Geopolymers are being studied extensively as a potential replacement for Portland cement. However, very few studies have focused on characterizing the setting and stiffening of geopolymers. The main objective of this research is to relate microstructural changes with the hardening of geopolymers, especially Class F fly ash-slag-based geopolymers. The raw materials were thoroughly mixed with potassium silicate and potassium hydroxide for activation. The hardening rate was studied using the Proctor penetration test (ASTM C430) and the shear wave ultrasonic wave reflection method. The microstructural changes responsible for hardening were studied by characterizing geopolymer solids using Fourier transform infrared spectroscopy (FTIR) and the x-ray diffraction method (XRD). It was observed that XRD could not detect the changes happening with time because the geopolymer product was amorphous. However, FTIR clearly showed a shift in the Si-O-7" (7"=Al, Si) asymmetric stretching band to lower wavenumbers, indicating the development of a geopolymer product with time. The main drawback of using FTIR was that Si-O-Tvibrations from the initial ash, the geopolymer product, and calcium silicate hydrate yield an overlapping spectrum resulting in a broad hump that was difficult to interpret. In order to resolve this, FTIR was performed on the solid residue after selective dissolution of calcium silicate hydrate and geopolymer product by salicylic acid and hydrochloric acid, respectively. This revealed the presence of calcium silicate hydrate in the samples at an early age.