During calcium carbonate precipitation from hard waters, 3 anhydrous polymorphs are observed: calcite, vaterite and aragonite, calcite being currently the most thermodynamically stable form. In high super-saturation conditions, additional hydrated forms and molecular associations have been identified. The thermodynamic equilibrium conditions of the different calcium carbonate varieties in contact with waters containing various ionic species were calculated and presented as equilibrium curves and "isoTAC curves" in pCO2 vs [Ca2+] diagrams. Scaling experiments were performed by bubbling a neutral gas within hard waters containing various excesses of foreign salts or inhibitors. The precipitation process start with the formation of (CaCO3)n associations observed by in-situ micro-Raman spectroscopy. The supersaturation conditions at the nucleation time were deduced from the variation of the pH and [Ca2+] values, and were compared with the thermodynamic predictions. It is shown that the molecular associations can be considered as the precursors of the hydrated calcium carbonate in a first step, and of the anhydrous final polymorphs in a second step. In addition, it is shown that the calcium carbonate monohydrate precursor is promoted by increasing the ionic strength, whereas an excess of calcium promotes the amorphous hydrated form. A fair correlation between the hydrated precursors, as identified by the thermodynamic approach and the crystal structure of the final anhydrous forms was observed.