O3) did not participate in the reaction because it has a stable crystal structure, as reported in a previous study [36]. The main amorphous phase is C-A-S-H gel, which corresponds to the C-S-H(I) peaks in Figure 4 [36].

Comparing the XRD patterns of slag activated by different concentrations of sodium hydroxide, it was found that as the concentration increases, more crystals are formed in the product [37]. As spinel is stable in the pore solution of alkali-activated material, it can be used as an internal standard material for semiquantitative analysis. For 2 M activated BFFS, the rate of product generation was slow. Hydrotalcite was not observed in the XRD spectrum until 3 d (Figure 3a). The calcite in the 2 M XRD patterns was not consumed at 28 d, indicating that the low concentration of 2 M NaOH does not have sufficient alkalinity to dissolve BFFS and provide sufficient ions for crystal growth. Compared with paste activated by higher concentrations of NaOH, few C-S-H(I) peaks were observed in the XRD patterns. This can also be explained by the low concentration of NaOH. With an increase in age, the peak of hydrotalcite in the 2 M pattern did not change significantly, indicating that further reaction in the 2 M paste is minimal. The phase composition development of the 2 M paste explains the poor pore structure, and the low strength development of the 2 M mortar.

**Figure 4.** XRD spectra of hardened pastes at different ages: (**a**) 2 M, (**b**) 5 M. (**c**) 8 M. (**d**) 10 M and (**e**) 12 M.

When a higher concentration of NaOH (>2 M) was used to activate the BFFS, the hydrotalcite peak was clearly observed at 1 d. At the same age, the peaks of crystalline products and C-S-H(I) were higher with an increase in concentration, indicating that high alkalinity can promote product formation. High alkalinity accelerates the dissolution of BFFS, increasing the ion concentration in the solution [38,39] and promoting crystallization and growth of the products. Unlike the 2 M paste, the reaction continued to 28 d, as the peaks of hydrotalcite and C-S-H(I) are sharper than the peak of spinel (Figure 4b–e). This explains the strength and pore structure development. Comparing Figure 4c and Figure 4e, it is observed that the peaks of C-S-H(I) are similar in intensity according to the spinel peaks. The hydrotalcite peak of the 12 M paste (Figure 4c) is more intense than that

of the 8 M paste (Figure 4e), indicating that with an increase in the NaOH concentration, more hydrotalcite was formed than C-A-S-H gel. C-A-S-H is the main product providing strength to alkali-activated material. The formation of hydrotalcite consumes more calcium and aluminum in the solution, reducing the Ca and Al content in the C-A-S-H gel, resulting in a low-degree C-A-S-H gel. Hydrotalcite and portlandite are preferred to enrich the particles and fine aggregates, forming an interfacial transition zone (ITZ) and reducing the compressive strength. As a high concentration of NaOH promotes generation of hydrotalcite and other crystals, the compressive strength of high-concentration NaOH is lower.

### *3.4. Isothermal Calorimetry*

Isothermal calorimetry was measured to analyze the reaction mechanism and production of the reaction products. Figure 5 shows the isothermal calorimetry results for the alkali-activated pastes within 96 h. Isothermal calorimetry was performed to evaluate the effect of concentration on the kinetics of alkali-activated BFFS. The heat evolution curves of the alkali-activated BFFS are shown in Figure 5a. The evolution peak occurred in the early stage of the reaction (within 4 h). With an increase in the concentration, the evolution peak became more intense. The first exothermic peak of alkali-activated materials can be interpreted as the heat of wetting and the heat of dissolution according to [40]. The results show that a higher pH of the solution increases the dissolution of BFFS. This is consistent with the XRD results. A fast and intense evolution indicates fast dissolution of the BFFS, which provides sufficient ions in the concentration for crystallization. The 2 M paste shows a low heat evolution in poor formation of product and pore structure. The 5 M curves in Figure 5a show a broad shoulder peak from 3–4 h. This curve is widely accepted as an acceleration peak. The shape of this curve is similar to that for Portland cement, although the temporal occurrences are different [41].

**Figure 5.** Isothermal calorimetry curves for paste: (**a**) heat flow and (**b**) cumulative heat.

Figure 5b shows the cumulative heat of the AA-BFFS. The total heat of the 2 M paste is significantly less than that of the others, indicating that the low pH of the 2 M NaOH reduces the reaction rate and the product formation of the paste (mainly C-A-S-H gel). This is direct evidence to explain the low strength and poor pore structure of the 2 M paste. However, for high-concentration pastes, the total heat within 72 h is similar, which is consistent with the XRD results. The cumulative heat and heat evolution indicate that the violent reaction at the early age hinders the later C-A-S-H production.

### *3.5. FT-IR Spectra of Hardened Pastes*

The chemical bonding and the polymerization degree of C-A-S-H was analyzed by FT-IR. Figure 6 shows the FT-IR spectra of hardened pastes hydrated for 1 d, 3 d, 7 d and 28 d. Three obvious peaks are observed. The most important peak is at approximately 950–1000 cm−<sup>1</sup> , corresponding to the Si–O asymmetric stretching vibrations (ν3) of Q2

units [41–43]. The peak at 870–875 cm−<sup>1</sup> is related to the asymmetric stretching (ν3) and out-of-plane bending (ν2) modes of CO2<sup>−</sup> 3 ions [41,44]. The weak peak at 650–700 cm−<sup>1</sup> corresponds to the Al–O–Si bonds [43]. M. This means the optimal concentration result in the higher polymerization degree of C-A-S-H gel. This explains the compressive strength and pore structure, which are consistent with the XRD patterns.

The chemical bonding and the polymerization degree of C-A-S-H was analyzed by FT-IR. Figure 6 shows the FT-IR spectra of hardened pastes hydrated for 1 d, 3 d, 7 d and 28 d. Three obvious peaks are observed. The most important peak is at approximately 950**–**1000 cm−1, corresponding to the Si**–**O asymmetric stretching vibrations (ν3) of Q2 units [41,42,43]. The peak at 870**–**875 cm−1 is related to the asymmetric stretching (ν3) and

The band at 875 cm−1 corresponds to calcite in the precursor. The shifting of this band can be interpreted as the dissolution of calcite and the formation of other unknown

indicating that calcite did not react in the 2 M paste. This is consistent with the results in

is weaker, disappears or shifts to other wavenumbers, indicating that calcite dissolved and transformed into other carbonates. The results indicate that higher concentrations promote crystal dissolution and new crystal formation, which is consistent with the XRD

The bands at 950**–**1000 cm−1 are different for different concentrations. At 28 d, with an increase in the concentration, the band at 950**–**1000 cm−1 first shifted toward a lower wavenumber and then back to a higher wavenumber. The lowest wavenumber was observed in 8 M activated BFFS (948 cm−1, Figure 6c), and is consistent with the compressive strength. The shifting of the band from 950**–**1000 cm−1 is caused by different Si/Al ratios in the Si-O-T (Si, Al) [45]. A higher wavenumber corresponds to an "Si-rich" bond with a lower degree. A lower wavenumber corresponds to an "Al-rich" bond with a higher polymerization degree [35]. Thus, the wavenumber trend from low concentration to high concentration can be interpreted as the Al/Si ratio and polymerization degree of C-A-S-H gel first increasing and then decreasing when the concentration is greater than 8

weaker, indicating that calcite dissolved. For the last two concentrations, the CO2<sup>−</sup>

3 ions [41,44]. The weak peak at 650**–**700 cm−<sup>1</sup>

3 bond of the 2 M paste remains at 28 d,

3 bond broadens and becomes

3 bond

*Crystals* **2021**, *11*, x FOR PEER REVIEW 8 of 11

*3.5. FT-IR Spectra of Hardened Pastes* 

out-of-plane bending (ν2) modes of CO<sup>2</sup><sup>−</sup>

carbonates. Figure 5a shows that the CO<sup>2</sup><sup>−</sup>

Figure 4a. For the 5 M and 8 M concentrations, the CO2<sup>−</sup>

corresponds to the Al**–**O**–**Si bonds [43].

results.

**Figure 6.** FT-IR spectra of hardened pastes at different ages: (**a**) 2 M, (**b**) 5 M, (**c**) 8 M, (**d**) 10 M and (**e**) 12 M.

The band at 875 cm−<sup>1</sup> corresponds to calcite in the precursor. The shifting of this band can be interpreted as the dissolution of calcite and the formation of other unknown carbonates. Figure 5a shows that the CO2<sup>−</sup> 3 bond of the 2 M paste remains at 28 d, indicating that calcite did not react in the 2 M paste. This is consistent with the results in Figure 4a. For the 5 M and 8 M concentrations, the CO2<sup>−</sup> 3 bond broadens and becomes weaker, indicating that calcite dissolved. For the last two concentrations, the CO2<sup>−</sup> 3 bond is weaker, disappears or shifts to other wavenumbers, indicating that calcite dissolved and transformed into other

carbonates. The results indicate that higher concentrations promote crystal dissolution and new crystal formation, which is consistent with the XRD results.

The bands at 950–1000 cm−<sup>1</sup> are different for different concentrations. At 28 d, with an increase in the concentration, the band at 950–1000 cm−<sup>1</sup> first shifted toward a lower wavenumber and then back to a higher wavenumber. The lowest wavenumber was observed in 8 M activated BFFS (948 cm−<sup>1</sup> , Figure 6c), and is consistent with the compressive strength. The shifting of the band from 950–1000 cm−<sup>1</sup> is caused by different Si/Al ratios in the Si-O-T (Si, Al) [45]. A higher wavenumber corresponds to an "Si-rich" bond with a lower degree. A lower wavenumber corresponds to an "Al-rich" bond with a higher polymerization degree [35]. Thus, the wavenumber trend from low concentration to high concentration can be interpreted as the Al/Si ratio and polymerization degree of C-A-S-H gel first increasing and then decreasing when the concentration is greater than 8 M. This means the optimal concentration result in the higher polymerization degree of C-A-S-H gel. This explains the compressive strength and pore structure, which are consistent with the XRD patterns.

### **4. Conclusions**

In this study, the properties of alkali-activated BBFS were investigated at different concentrations of NaOH (2 M, 5 M, 8 M, 10 M, 12 M). Based on the results, the following conclusions can be drawn.


FT-IR result indicates that the polymerization degree of C-A-S-H gel is consistent with the compressive strength, the optimal concentration of 8M shows the highest polymerization degree. The result indicates both low and high concentration reduce the polymerization degree of C-A-S-H which also do harm to the compressive strength.

**Author Contributions:** Z.H.: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing—original draft. Y.Z.: Investigation, Methodology. Y.C.: Conceptualization, Formal analysis, Funding acquisition, Supervision, Validation, Writing—review & editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by [National Natural Science Foundation of China] grant number [No. 51822807].

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The data presented in this study are available within the manuscript.

**Acknowledgments:** The authors would like to acknowledge the National Natural Science Foundation of China (No. 51822807).

**Conflicts of Interest:** The authors declare no conflict of interest.
