Natural and modified polyflavonoid tannin extracts have been shown to behave as super plasticizers of cement and concrete mixes, improving fluidity without any significant retardation of the onset of hardening. This was shown by applied tests, by conduction calorimetry, and by X-ray diffraction analysis. The tannin extracts’ behavior was ascribed to the balance of different effects, namely, (1) their increase in molecular mass induced by the presence of the silicate and aluminate component of cement, (2) the decrease first and then stabilization of the molecular mass and improved solubility induced by introduction of sulfonic groups in the structure of the tannin, and (3) the stabilization in the molecular mass induced by urea through its hindrance to tannin autocondensation and its decrease of tannin extract colloidal association in a water solution.