The effect of specimen composition on the physicochemical processes in cemented paste backfill is studied using small perturbation electromagnetic waves. The factors that are tested include binder type and content, selected chemical admixtures (superplasticizers), mineral additives (e.g., fly ash), and pore fluid chemistry (e.g., ionic concentration and pH). Two measurement techniques are used: a two-electrode capacitor-type system at low frequencies (100 Hz to 100 kHz) and an open-ended coaxial termination probe at high frequencies (200 MHz to 1.3 GHz). The electromagnetic measurements are sensitive to changes in the CPB composition. The temporal variations in the effective conductivity reflect the various hydration stages, while the decrease in the real relative permittivity with time is due to a reduction in free water content.