In this manuscript a multi-modal non-destructive testing (NDT) approach is presented for hydration monitoring of fresh mortar based on ultrasonic pulse velocity (UPV) and electromagnetic (EM) transmission measurements. The EM millimeter wave velocity and permittivity are measured for the first time throughout cement hydration and accurately demonstrate the conversion of mortar's free water to chemically bound. UPV and isothermal calorimetry complementarily correlate with the stiffness development and the heat of the exothermal reaction, respectively. The common trends of all methods are benchmarked in this multi-modal proof-of-concept study that reveals unknown links between the EM and elastic properties of the material, enabling contactless monitoring of stiffness development during hydration.