Mixturing of various additives to cement paste is known from the very beginning of building constructions. These additives were mainly substances of natural origin and their use has improved, for example, properties of fresh cement or influenced properties of the final product. In connection with retardation of the solidification process of the paste were certain organic compounds already studied in the early twentieth century. Nevertheless, the exact mechanism of their influence is not yet fully understood. At the starting phase of the hydration process within the cement paste, the portlandite crystals are formed and then the vast generation of the CSH phases followes. The inner fluctuations cause the portlandite clusters formation creating the metastabile system. This system is, due the nucleation, saturated by released positive ions of calcium. In this paper is investigated the effect of citric acid and tartaric acid on the hydration process of the fresh cement paste. There was carried out determination of the Ca2+ ions during the first induction period. A method developed by the Department of Physics at the Faculty of Civil Engineering Czech Technical University in Prague was used. Consequently, we observe the mechanical properties of cement pastes which are determined by the samples old 1, 3, 7, 14, and 28 days. The observation is performed on the three-point-flex test of beams on which the flexural strength is studied. Further we study compressive strength too.figureFig. 1 – Development of Compressive Strength This study was supported by SGS12/103/OHK1/2T/11. The mechanical tests were performed in the Klokner Institute of Czech Technical University in Prague.