<p>In addition to their mechanical properties, geopolymer structures are considered to be environmentally and economically friendly structures due to the non-requirement of calcination in their binding process, as compared with ordinary Portland cement structures. To take advantage of these properties, the water reaction occurring inside the precursors, which are used to supplement sufficient alkali activation during the binding process (geopolimerization), needs to be analyzed. In this study, microwave reflection and transmission measurements are applied as a non-destructive tool in the detection of water reaction with geopolymer paste samples with fly ash (FA), ground granulated blast furnace slag (GGBFS), and calcinated clay or metakaolin (MK) precursors having different sodium silicate/sodium hydroxide ratios. Our measurements indicate the following results. First, microwave transmission properties are more useful than and preferable to microwave reflection properties for the analysis of water reaction within all test geopolymer paste samples (FA, GGBFS, and MK). Second, reflection measurements (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(|S_{11}|\)</EquationSource> </InlineEquation>) exhibited minimal sensitivity to specimen length, with differences below 0.45 dB for thicknesses between 10 and 20 mm. Third, conversely, transmission responses (<InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(|S_{21}|\)</EquationSource> </InlineEquation>) showed pronounced variations of up to 35 dB, underscoring their enhanced sensitivity to internal water reaction mechanisms. Fourth, dielectric characterization indicated that FA powder possessed substantially higher dielectric loss (<InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\epsilon _{r}^{\prime \prime } \approx 0.50\)</EquationSource> </InlineEquation>) relative to GGBFS (<InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\approx 0.08\)</EquationSource> </InlineEquation>) and MK (<InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\approx 0.03\)</EquationSource> </InlineEquation>), which directly governed the magnitude of signal attenuation. Finally, the negligible mass loss observed across all prepared samples (&lt;2.5%) confirms that the evolution of <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(|S_{21}|\)</EquationSource> </InlineEquation> was dominated by the transformation of free water into bound water within the matrix rather than by evaporative losses. Scanning electron microscope micrographs and energy dispersive spectroscopy tests were also performed to examine the effect of water reaction within the prepared samples.</p>