Until now, the standardized methods developed for the detection of initial and final setting are destructive and non-continuous. The well-established ultrasonic pulse transmission velocity (UPV) method can be correlated to the setting process of mortar by the measurement of P-waves. However, S-waves seem to be better indicators of the setting of cement-based materials, since they are more sensitive to the solid matrix connectivity. This study proposes a new methodology, based on the combined monitoring of P- and S-waves, to determine the setting time of mortars containing two types of fly ashes. The S-waves velocity and dynamic elastic properties seem to be the most accurate indicators of the setting process, allowing capturing microstructural evolutions that isothermal calorimetry or usual UPV methods do not detect.