Alkali-activated fly ash geopolymers are structural materials that can be used as a sustainable alternative to ordinary portland cement concrete in infrastructure applications. However, the widespread use of geopolymers by the construction industry has been limited in part by a lack of understanding about the fundamental reaction mechanisms that occur during their formation. Since microwave signals are very sensitive to the presence and binding state of water, microwave materials characterization is used to investigate the role of water in two fly ash geopolymers during reaction at early ages. The results indicate that this measurement technique has a promising potential for assessment of changes in geopolymer material properties during their formation. This will allow for better prediction and control of the setting behavior and mechanical and durability properties of geopolymers.