Geopolymer cements have not found widespread use as a Portland cement replacement, in part because of the difficulty in proportioning mixtures in a reliable manner. Unlike Portland cements, which are manufactured materials with relatively consistent compositions, geopolymers are made from natural or waste aluminosilicate powders that come from a variety of sources and have highly variable compositions. These powders are mixed with caustic solutions, which must be selected carefully to optimize strength and durability. Geopolymer cement can be designed by tailoring caustic solution composition to the reactive phase composition of the solid component of the mixture; however, assessing which phases are reactive is challenging for complex and heterogeneous solids, such as fly ash. The work presented here focuses on applying a scanning electron microscopy and multispectral image analysis (SEM-MSIA) method to identify and quantify the reactive glassy phases in fly ash and to determine how these phases dissolve over time in a caustic activating solution. The fly ash was selected based upon its oxide contents and was analyzed for phase content using X-ray diffraction and Rietveld analysis (RQXRD) and SEM-MSIA, which identified multiple glassy phases in the fly ash. Next, the fly ash was suspended in 8M NaOH, and tested at various time intervals with SEM-MSIA to track changes in the amounts of each individual glassy phase initially identified in the fly ash. The results showed that an aluminosilicate glass (C-A-S) with a moderate amount of calcium appeared to be the most reactive between 0 and 28 days for a Class F fly ash. Other phases that were identified in the fly ash included a high-Ca C-A-S glass, two aluminosilicate glasses with different S/A ratios, two alkali-modified A-S phases, and an iron-containing glass.