A guided wave approach to characterize the evolution of fresh mortar during the first twenty-four hours of hydration is presented. Using a through-transmission system, the method measures the energy leakage of the first torsional wave mode from a circular steel bar to the surrounding mortar. The evolution of mortar properties are discussed and correlated with the attenuation of the guided wave. The study shows that the technique is useful for monitoring the microstructure development and the curing rate of varying water-cement ratios (w/c = 0.40, 0.50, and 0.60). The effects of chemical (accelerant and retardant) and mineral (silica fume and fly ash) admixtures on the guided wave energy leakage are also presented and discussed. The guided wave approach is sensitive to changes in the water-cement ratio and to the addition of admixtures. Mortar compressive strength measurements at 12, 18, and 24 hours of hydration for the different mortar mixtures are presented. A correlation relating the energy leakage of the guided wave the strength development of the mortar is discussed.