The suggested method is based on the exploitation of the ultrasonic shear waves reflection phenomenon from an interface which consists in an elastic reference medium and a viscoelastic material of interest. We applied this method to the geopolymer; this polymer-like material is inorganically synthesized by the alkaline activation of an aluminosilicate source (metakaolin) in an aqueous solution. This work aims to understand the geopolymerization mechanisms from the early fresh paste up to the hardened state. A dedicated self-made cell was built to transmit a broadband pulse in the mega-hertz range to study the reflected shear wave from the elastic/viscoelastic interface. An important issue of this method is the accurate determination of the phase angle and the module of the acoustic reflection coefficient. Both parameters are directly linked to the viscoelastic evolution and, thus, to physicochemical properties. The complex shear modulus is obtained with this method. At present, the percolation time is well correlated with standard dynamic rheology characterization. © 2013 IEEE.