Cementitious materials are aging materials which transform from a fluid-like state to a solid. These changes are brought about by the hydration reaction between cement and water, which results in a continuous evolution of the microstructure of the material. There is a continuous decrease in the porosity and an increase in elastic stiffness of the matrix with time. Determination of the changes in the microstructure of hydrating cement through setting, and early strength gain of the material have remained major experimental challenges. Methods used for studying cement microstructure either alter the microstructure during sample preparation or sample preparation time exceeds the time associated with changes in microstructure. An ultrasonic test procedure for continuously monitoring changes in hydrating cementitious materials through setting using horizontally polarized shear waves (SH) is presented. A theoretical framework based on the poro-elastic idealization of the hydrating cement paste is developed for interpreting the ultrasonic reflection data. The wave response obtained from the poroelastic two-phase idealization of hydrating mortar as a water-filled solid matrix is shown to provide a realistic interpretation of the experimental response. The observed changes in the ultrasonic signal are shown to be related to changes in the porosity and the elastic modulus of the skeleton within the aging material. © 2013 American Society of Civil Engineers.