*3.5. Fourier Transform Infrared (FTIR) Analysis of Cement Pastes*

As illustrated in Figure 7, all the paste samples show appearance of IR bands at the same location; however, their intensities differ. This is attributed to the formation of hydration products such as C-S-H and C-H [64]. The peaks from 900 to 1100 cm−<sup>1</sup> are associated with vibrations of Si-O bands in C-S-H phase [65]. The IR bands show a higher relative intensity of the Si-O band in paste samples containing 20% WSA (WSA20). Further, the samples of ternary blends having WSA and SF showed better results than those of control and binary mixes. This shift in the Si-O band is associated with polymerization of silica. A slight shift (960 cm−1) was detected in WSA20. On the other hand, the ternary concrete samples with different percentages of SF (5%, 7%, and 10%) showed a broader and significant shift (980 cm<sup>−</sup>1). A shift toward a higher wavenumber in the spectrum of binary and ternary pastes suggests formation of a high amount of C-S-H gels. A large amount of C-S-H gels are produced from the nucleation sites provided by the fine particles of SF, as also observed earlier through SEM–EDS analysis. The development of high compressive strength in these mixes can be linked to the formation of a large amount of C-S-H gels. Moreover, the peaks at 720, 875, and 1415 cm−<sup>1</sup> are associated with calcite formed as a result of carbonation [66].

**Figure 7.** FTIR spectra of control, binary (C/WSA), and ternary (C/WSA/SF) paste samples after 91 days of curing.

In all the paste samples, the peak at 3645 cm−<sup>1</sup> indicates the presence of free OH groups, which suggests the presence of the portlandite phase. The control sample exhibits a wider and more visible peak as compared to all other binary and ternary samples. However, this portlandite peak was reduced in all the binary mixes, which indicates the extent of portlandite consumption caused by the presence of amorphous silica in WSA. Interestingly, in ternary mixes containing WSA and SF, the peak remained very small or almost disappeared, indicating a high pozzolanic reactivity, which consequently results in greater consumption of C-H. This leads to formation of more C-S-H gels in these mixes [67]. As discussed in the preceding section, similar evidence of this large amount of C-S-H gels in these mixes was also noticed in SEM–EDS analyses.
